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General Automotive Wiring Guidelines

There are a number of tools and techniques you will need to wire MicroSquirt^® to your vehicle. You will also need some specialized knowledge. We will try to present an overview of everything you need here (if you have questions, ask them on the forums at www.microsquirt.com):

Tools:

• Crimper: These come in a wide variety of styles and quality, and you often get what you pay for. The cheapest are stamped from flat steel, and these often distort (the jaws slip off each other) before a good crimp is made. As a minimum, it is worth getting a forged steel crimper that has wide jaws (~½") that hold the butt splice connectors in place while the connector is crimped, resulting in a strong and reliable joint. These can be found for around $20 or so. Remember that you will be relying on these connections every minute you are driving your vehicle, and under all conditions. You will want them to be as solid as possible, as deficient connections are often difficult to troubleshoot.
  

• Multimeter: You will need a digital (or analog) multi-meter that can check potential (Volts), current (Amps), and resistance (Ohms). You do not need an expensive meter as you will be mostly checking to see if things are approximately correct, and lower cost digital multi-meters (DMM) can often be found for under $20.
• Lighter/heat source: This is handy for shrinking heat shrink tubing over butt splice connectors or solder joints. You can do it with matches, but this will be slow and sometimes painful. For a heat source, you can use a cigarette lighter (in well ventilated conditions well away from any gasoline), or a heat gun that blows hot air.
• Vehicle Wiring Diagram: You will need to know which wires go where on your vehicle in order to connect MicroSquirt^® appropriately. The best wiring diagrams and other information are generally found in a factory service manual for your vehicle, but of course there are other sources (aftermarket manuals, Internet, etc.).
• Soldering Iron or Gun (optional): If you choose to solder wires together , rather than using butt splice connectors, you will need a soldering iron or gun. This should be rated to at least 25 Watts, as smaller irons may not heat the joint hot enough to provide a good connection. Before using the iron/gun, plug it in and let it heat up for several minutes to help ensure good connections.

Supplies:

• Wire: to extend the supplied pigtail as and when necessary. For the injector or ground leads, this should be at least 18 gauge wire, for the remaining connections 20 or 22 gauge is sufficient (large gauge numbers mean thinner wire).
• Heat shrink tubing: This can be used in short section to cover soldered or crimped connections to make them more 'weather proof' and give additional mechanical strength to the connection.
• Connectors: These can be butt-splice connection to join wires (such as the supplied harness pigtail to a vehicle harness), or specific connectors for things like injectors, sensors, and ignition modules, etc. Crimp-style butt splice connectors are supplied with MicroSquirt^® to join the harness to connector pigtails for external components. MicroSquirt^® does not come with the external connectors you need, since there are thousands of different types, a many users will already have these as part of their OEM installation in any case. If you need specific connectors, try an auto parts store or one of the MicroSquirt^® distributors.
• Solder (optional): You can solder the wires instead of using butt splice connectors. Use standard 60/40 rosin core solder (silver solder is not required).
• Nylon ties: These are very useful to make sure wiring isn't dangling loose, and can't contact moving or hot vehicle components. Make sure you allow some slack when wiring between the engine and body/frame, as they move relative to each other, sometimes quite a bit (up to several inches).
• Electrical tape: This can be used in the place of heat shrink tubing, and even nylon ties. It is also useful for 'wrapping' sections of harness wiring to keep it tidy and bundled.
• Fuel injection system components: MicroSquirt^® controller is just the controller, so you will need injectors, fuel pump, coolant and air temperature sensors, a throttle position sensor, a MAP sensor, an O2 senor (optional but helpful), a fast idle valve (optional), and a number of relays, fuses, and other automotive electrical components to suit your application. The relay board can reduce the complexity of some of the wiring.

Of course you will also need a PC computer to run the tuning software. Any computer that can run Windows 98 or higher (i.e., WinMe, WinXP, WinVista) will work. Ideally the computer will have a serial port (9-pin male connector), but many USB/Serial adapters have been used successfully. A laptop computer is easiest to use in the vehicle, of course, and cheap older laptops with Win98+ and a serial port are commonly available at low cost.

Wiring Procedure

NOTE: The wire labeled "Vref" is the +5V voltage source for the MAP sensor and the TPS sensor. This wire only connects to these components (and possibly the LEDs, if used), so be very careful to not connect this directly to ground or (worse) +12V battery. Bad things will occur if you mis-wire this one - double check your work.

When wiring up a vehicle, we recommend performing this in discrete and deliberate stages - this is how to approach this:

1. You may wish to use the MicroSquirt^® access board, which can simplify the wiring for many installations.

2. Always make sure the dangling wires on the MicroSquirt^® harness are not touching each other or any metal (ground return path). Next, wire up all of the sensors (TPS, EGO, coolant, intake air temp, MAP) and the signal return grounds (use the sensor ground return wire for these). Then hook up the main ground wires to the engine block and the switched +12V for the +12V lead. Check again to make sure the remaining dangling wires (which includes the outputs - you have not hooked them up yet) are insulated from each other and anything else - tape these if need be.

