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2G 2G basic ECU MFI/MPI relay circuit function

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Like the 1G, the 2G DSM has two main electrical circuits for the engine. The MPI/MFI circuit and the Ignition circuit.

The ECU is connected to both circuits but is powered from the MPI/MFI circuit and controls it. The MPI/MFI circuit also powers the MAF, CAS, ISC, Injectors, O2 sensor heater, the Boost Control, Purge Control, EGR Control and Fuel Pressure Solenoids. The Ignition circuit powers the Ignition Coils, Fuel Pump, and Starter Solenoid.

There are two variants of the 2G MPI/MFI circuit, the 95 and the 96-99. In 96 the single MPI/MFI relay was replaced by two identical relays attached to a metal bracket.

1995 MPI Relay MR161519 (E8T08474)
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1996 - 1999 MFI Relay MR224169 (E8T101)
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On the 95 cars, power for the MPI circuit comes from the 20A "ENGINE" fusible link #4 to pins 4 and 8 (Red/Black wires) on the MPI relay and should be present all the time (even with the car off). Power for the Fuel Pump and the rest of the Ignition circuit comes from the 30A Ignition fusible link and winds up on pin 7 (Black/White wire) of the MPI relay.

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On the 96-99 cars, power for the MPI circuit comes from the 20A "ENGINE" fusible link #4 to pins 3 and 4 (Red/Black wires) on one relay and should be present all the time. Power for the Fuel Pump and the rest of the Ignition circuit comes from the 30A Ignition fusible link and winds up on pins 3 and 4 (Black/White wires) on the other relay. (The MFI relay has the mainly red wires and the FP relay the mainly black wires so you can tell which is which)

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MPI Relay function:

The 2G ECU receives backup power on pin 80 from Dedicated Fuse #11 (room) in the relay boxin engine bay. This is used to maintain the RAM in the ECU's CPU and allow the ECU to power itself up. It also provides power to the radio to maintain it's settings and to the cabin lights. Like MPI relay input pins the backup power on ECU pin 80 should be present all the time. In other words it's unswitched.

The ECU sits dormant in a powered down state waiting to see pin 82 (IG1 from ignition switch) go high (to battery voltage). When the IG1 from ignition switch goes high the ECU pulls pin 38 low (to ground). This causes the the MPI/MFI relay to activate and switches the MPI power from the input to the output pins providing power to the ECU on pins 12 and 25 along with various sensors, solenoids, injectors, ISC, and O2 Heater in the engine bay.

Shortly after battery voltage shows up on ECU pins 12 and 25 the reset circuit in the ECU wakes the ECU up, the CEL will come on for about 5 seconds and then turn off, the boost gauge will display 0 (Stock ECU software on turbo cars) and the ISC on the throttle body will move in and out to rehome.

The next interesting condition is when ECU pin 71 (indirectly START from the ignition switch) goes high (to battery voltage) causing the ECU to do the things it needs to do the start the engine.

Once the engine is spinning (CAS is pulsing) the ECU pulls pin 8 (The Fuel Pump Control signal)) to ground and continues to hold it low until you turn the car off or the engine stops (CAS stops pulsing for 10 seconds), to keep the fuel pump powered.

MPI Relay testing:

95


1) Black/Blue - Fuel Pump Power output
2) Red - MFI/MPI Power output
3) Black/Red - MFI/MPI Power output
4) Red/Black - MFI/MPI Power input
5) White/Red - ECU Fuel Pump Relay control
6) Blue/Green - ECU MFI/MPI Relay control
7) Black/White - Fuel Pump Power input
8) Red/Black - MFI/MPI Power input

The ECU control lines are grounded by the ECU to activate the relay.
The MFI/MPI Power input comes from the MFI/MPI fuse.
The Fuel Pump Power input comes from the Ignition Switch.

Grounding pin 6 should turn on the MFI/MPI power circuit. The MFI/MPI Relay should click and switch the incoming power on pin 4 to pins 2 and 3.

Grounding pin 5 while the ignition switch is on should turn on the fuel pump. The MFI/MPI Relay should click and switch the incoming power from the ignition circuit on pin 7 to pins 1.

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96-99

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Last edited by a moderator:
Looks like the 2G Spyder is slightly different but works much the same with the added complexity of the Fuel Pump relay module and I assume some sort of PWM by the ECU to control the pump. On the 2.4L Spyder it looks like the power to drive the MFI relay coil comes from the ignition circuit (ignition key) not the MFI circuit like the turbo cars. So both need to be good to turn on the ECU.

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Last edited:
I see that I could have done a better job explaining the fuel pump operation regarding the MPI relay. The various 2G configurations are more complex than the 1G's. I'm not going to discuss 2gNT's here at all. Please refer to the diagram for your year 2G for pin numbers due to the difference in relays.

As a reminder there isn't a function to prime the pump as it comes from the factory.

Power:

The fuel pump gets power from the car's ignition fusible link when the ignition key is in the run (IG1) or start position, on the large Black/White wires to the MPI relay. When the fuel pump side of the MPI relay is activated that power exits on the large Black/Blue wire from MPI relay and runs to two places,the fuel pump check connector A-58 in the engine bay, and to the fuel pump. The other side of the fuel pump is connected to ground using the large Black wire.

Control:

The 2G only has one control input to the fuel pump side of the MPI relay unlike the 1Gs.

When the ECU is seeing CAS signals indicating that the engine is rotating, it will pull ECU pin 8 to ground. That is connected by a White/Red wire to MPI Relay activates the fuel pump side of the MPI relay sending power to the fuel pump.

Once the ECU see that the CAS pulses has stopped for 10 seconds it will release ECU pin 8 and let it float causing the relay to deactivate and the fuel pump to stop.

Testing:

Much like the ECU side of the MPI relay, testing the fuel pump side is done by applying battery voltage to the coil and making sure the relay activates.

Checking the whole circuit can start from either end by looking for battery voltage at the fuel pump when it should be running or from the battery end and work to the pump. The normal conditions are described above and you're looking for what isn't normal.

If you follow the 2.4L Spyder diagram you'll see that there is a slight difference with the addition of the fuel pump relay module which controls the speed of the fuel pump under ECU feedback.
 
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