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 circuits 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 oon the large Black/Red (1.25-BR) wire 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 the large Black/White wire (2-BW) 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).
1990 FSM MPI Circuit. * 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 from the output of the MPI relay pin 2 to ECU pin 13 *
MPI Relay function:
The ECU receives backup power on the Red/Black wire to ECU pin 103 from Junction Box fuse #19 (room) in the drivers footwell fusebox. This is used to maintain the memory 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 (1.25-BW wire IG1 from ignition switch) go high (to battery voltage). When pin 110 goes high the ECU pulls pin 63 (Black/Blue wire to 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 (1.25-Red wires) 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 large Black/Yellow wire ) goes high (to battery voltage). The START signal causes the ECU to do the things it needs to do to start the engine, it also goes to the fuel pump side of the MPI relay coil pin 9 (2-BY wire) to power up the fuel pump since the other end of this coil is grounded at MPI relay pin 6 (2-B wire).
Once the engine is spinning (CAS is pulsing) the ECU pulls pin 56 (White/Red wire to MPI relay pin 7 ) to ground and continues to hold it low until you turn the car off or the engine dies (CAS stops pulsing for 10 seconds), this keeps 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 (MPI and Ignition) on the positive 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, there is some wire issue, 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. All 1G ECUs have a common issue with capacitors leaking and ruining the ECU.
The ECU is connected to both circuits 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 oon the large Black/Red (1.25-BR) wire 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 the large Black/White wire (2-BW) 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).
1990 FSM MPI Circuit. * 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 from the output of the MPI relay pin 2 to ECU pin 13 *
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MPI Relay function:
The ECU receives backup power on the Red/Black wire to ECU pin 103 from Junction Box fuse #19 (room) in the drivers footwell fusebox. This is used to maintain the memory 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 (1.25-BW wire IG1 from ignition switch) go high (to battery voltage). When pin 110 goes high the ECU pulls pin 63 (Black/Blue wire to 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 (1.25-Red wires) 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 large Black/Yellow wire ) goes high (to battery voltage). The START signal causes the ECU to do the things it needs to do to start the engine, it also goes to the fuel pump side of the MPI relay coil pin 9 (2-BY wire) to power up the fuel pump since the other end of this coil is grounded at MPI relay pin 6 (2-B wire).
Once the engine is spinning (CAS is pulsing) the ECU pulls pin 56 (White/Red wire to MPI relay pin 7 ) to ground and continues to hold it low until you turn the car off or the engine dies (CAS stops pulsing for 10 seconds), this keeps 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 (MPI and Ignition) on the positive 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, there is some wire issue, 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. All 1G ECUs have a common issue with capacitors leaking and ruining the ECU.
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