3. Hook up the laptop and run MegaTune and establish communications - remember that with the latest MS-II embedded code (V2.881 and later) you need to set the "ECU Type" to MicroSquirt^® the first time you run (click the "burn to ECU" button for this and any other change to stick). Until you set the ECU Type, all the other parameters will be grayed out.

4. Let MegaTune and the MicroSquirt^® just run for a while and talk to each other, at least for several minutes. If there are any communications burps/hiccups, this is the time to find them out and correct (it's a nice, clean electrical environment at this point). Do not worry about the MicroSquirt^® sucking down the vehicle battery, its current draw is around 40 milliamps, so it is a very small load - the dome light above your head draws many times more current. This is also a good time to get familiar with MegaTune - set up the number of cylinders, REQ_FUEL, ignition trigger type, etc. - and, maybe save off a MSQ or two, just to get the feel for how it all works. Read Configuring MegaSquirt-II for information on configuring MicroSquirt^® and MegaTune. You should also:
   • check the sensor readings in the real-time display ('Tuning/Realtime Display' in MegaTune) to see if you have reasonable values returned by the sensors,
   • calibrate the TPS,
   • set the MAP sensor type,
   • set the EGO sensor type, etc.
   If this works as expected (the sensor values look good, the TPS values aren't backwards, you have set the correct MAP sensor type, etc.), you are in great shape, and you are potentially minutes away from a running engine.

5. Also, in this state, with no ignition or injectors hooked up (and no fuel pump active), turn over the starter motor with the MicroSquirt^® turned on and MegaTune running. This is a big current draw and if there are grounding issues you will see the sensor readings jump all over the place - if so then check your engine-to-battery-to-chassis grounds (see the sticky on this). Less of an influence but perhaps more fun is to turn on the windshield wipers, heater fan, headlights, and even blow the horn - each of these can produce noise on the +12V feed and it is better to find this out now so you can rectify instead of tracking down, say, a random tach glitch that is really caused by battery feed noise - it happens.

6. Next, hook up the tach input source, and check for tach signal with MegaTune with the starter cranking. If you are using EDIS, GM DIS, or HEI module it is often better to test this along with the outputs (next step).

7. Finally, hook up the fuel pump relay, injectors, and ignition. And then start it up (they always start right up, or at least do something), tweak the tune, and drive away! Read Tuning MegaSquirt-II for information on tuning.

For each circuit, you will need to:

1. Choose the order of circuits you will work on. Generally, this should be:
   1. Power and ground circuits. Completing this step will let you power up MicroSquirt^® EFI controller, and test the connection to MegaTune on your tuning laptop.
   2. Sensors:
      • Coolant and Intake Air Temperature sensors (CLT and IAT),
      • Throttle Position Sensor (TPS),
      • Exhaust Gas Oxygen (EGO, aka O2) sensor (if used),
      • Manifold Absolute Pressure (MAP) sensor.
   3. Actuator and Injector Outputs: These include:
      • Fuel Pump,
      • Fast Idle (if used),
      • Ignition control module (if used),
      • Injectors.
   4. Ignition Inputs and Outputs:
      • Ignition Inputs ("tach" signal(s)),
      • Ignition Outputs (coils or ignition module driver(s))
   (Many of these are broken out separately from the main wiring diagram below to make it easier to follow.)
2. For each item in your list, identify the wires you need to connect from the list and wiring diagram below and your vehicle wiring diagram.
3. For each wire, crimp or solder the connection:
   • Crimp:
     • Strip the insulation off the end of the harness pigtail and the end of the wire you are connecting to by about ¼" (6mm). Crimping tools often have wire strippers built in to them.
     • Crimp a butt splice connector onto the end of one of the wires you stripped, and give it a tug to make sure it won't pull loose easily (if it does, cut the butt splice connector off and use a new one to redo the connection),
     • Slide a piece of heat shrink tubing (that is large enough to pass over the butt splice connector and approximately twice as long as the butt slice connector) onto the other wire,
     • Position the butt splice connector on the second wire, and crimp it, then give it a tug to make sure it won't come loose.
   • Solder:
     • Strip the insulation off the end of the harness pigtail and the end of the wire you are connecting to by about ½" (12mm),
     • Slide a piece of heat shrink tubing about 1 to 1½" long over one of the wires,
     • Twist the wires together so that they are tighten wound in a spiral pattern,
     • Heat the twisted wires with a hot soldering iron/gun, until the wires are hot enough to melt the solder,
     • Touch the solder to the wires (not the iron or gun) until it melts and is wicked into the joint,
     • Feed in enough solder to completely saturate the wire connection,
     • Let the joint cool.
4. Slide the heat shrink tubing over the connection, center it over the connection, then shrink it into place with a heat source.
5. Verify that you have made the correct connection(s) at each step.
6. After each step, test the function of the unit. For example, you can see if:
   • your MicroSquirt^® communicates with MegaTune on your laptop after connecting the power and ground wires,
   • you get 'sensible' readings from the temperature and MAP sensors after hooking these up (temperatures should be about ambient, MAP should be between 90 and 103 in most cases - except at high elevations where it might be lower.)
   • your fuel pump comes on after hooking it up and powering up MicroSquirt.

Wiring MicroSquirt^® to the external harness is much the same as with MegaSquirt-II, and you can read much more about that here: www.megamanual.com/v22manual/mwire.htm. You use the same sensors, etc.

