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2G 2G basic ECU MPI circuit function

steve

DSM Wiseman
13,748
834
Feb 3, 2002
Boulder, Colorado
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 Junction Box fuse #19 (room) in the drivers footwell fusebox. 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.

96-99

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