1G basic ECU MPI circuit function
The 1G DSM has two main circuits for the engine. The MPI circuit and the Ignition circuit.
The ECU is connected to both but powered from the MPI circuit.
Power for the MPI circuit comes from the 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 Ignition fusible link on the positive battery terminal and for the fuel pump winds up on pin 3 on the MPI relay.
The MPI relay has two actual relays inside, one for the MPI power and one for the fuel pump power. 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 to either relay is activated the relay switches the power from the source to the output pin(s)
The ECU receives backup power on pin 103 from 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. 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.
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 (indirectly 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), to keep the fuel pump powered after you release the ignition.
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 and the room fuse. 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 and MPI relay pins. 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 power 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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