However, because of the change to the AMPSEAL connector (from the DB37), and the absence of the DB9 connector, the external connections are somewhat different. The AMPSEAL connections are:

To wire MicroSquirt^® EFI controller, you need to make the following connections:

Pin	Name	Function	Harness Wire Color(s) (main/stripe)
Pin1	12 Volt Supply	This connects to a switched 12 Volt supply. Use the ignition switch to control a relay that provides 12 volts to both the MicroSquirt® and the injectors.	Red
Pin2	CAN High	This is one of two Controller Area Network (CAN) communications lines, used to communicate with MegaSquirt peripherals.	Blue/Yellow
Pin3	CAN Low	This is one of two Controller Area Network (CAN) communications lines, used to communicate with MegaSquirt peripherals.	Blue/Red
Pin4	VRIN2+	Second ignition input signal (Input 2) for a cam position sensor or second crank sensor with the Dual Spark option.	Brown/White
Pin5	Spare Input	This is a second spare ADC(ADC7). It is used for alternate MAF pin or knock sensing, but it can be used to record whatever 0-5 Volt signal is put on it if neither the MAF option nor the knock option uses it. In this case it is stored in outpc.knock.	n/a
Pin6	Flex Fuel	For input from a ethanol Flex Fuel sensor.	Purple/White
Pin7	FIDLE	This is the output for controlling an ON/OFF style FIdle or a PWM style IAC (usually found on Fords). You cannot use a stepper type IAC (with 4 pins), only FIdle (on/off) or PWM style idle valves (2 pins) are supported.	Green
Pin8	FUEL PUMP	This output controls a fuel pump relay by providing a ground when the fuel pump should be running. It cannot control a fuel pump directly.	Purple
Pin9	INJECTOR 1	This pin connects to the injectors. You connect half the injectors on one bank (INJECTOR 1) and half on the other (INJECTOR 2). These connect to one pin on each connector. The other pin on each connector is supplied with 12 Volts through the main relay (see diagram) and the INJECTOR pins ground the injectors to 'fire' them and squirt fuel.	Green
Pin10	INJECTOR 2	This pin connects to the injectors. You connect half the injectors on one bank (INJECTOR 1) and half on the other (INJECTOR 2). These connect to one pin on each connector. The other pin on each connector is supplied with 12 Volts through the main relay (see diagram) and the INJECTOR pins ground the injectors to 'fire' them and squirt fuel.	Blue
Pin11	IGNITION OUTPUT #2	Second ignition output signal. Note that this wire can be used to directly control one coil, and it can carry up to 7.5 Amps of current (be sure to read MicroSquirt® Direct Coil Control). The coil's primary current can generate a lot of noise inside the MicroSquirt. The whole MicroSquirt® setup is very compact and noise coupling in the wires can be of concern. Running this wire (and the IGNITION OUTPUT #1 where appropriate) in their own shielded "conduit", along with the injector outputs, can substantially reduce noise injection back into the tach circuit. You can buy shielded wire for this, or wrap 2 to 3 layers of aluminum foil (have a look into your kitchen) around the wires and connect the shield to ground.	White/Red
Pin12	IGNITION OUTPUT #1	Ignition output signal. Note that this wire can be used to directly control one coil, and it can carry up to 7.5 Amps of current (be sure to read MicroSquirt® Direct Coil Control). The coil's primary current can generate a lot of noise inside the MicroSquirt. The whole MicroSquirt® setup is very compact and noise coupling in the wires can be of concern. Running this wire (and the IGNITION OUTPUT #2 where appropriate) in their own shielded "conduit", along with the injector outputs, can substantially reduce noise injection back into the tach circuit. You can buy shielded wire for this, or wrap 2 to 3 layers of aluminum foil (have a look into your kitchen) around the wires and connect the shield to ground.	White
Pin13	SERIAL Rx	This pin is used to receive data from the laptop computer serial port (usually a DB9 or DB25). It is connected to Pin3 on a DB9 connector (Pin2 on a DB25) to plug into your laptop/notebook computer.	Red (pre-wired to jack)
Pin14	SERIAL Tx	This pin is used to receive data from the laptop computer serial port (usually a DB9 or DB25). It is connected to Pin2 on a DB9 connector (Pin3 on a DB25) to plug into your laptop/notebook computer.	Orange (pre-wired to jack)
Pin15	BOOTLOADER	This pin is grounded to enter bootloader mode, which allows you to update the embedded code in MicroSquirt. You can connect this lead to a switch, with the other side of the switch connected to ground. When you close the switch and reboot MicroSquirt® EFI controller, you will be in bootloader mode.	Purple/Black
Pin16	ACCEL LED	This is the supply for an 'acceleration indicator LED'. You connect the cathode (short lead) of and LED (light emitting diode) to this wire. The longer lead of the LED you connect to a 330 Ohm resistor, then connect the other side of the resistor to the Vref wire (5 Volts). Be certain that Vref will not be grounded or connected to 12 Volts. Or you could use some other voltage source beside Vref. For example, you could use the battery supply voltage (nominally 12 Volts) as long as you increase the current limiting resistor value to 1K Ohms (to keep the current within limits). The LED will then light whenever accel enrichment is in effect. When used as a spare port, this output can drive up to 5 Amps absolute maximum (4 Amps is a safer limit)	Yellow/Black
Pin17	WARM-UP LED	This is the supply for an 'warm-up indicator LED'. You connect the cathode (short lead) of and LED (light emitting diode) to this wire. The longer lead of the LED you connect to a 330 Ohm resistor, then connect the other side of the resistor to the Vref wire (5 Volts). Be certain that Vref will not be grounded or connected to 12 Volts. Or you could use some other voltage source beside Vref. For example, you could use the battery supply voltage (nominally 12 Volts) as long as you increase the current limiting resistor value to 1K Ohms (to keep the current within limits). The LED will then light whenever warm-up enrichment is in effect. When used as a spare port, this output can drive up to 5 Amps absolute maximum (4 Amps is a safer limit)	Yellow/White
Pin18	N/C (ground)	Not Used	n/a
