| Pin | Name click for help | 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. This goes to one of the two VR sensor wires, or to a Hall sensor's signal; wire. Note that you must connect pin #21 appropriately to match your sensor. | Brown/White |
| Pin5 | Spare Input 2 | This is a second spare ADC(ADC7). It is used for alternate MAF pin or knock sensing. | TBD |
| 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 idle air controller (IAC). | 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). | 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). | Blue |
| Pin11 | IGNITION OUTPUT #2 | Second logic level ignition output signal. | White/Red |
| Pin12 | IGNITION OUTPUT #1 | Logic level ignition output signal #1. | White |
| Pin13 | SERIAL Rx (pre-wired) | This pin is used to receive data from the laptop computer serial port. | Red (pre-wired to jack) |
| Pin14 | SERIAL Tx (pre-wired) | This pin is used to transmit data to the laptop computer serial port. | 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. | 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. It can also be used as a spare port. | Yellow/Black |
| Pin17 | Warm-Up LED | This is the supply for an 'warm-up enrichment LED'. You connect the cathode (short lead) of and LED (light emitting diode) to this wire. It can also be used as a spare port. | Yellow/White |
| Pin18 | 2ND SENSOR GROUND | Pin 18 is a second sensor ground (in addition to pin 20) for those who require it. Pin 18 may not be populated on your harness, but this pin can be added later if needed. | Black |
| Pin19 | SERIAL GROUND (pre-wired) | 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 the possibility of electrical noise. | White/Black |
| Pin21 | VR2IN- | This 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. | Black |
| Pin22 | GROUND | This is one of several ground wire pins. The ground wires from pins 22 and 23 should be run to the same spot on the engine (to avoid ground loops). | Black |
| Pin23 | GROUND | This is one of several ground wire pins. The ground wires from pins 22 and 23 should be run to the same spot on the engine (to avoid ground loops). | Black |
| Pin24 | MAP | This pin is connected to the signal from a MAP sensor that puts out a 0 to 5 Volt signal (roughly) proportional to absolute pressure. | Green/Red |
| Pin25 | CLT | This is for the coolant temperature sensor (The other wire from the sensor, if it has one, goes to the sensor return ground (pin 20). The default sensors are General Motors sensors. | Yellow |
| Pin26 | IAT | This is for the intake air temperature sensor (The other wire from the sensor, if it has one, goes to the sensor return ground (pin 20). The default sensors are General Motors sensors. | Orange |
| Pin27 | TPS | This is a pin for the 0 to 5 Volt signal from a throttle position sensor. | Blue |
| Pin28 | Vref | The wire labeled "Vref" is the +5V voltage reference for the MAP sensor (and/or MAF 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, the +12V supply). | Gray |
| Pin29 | Spare Input 1 | 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 a coil (non-CDI only), see this page. | 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, see this note. | 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) | TBD |
| Pin33 | VRIN1- | 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 ignition input (Input 2), use VR2IN+ (Ampseal pin 4) | TBD |
| 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). | Green/Yellow |