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1G 1G basic ECU MPI circuit function

The 1G DSM has two main circuits for the engine. The Multi Port Injection (MPI) circuit and the Ignition circuit.

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

Power for the MPI circuit comes from the 20A MPI fusible link on the positive battery terminal to pin 10 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 on the positive battery terminal, through the ignition switch, and for the fuel pump winds up on pin 3 on the MPI relay.

The Gold MPI relay box has two actual relays inside, one for the "MPI" circuit to the ECU and sensors, and one for the "Fuel Pump" (FP) circuit. The FP side has two trigger inputs, the MPI side one input. MPI Relay pin 10 is the source for the MPI power and pin 3 is the source for the FP power. When one of the control lines is activated that relay switches the power from the source to the output pin(s).

MPI Circuit from 1990 FSM. * Note that this is a early drawing and is missing the fuel pump check connector and the AWD fuel pump connector. The 2-BW wire comes from the ignition switch like shown in the later diagram. The 1991 FSM MPI drawing is missing the feedback to ECU pin 13 *
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MPI Relay function:

The ECU receives backup power on pin 103 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 the setting and to the cabin lights. Like MPI relay pin 10 the backup power on ECU pin 103 should to present all the time even with the car off. In other words it's unswitched. Without power to 103 I don't usually see the ECU able to power IC110 inside that controls the MPI relay.

The ECU sits dormant in a powered down state waiting to see pin 110 (IG1 from ignition switch) go high (to battery voltage). When pin 110 goes high the ECU pulls pin 63 (MPI relay pin 8) low (to ground). This causes the the relay to activate and switches the MPI power from MPI pin 10 to MPI pins 4 and 5 providing power to the ECU on pins 102 and 107 along with various sensors, solenoids, injectors, ISC, and O2 Heater in the engine bay.

Shortly after battery voltage shows up on ECU pins 102 and 107 the reset circuit in the ECU wakes the ECU up and if it boots up successfully you'll get a heartbeat on ECU pin 1, 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 108 (START from the ignition switch) goes high (to battery voltage).

The START signal also goes to the FP side of the MPI relay (pin 9) to power up the fuel pump, and to the ECU (pin 108) 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 it's pin 56 (MPI relay pin 7) 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 running after you release the starter.

MPI Relay debugging:

The first thing you should do if your ECU doesn't power up (CEL on for about 5 seconds, the boost gauge displays 0) is check the two fusible links on the battery terminal and the 10A room fuse #19. Pull them and measure the fuses for continuity (0 ohms resistance) rather than look at them since looking isn't an accurate test. Then check for battery voltage at the ECU backup power (Pin 103) and MPI relay (Pin 10). If you don't have power to these make sure you have a good connection to the battery terminal and check the fuses again.

Measure the ground continuity to ECU pins 101, 106 and MPI relay pin 6.

Remember that the ECU doesn't get power on pins 102 and 107 until the MPI relay is activated by turning the ignition key to RUN or START. If the MPI relay doesn't activate try grounding its pin 8 and check that battery voltage shows up on its pins 4 and 5. If that works then either the ECU isn't seeing the IG1 signal or the ECU is damaged.

If battery voltage is present on ECU pins 102/107, 103, the ECU grounds are good, and the ECU still doesn't power up it's likely damaged.

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DSM Wiseman
Feb 3, 2002
Boulder, Colorado
Testing the 1G MPI relay (MD119202) is a matter of applying power to the coil windings and making sure the contacts close. Inside the metal box are two relays, one for the MPI power and one for the fuel pump power. The fuel pump relay has two coil windings and energizing either should close the contacts.

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This is the circuit for an 1G automatic car taken from the 1G tech manual. The manual cars have the starter relay and clutch safety switch replacing the AT Inhibitor switch for providing power to the second fuel pump coil to activate the pump while cranking the engine.

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Applying +12v to pin 10 and the ground (-battery) connection to pin 8 should make 12v show up at pins 4 and 5. Same deal for pin 3 and 7 causing 12v to show at pin 2. Applying 12v across pins 9 and 6 will cause continuity between pins 3 and 2. (on relays with the second input)

As mentioned above the ECU is set up to provide the ground to the coils to cause current to flow from the battery to the fuse to the relay coil through the ECU transistor to ground.

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