Pin19	SERIAL GROUND	Serial communications dedicated ground.	Green (pre-wired to jack)
Pin20	SENSOR GROUND	This is a dedicated sensor ground wire. All of the sensor grounds can be connected to this wire to reduce then possibility of electrical noise.	White/Black
Pin21	GROUND	This is one of several ground wire pins (18 through 23). All of the ground wires should be run to the same spot on the engine (to avoid ground loops). Make sure you have a good ground connection from the batteries negative terminal to the engine, and from the engine to the frame as well.	Black
Pin22	GROUND	This is one of several ground wire pins (18 through 23). All of the ground wires should be run to the same spot on the engine (to avoid ground loops). Make sure you have a good ground connection from the batteries negative terminal to the engine, and from the engine to the frame as well.	Black
Pin23	GROUND	This is one of several ground wire pins (18 through 23). All of the ground wires should be run to the same spot on the engine (to avoid ground loops). Make sure you have a good ground connection from the batteries negative terminal to the engine, and from the engine to the frame as well.	Black
Pin24	MAP	This pin is connected to the output from a MAP sensor that puts out a 0 to 5 Volt signal (roughly) proportional to absolute pressure. General Motors MAP sensors are suitable (Pin24 connects to pin B on the sensor, pin A is grounded, pin C gets 5 Volts from Vref (Pin28 on MicroSquirt)). You can also use the original MPX4250 MAP sensor used with MegaSquirt, you connect the pion #24 to the sensor's Pin1, Pin2 is grounded, and 5 Volts (Vref) is connected to pin#3.	Green/Red
Pin25	CLT	MicroSquirt® uses the coolant temperature to determine the warm-up enrichments. The sensor must be a negative temperature coefficient (meaning the resistance decreases as the temperature increases). Most automotive temperature sensor are this type, however the default sensors are General Motors sensors.	Yellow
Pin26	IAT	MicroSquirt® uses the intake air temperature (aka. manifold air temperature = MAT) to determine the air density for fuel calculations. The sensor must be a negative temperature coefficient (meaning the resistance decreases as the temperature increases). Most automotive temperature sensor are this type, however the default sensors are General Motors sensors.	Orange
Pin27	TPS	This is a pin for the 'sense' connection on a throttle position sensor. To hook up your throttle position sensor (TPS), disconnect the TPS, and use a digital multi-meter. Switch it to measure resistance. The resistance between two of the connections will stay the same when the throttle is moved. Find those two - one will be the +5 Vref and the other a ground. The third is the sense wire to MegaSquirt. To figure out which wire is the +5 Vref and which is the ground, connect your meter to one of those two connections and the other to the TPS sense connection. If you read a high resistance which gets lower as you open the throttle, then disconnected wire is the one which goes to ground, the other one which had the continuous resistance goes to the +5 Vref from the MegaSquirt, and the remaining wire is the TPS sense wire.	Blue
Pin28	Vref	This is a 5 volt supply for the throttle position sensor and the MAP sensor. DO NOT connect it to 12 Volts! The wire labeled "Vref" is the +5V voltage reference for the MAP sensor and the TPS sensor. This wire only connects to these components, so be very careful to not connect this directly to ground or (worse) +12V battery. Bad things will occur if you miswire this one - double check your work. If your MicroSquirt has processor resets once installed, hooking up an external capacitor to Vref helps. Anything from 47µFarads up to 1000µF on the Vref to sensor ground return will help.	Gray
Pin29	SPARE ADC	Can be used for external baro MAP sensor.	Orange/Green
Pin30	OPTOIN+	Square wave ignition signal input positive connection for Input 1. Note that you can use only one of OPTIN+/- or VRIN+/-, because they connect to the same processor pin. For the second ignition input (Input 2), use VR2IN+ (Ampseal pin 4). If you are triggering directly off the coil (instead of something like a missing tooth wheel or another relatively low-voltage signal), connect OPTOIN+ (Ampseal pin #30) to the coil's negative terminal as usual, but connect the OPTOIN- (Ampseal pin #31) to a 12V source rather than grounding it. This then uses the flyback spike off the coil as a trigger (rather than the 12 Volt signal). Doing this reduces the energy dissipated in the circuit, prevent a thermal 'melt-down' of the components inside MicroSquirt®.	Gray/Red
Pin31	OPTOIN-	Square wave ignition signal input negative for Input 1. Note that you can use only one of OPTIN+/- or VRIN+/-, because they connect to the same processor pin. For the second ignition input (Input 2), use VR2IN+ (Ampseal pin 4). If you are triggering directly off the coil (instead of something like a missing tooth wheel or another relatively low-voltage signal), connect OPTOIN+ (Ampseal pin #30) to the coil's negative terminal as usual, but connect the OPTOIN- (Ampseal pin #31) to a 12V source rather than grounding it. This then uses the flyback spike off the coil as a trigger (rather than the 12 Volt signal). Doing this reduces the energy dissipated in the circuit, prevent a thermal 'melt-down' of the components inside MicroSquirt®.	Gray/Black
Pin32	VRIN1+	Variable reluctor A/C signal input for Input 1. Note that you can use only one of OPTIN+/- or VRIN+/-, because they connect to the same processor pin. For the second ignition input (Input 2), use VR2IN+ (Ampseal pin 4)	Silver (coax center wire)
Pin33	VRIN-	Variable reluctor A/C signal input (ground) for Input 1 & Input 2 (if used). Note that you can use only one of OPTIN+/- or VRIN+/-, because they connect to the same processor pin. For the second igntion input (Input 2), use VR2IN+ (Ampseal pin 4)	Silver (coax shield)
Pin34	O2	This is connected to the oxygen sensor output wire. This may be a 0 to 1 Volt signal directly from a narrow band sensor, or a 0 to 5 Volt signal from a wide band sensor controller.	Pink
Pin35	TACH OUTPUT	This connection can be used to drive a standard tach (OEM or aftermarket). For details on how to hook it to your tach, see your tach installation manual or OEM service manual.	Green/Yellow

