No start

[ZIO]

no fuel pump noise, no ecu light when key engaged (does the 1990 GST have a ecu light?)

mst relay clicks to on before starting

no voltage on fuel pump check point in engine bay which confirms no fuel.

car turns over but doesn't start and no smell of gas

had ecu repaired with new caps because of leaky faulty ones, foil looks ok, all chips look ok, all other things on the board look ok.

everything else sees to work, lights, etc, it just won't start.

my manufacture date says sept 1990, the ECU is a MD 128625

need help!

 

[MY1GDSM]

Taken from the the second newest How to off to the right of the screen ->> 1G basic ECU MPI circuit function

Have you tested the MPI relay?

Also,

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.

 

[ZIO]

Vroom Vroom!

I had to make a schematic from the other ones to understand it fully and I found a few problems during my adventure.

I first had 3 electrolytic (leaky) and a surface mount (shattered but replaced with better rated capacitor) and another weirdo diode replaced and some foil fixed.

Then to testing:

Using a method of elimination I went through this checklist:

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.

My final conclusions were: the manufacturer's diagrams were adequate for wiring harness troubleshooting but not adequate for the MPI relay tie-in troubleshooting all on one page, so I created one that I could understand better based upon cause and effect.

1. battery backup power on pin 103 was .4 volts even after replacing the fuse on #19 in the interior fusebox. (I bypassed to pin 10 of the MPI relay for an always on connection)

2. The ECU was spitting out a constant on signal from pin 66 which would explain the previous owner's complaint of the battery that dies quickly (for some reason) which turned out to be from the fuel pump that never shut off. I put positive feed lines 4 & 5 from the MPI (A, 102, 1.25-R) on on a kill switch for added security and to be able to start or turn off the fuel pump and all the sensors, etc.

3. There was no power to pins 107 or 102 from failed contacts in the MPI relay, these both were powered through the grapevine by tying the 2 lines to pin 10 on the MPI relay through the kill switch.

Everything else coming in/out of the ECU seemed to be operating correctly including the starter relay signal and the functions active when the key was turned into different positions.

After all was said and done it appears that this was a non-turbo to turbo conversion from the lack of a turbo gauge on the dash. So likely they used the right ECU on the wrong harness or something to that effect. Either way I trouble-shot the pin-out provided by someone awesome out there and with the three documents, My MPI relay diagram, the ECU pin-out diagram, and the step-by step what to look for list it was a fun adventure in car repair. Afterwards it started as it should have with all the voltages where they should have been.

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