V3 MicroSquirt® - QuickStart Guide

There are a number of tools and techniques you will need to wire MicroSquirt^® to your vehicle. You will also need some specialized knowledge. Our general automotive wiring guide presents an overview of some things you need know for wiring and soldering, so read it first if you are not experienced in wiring aftermarket EFI systems (if you have questions, ask them on the forums at www.microsquirt.com):

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.

To wire your MicroSquirt® controller to your vehicle, 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.

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

Click image for full-size version. Note that some wire colors may change over time (ex. VRIN1+/- and VRIN2+/-). Always verify the pin number for any connection.

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

Your MicroSquirt® controller requires a 12V power source (normally the vehicle's battery/alternator system). The main supply for your MicroSquirt® controller is through Ampseal connector pin#1 (red wire). The main grounds are on pins 22 and 23. The 12V supply to your MicroSquirt® controller must come through a relay that also powers the injectors.

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.

In addition to the relay, you must have fuses on the power supplies. Generally a 2 Amp fuse is sufficient for your MicroSquirt® controller, and each of the injector power supplies needs a fuse of 1.5 to 2.5 Amps per injector.

Note that low impedance injectors (DC resistance less then 10 Ohms) require current limiting resistors, see: Injector Resistors).

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 22, & 23) - these are the returns for the fuel injector and ignition drivers, and the fuel pump/fast idle/spare output drivers. There are two wires on the connectors for these, going to pins 22 and 23 on the AMPSEAL - these are for the high power ground path. Both of these wires need to ground direct to the engine block. It is important to run both 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 grounds (Ampseal pin 20 and 18, if populated) - 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. 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

Your V3 MicroSquirt® controller has 'logic-level' ignition outputs. That means it can only supply a 0 to 5Volt signal at a low current. This is the perfect signal for many ignition modules, however.

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. VR1in/VR2in: These circuits are designed for use with alternating current variable reluctor AC signals that have both positive and negative voltage components OR square wave inputs from a Hall sensor. For more info, see the ignition pickup page. This circuit has two sensor connections:
   • VRIN+ on pin 32, and
   • VRIN- on pin 33.
3. VR2in: This has two connections as well for a camshaft position sensor or other secondary position sensing device.
   • VR2IN+ is on pin 4, and
   • VRIN2- is on pin 21.
   VR2IN- is the ground pin for the second variable reluctor input (typically for a VR or Hall camshaft sensor). For use with a variable reluctor sensor (that has an AC signal) connect this wire to the second VR sensor wire. For use with a Hall sensor, leave this wire 'floating' by snipping it off and sealing it. Do not ground it.

Note that you connect either VR1in 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 VR1in+/VR1in- (aka. Input 1), and the camshaft sensor to VR2in+ (and leave VR2in- floating). You do not wire the crankshaft VR sensor to OptIn, since this cannot be used simultaneously with VR1in.

The Hall sensor will trigger the VR2in circuit without issues if you add a 10K to 100K Ohm pull-up to either 5Vref or 12 Volts 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 at 1.8 volts (not at zero like the first channel), and 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!)

Mass Air Flow (MAF) Wiring

Your V3 MicroSquirt® controller can be connected to a mass air flow sensor. The sensor can be either a voltage signal type (common with Ford and Nissan MAF sensors) or a frequency signal type (common on GM products). Your MicroSquirt® controller can take a mass air flow sensor's signal on one of three pins:

1. MAF on MAP pin (Ampseal pin 24): This is for MAF only configurations (no MAP).
2. MAF on Baro Pin (Ampseal pin 29): Use this for MAP + voltage MAF configuration.
3. MAF on Knock Pin (Ampseal pin 5): Use this for frequency MAF configuration, with or without MAP

Note that some MAF sensors require a 5Volt supply, while others require a 12V supply - check to be sure you connect the correct voltage.

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).