AMPseal 35 pin Connector

The AMPseal connector has 35 pins, numbered in rows from 1-2, 13-23, and 24 to 35. They are numbers from left to right as viewed looking in to the connector on MicroSquirt. These numbers are also imprinted in the connector itself, if you look carefully.

Injector and Power Wiring

Note that MicroSquirt^® and the injectors MUST be powered off the same relay (the 'main relay'). If the injectors are powered while MicroSquirt^® controller is not, the injectors might be grounded and flood the engine with fuel.

Grounds

Make sure the grounds are correct. When running the grounds on MicroSquirt^® EFI controller, it is important to remember that there are different "types" of grounds. These are:

1. High power grounds (Ampseal pin 21,22,23) - these are the returns for the fuel injector and ignition drivers, and the fuel pump/fast idle/spare output drivers. There are three wires on the connectors for these, going to pins 21, 22, and 23 on the AMPSEAL - these are for the high power ground path. All three (3) of these wires need to ground direct to the engine block. It is important to run all three wires because it will reduce both the resistance and the overall inductance of the ground return path. Each wire has a resistance, and using three of them in parallel reduces the overall resistance. Equally important, each wire has an inductance, and inductances do not "like" fast-changing signals (like a pulse from a spark) and can cause very brief voltage offsets in the ground path. By having multiple wires it is the same as having multiple inductors in parallel, resulting in an overall lower inductance. At the point on the engine block where the grounds hook together, it is a good idea to run a separate wire from this junction back to the battery as well. This is redundant, however it often cleans up noise from sources like the starter motor. And, if it does help then you should take another look at your big positive and negative wire on the battery. Since we are talking battery - the point where you pick up the +12V to power the MicroSquirt^® controller is very important. This will go through a relay in order to turn on and off the MicroSquirt^® EFI controller, and the power source for the +12V on the relay needs to go back to the battery, or a path that leads direct to the battery without a long run of wiring. Just like for the grounds, run a separate wire from the relay direct back to the battery just to be sure.
2. Sensor ground (Ampseal pin 20) - the coolant sensor, intake air temperature sensor, throttle position sensor, and external MAP sensor needs to be grounded back to pin 20 on the AMPSEAL. This is the low-current sensor return path and it needs to be kept away from the high power ground. This wire hooks directly to the sensors only and not to the engine block - it is its own return path.
3. VR return ground (Ampseal pin 33) - there is a separate VR(-) input on the AMPSEAL, this needs to be connected to the VR sensor(s). If you are using two VR sensors, return both back to this wire (these are low current and can be shared on the one wire return path) Do not ground the VR sensor anywhere else, return the ground back to the VR(-) terminal. On the MicroSquirt^® EFI controller, this return goes directly back to the VR input circuit's transistor/op amp and not to the ground plane, this keeps the high amplitude VR voltages (and resulting currents) isolated to the VR circuit.
4. Serial return (Ampseal pin 19) - the serial cable on the MicroSquirt^® has a separate ground return path thru the AMPSEAL connector. This return goes direct to the RS-232 transceiver (and not thru the ground plane, keeping the noise off...).

With the small size of MicroSquirt^® EFI controller, keeping the grounds straight is important. It is not hard, just keep things in logical groups - high power stuff goes to engine block, sensors on their own ground loop, and the VR sensor is also separate. The ignition coil current path is from the battery to the ignition coil to the MicroSquirt^® driver and back. The same goes on for the injector and general purpose outputs. Lots of juice flowing on this path, it needs to stay away from the sensors. It also needs a low resistance/inductance loop. The Vref is the reference voltage generated by MicroSquirt^® EFI controller, it passes thru the sensors and the return ground path comes back to the MicroSquirt. Only one return path is required for the sensors because it is comparatively low current, and we all know that voltage drop across a wire is driven by Ohm's law (V=I*R).

Ignition Wiring

In general, MicroSquirt^® can be wired the same as MegaSquirt-II for ignition input (tach) signals. However, there are a few of things to note for those installs that are direct-driving ignition coils with MicroSquirt^® (i.e. not using an ignition driver module but the internal VB921 drivers in the MicroSquirt):

• Ignition Outputs:
  
  1. Make sure to set the Spark Output to 'going high (inverted)'. This sets the correct polarity for the output. By the way, for those just curious on why this particular nomenclature, the answer is that the driver IGBT in the MicroSquirt^® inverts the signal in its configuration. The processor output on the MicroSquirt^® (you do not have access to this wire, it is hard-wired to the IGBT) goes high in order to turn on the IGBT driver, which then pulls the output low (i.e. inverts).

  2. You need to have a handle on your dwell setting for the coil you are using. All the dwell number represents is the amount of time (in milliseconds) it takes to charge up the coil. Because of a parameter known as inductance (the tendency of a current not to change due to the energy stored in a magnetic filed surround an inductor) it takes time to bring the coil up to a desired energy.

     The dwell number is driven by the primary inductance and resistance. Chances are you do not know the inductance or resistance. If you do not then you can do one of two things:

     1. See if you can look up the recommended dwell (or, alternatively, the resistance and inductance) on the manufacturer's web site or the service manual.
     2. Ask on the MicroSquirt^® or MegaSquirt forums for suggested settings from someone else is already using the same coil.
     3. You can measure the inductance and resistance yourself. You will need an ohmmeter, which you probably already have. The other item is an inductance meter, which you may not have. Go find one of your geeky friends and ask to borrow their inductance meter. You can also become a geek yourself and purchase an inductance meter, you can get one on Ebay often for little money. With the inductance and ohmmeter, grab the following PC application:
        www.megamanual.com/files/software/IgnCoilAnalyze.zip

        Follow the instructions included with the application and you will quickly determine the ignition coil dwell time. One item to note is that the IGBT driver used in MicroSquirt^® saturates at 7.5 amps, so use this number as the charge current in the application.

     4. Guess on the dwell number! You can start with a number, like 3 milliseconds and work from there. In fact 3 milliseconds should be enough to get the engine running and is an all-around good starting number. There are tips on tuning the dwell value here: www.megamanual.com/ms2/configure.htm#dwell
     5. Obtain a 0.1 ohm resistor rated at 10 Watts or higher and put inline with the +12V going to the coil, and use a oscilloscope to measure the voltage across the resistor. Using Ohm's law, the current is equal to the voltage divided by the resistance (I=V/R). What you will see is the scope trace ramp up as the coil charges. If you keep on increasing the dwell number in the software, you will get to a point where the ramp levels off - at this point is the target dwell number to use.

  3. Realize that driving an ignition coil generates a lot of noise. This noise can get back into the other MicroSquirt^® circuitry and cause false tach signals, processor resets, sensor noise, etc. On MicroSquirt^® EFI controller, everything is packed tightly together, so it has more chance for noise coupling. A few things to help with this:
     1. Keep the ignition drive wires (and injector) away from the other wires on the MicroSquirt^® connector. Other nearby wires may pick up noise, especially through the serial link.
     2. Finally, it is wise to dress the wires such that the high current "stuff" stays away from the sensor/VR/serial wires. Remember that an ignition coil flyback pulse is 350 volts with a current of 7 amps or so, a nice ElectroMagnetic Pulse (EMP) generator! If this gets into the sensors, VR, or serial connections then you will get all sorts of weird stuff going on...

        One thing that you can do to significantly reduce noise is to use a 'snubber' capacitor. Remember in the old days with points (Kettering) ignition? With the points was the condenser. And everyone knows that the condenser was there to keep the points from arcing and burning out.

        Here is why the condenser (a.k.a. capacitor) reduced the sparking. A capacitor blocks direct current (DC) but will pass AC current. When the points open there is a fast transient in voltage and current. What the condenser would do is to briefly shunt the current around the points and complete the path. But it would only do this for a brief moment, but enough to reduce the arcing.

        The same thing can be done with solid state ignition drivers - in fact systems like the Ford EDIS and GM HEI use capacitors located at the ignition coil. These capacitors will briefly shunt the flyback current, bypassing the driver, for a brief moment. What this does is reduce the stress on the driver AND reduce the radiated noise emission (EMI). Its a significant benefit. The capacitor does change the discharge pattern a bit (it sets up a L-R-C circuit) but not enough to notice.

        You can easily do the same thing. On the IGN signal from the MicroSquirt^® going to the ignition coil, connect a 0.01 microFarad (µF) capacitor rated at 630 Volts or so, connect the other terminal to ground. Connect this up right at the ignition coil (EDIS does this) such that the shunt path is contained right at the ignition coil. You need to use a Polypropylene (PP) capacitor rated for pulse discharge operation. A good choice is the Wima MKP-10 series:

        www.wima.com/EN/mkp10.htm

        You can also use an equivalent, like Epcos B32621A6103J (you can get this at www.digi-key.com).

  IMPORTANT! You can directly drive a maximum of one coil per ignition driver with MicroSquirt! This is because the coil driver (the VB921) can only sink enough current for one coil. That one coil can be a double ended 'wasted spark' type coil for two cylinders/coil, and it will be fine. But you CANNOT 'direct drive' two separate coils in wasted spark configuration driven directly off of one ignition driver (see below).

  

  If you want to drive more than 2 coils, you might look at the 4-coil Bosch 211 igniter or the LSx series of coils with their built in igniters (and logic level signals on the ignition outputs).

• Ignition Inputs:
  

  There are possible three ignition input circuits (to two input pins on the processor) for the MicroSquirt^® controller:

  1. OptIn: This circuit feeds Input 1 (same as OptIn), so it cannot be used simultaneously with OptIn. This has two sensor connections: OPTIN+ on pin 30 and OPTIN- on pin 31. It is designed for use with 5 to 12 Volt (nominal) square wave signal, such as those from a Hall or optical sensor, points or coil negative terminal. See this page for more info. This circuit feeds Input 1.
  2. VRin: This circuit is designed for use with alternating current variable reluctor AC signals that have both positive and negative voltage components. For more info, see the ignition pickup page. This circuit has two sensor connections:
     • VRIN+ (coax cable center wire) on pin 32, and
     • VRIN- (coax cable shield) on pin 33.
     (Some older diagrams may show these reversed - the text here is correct! It is also possible that some early production MicroSquirt^®'s were mis-wired from the factory. Check the coax cable against the Ampseal pins to verify yours are correct: shield to pin 33, center wire to pin 32. If they are backwards, you do not need to swap them, just use the appropriate pins, i.e. the shield for the input (pin 32), the center wire for the ground (pin 33). If you prefer, you can swap the coax pins to be 'correct'. It takes 3 minutes to do. All you have to do is pull off the red clip on the harness connector, this will expose all of the pins. Next, find the two pins for the coax shield (pin 32 if it is backwards) and center conductor (pin 33) and *gently* wiggle them while gently pushing the terminal - it will dislodge and push back. then switch the locations and push back in till it clicks in place. Then reinstall the red clip (line up all of the terminals).)
  3. VR2in: This has one connection: VR2IN+ on pin 4, use VRIN- (pin 33) for the other lead. This circuit is designed for use as Input 2 with Hall sensor signals and 'dual spark mode' (for either the second of two crank sensors in 'Independent Dual Ignition' mode, or for a camshaft position sensor). It will also work with VR sensors, however. For more info, see this page.

  Note that you connect either VRin OR OPTin, not both. They connect to the same processor input port (PT0, aka. "Input 1"), so both CANNOT be used simultaneously for different sensors.

  The OptIn circuit would be used for a square wave input (from a distributor or sometimes a crank wheel, or a module like the EDIS), while the VRin circuit would be used for a VR sensor (either in a distributor or a crank wheel).

  Typically, where both crank and camshaft wheels are used with a MicroSquirt^® controller, the crank wheel will have a VR sensor, and the camshaft wheel will be a Hall sensor. This is because the VR sensor is especially good at providing a clean signal at reasonable engine speeds (the voltage rises with tooth speed, and the crank wheel can be made quite large, generating a good signal at modest engine speeds), while the cam wheel must work with the much smaller camshaft (or distributor wheel) diameter which is only spinning at ½ crank speed, making the VR sensor a less reliable trigger.

  Where you have a VR sensor on the crank and a Hall sensor of the cam, you would wire the crank sensor to VRin (aka. Input 1), and the camshaft sensor to VR2in (aka. Input 2). You do not wire it to OptIn, since this cannot be used simultaneously with VRin.

  The Hall sensor *will* trigger the VR2in circuit without issues (it has been designed with a voltage offset especially suited for Hall sensors and other square wave signals). A Hall sensor is normally a open-collector setup, and since there is a 1 mega Ohm pull-up on the VR2IN+ input (second channel) all the user needs to do is hook the sensor's collector to the VR2IN+ input (pin 4 on the Ampseal), and the emitter to the VRIN- input (pin 33, this ground is shared with the VRin return signal). Also, the VR2IN channel is biased to transition but at 1.8 volts (not at zero like the first channel), as this is perfect for a Hall sensor setup. See the dual spark page for more information.

If you are triggering directly off the coil (instead of something like a missing tooth wheel or another relatively low-voltage signal), connect OPTOIN+ (Ampseal pin #30) to the coil's negative terminal as usual, but connect the OPTOIN- (Ampseal pin #31) to a 12V source rather than grounding it. This then uses the flyback spike off the coil as a trigger (rather than the 12 Volt signal). Doing this reduces the energy dissipated in the circuit, prevent a thermal 'melt-down' of the components inside MicroSquirt^®.

Manifold Absolute Pressure (MAP) Sensor Wiring

This sensor is critical to how MicroSquirt^® functions. the MAP sensor tells MicroSquirt^® the intake vacuum (or boost) the engine has, and use this to scale the fuel injected. These generally have three electrical connections: 5 Volts, ground, and a signal. In addition to the electrical connections, the MAP sensor also has a vacuum connection the intake manifold - this must be downstream of the throttle(s), in the plenum area.

A separate baro sensor (for full-time barometric pressure sensing, rather than just using the startup value) is connected the same way, but the signal wire is connected to AMP pin 29 via the orange wire with the green stripe. Vref and ground can be shared.

For the popular GM MAP sensors (which come in 1, 2 and 3 bar versions) the wiring is:

and has:

(Note that ABC are swapped from the previous diagram!)

Coolant Temperature Sensor Wiring

The temperature sensors that are default sensors for MicroSquirt^® controllers are General Motors two-pin sensors. A lot of temperature sensors (aka. "senders") designed for gauge use have one end grounded to the threads for contact in the engine block. This is fine for gauge work, but for EFI you will notice that the majority of the sensors have two terminals (and insulated from the case/thread) such that a separate ground to the ECU can be implemented. This to eliminate the possibility of an erroneous ground path introducing errors in the readings.

One of the pins is ground, and the other goes to MicroSquirt. It doesn't matter which pin you use for which function. You can use other temperature sensors by inputting appropriate values into MegaTune (see: www.megamanual.com/megatune.htm#oh). If you have substituted a one pin connector, the body of the sensor is grounded to the engine, and the pin is connected to MicroSquirt.

Intake Air Temperature Sensor Wiring

The temperature sensors that are default sensors for MicroSquirt^® controllers are General Motors two-pin sensors. One of the pins is ground, and the other goes to MicroSquirt. It doesn't matter which pin you use for which function. You can use other temperature sensors by inputting appropriate values into MegaTune (see: www.megamanual.com/megatune.htm#oh). If you have substituted a one pin connector, the body of the sensor is grounded to the engine, and the pin is connected to MicroSquirt.

The IAT sensor must be placed to measure the temperature of the air entering the manifold. In a normally aspirated engine, this can be almost anywhere in the intake tract (air cleaner, throttle body, etc.) since air temperature does not change a lot. For a boosted application (blower of turbocharger), the intake air temperature should be measured in a boosted part of the tract (since compressing the air raises its temperature), and you should use an 'open element' IAT sensor: www.megamanual.com/v22manual/mwire.htm#clt

Fuel Pump Wiring

MegaSquirt controls the fuel pump operation. This is to shut the fuel pump down if the engine stalls, preventing the pump from running unnecessarily (or in the case of a crash).

Exhaust Gas Oxygen Sensor Wiring

An exhaust gas oxygen sensor (EGO) is optional but useful for tuning with MegaSquirt. You can use either a narrow-band sensor, or the more useful wide-band sensor (with a controller).

Fast Idle Valve Wiring

The fast idle valve is used to increase engine speed when the engine is cold, preventing it from stalling. MicroSquirt^® can use either a 'solenoid'-style ON/OFF valve, and a variable pulse width modulation valve. See: www.megamanual.com/ms2/IAC.htm#fidle

Throttle Position Sensor Wiring

To hook up your throttle position sensor (TPS), disconnect the TPS, and use a digital multi-meter. Switch it to measure resistance. The resistance between two of the connections will stay the same when the throttle is moved. Find those two - one will be the +5 Vref and the other a ground. The third is the sense wire to MegaSquirt. To figure out which wire is the +5 Vref and which is the ground, connect your meter to one of those two connections and the other to the TPS sense connection.

If you read a high resistance which gets lower as you open the throttle, then disconnected wire is the one which goes to ground, the other one which had the continuous resistance goes to the +5 Vref from the MegaSquirt, and the remaining wire is the TPS sense wire.

Accel and Warm-Up LED Wiring

These circuits ground the LED to light them. You can use the Vref 5 volt supply on Pin 28 to power the LEDs, or you could use some other source. For example, you could use the battery supply voltage (nominally 12 Volts) as long as you increase the current limiting resistor value to 1K Ohms (to keep the current within limits).

Using the Relay Board with MicroSquirt

Check the MicroSquirt^® access board, which may be a better choice than the relay board for some installations.

You can use MicroSquirt^® with a relay board, if you like. You will need to solder many of the leads from the suppled harness (extending them as necessary) to a female DB37 solder cup connector (like Digi-Key 2237F-ND, and a suitable hood (937GME-ND), if desired and if temperatures permit). The connecting cable should be wired like this:

Note:

• There are 5 'spare' terminals, but six possible connections, you will have to decide which to use for what function,
• If you are using direct coil control (instead of an ignition module), you should by-pass the relay board and wires the appropriate pins directly from the supplied harness to the coil's negative terminal(s).
• The serial cable, CAN connections, bootloader switch lead, and a few other connections should no be run to the relay board, they are intended to be used separately from the engine wiring.
• Wires should be soldered at the solder cup, and the joint completely covered with a piece of heat shrink tubing about 1/2" (12mm) long (which must go on before the wire is soldered, obviously!).