1G No Power to ECU

[wishihadatalon]

Alright guys, I did a search and wasn't able to find anything helpful to my situation. The story goes like this. I received this car (90 gsx) from my co-worker for free with a blown motor and half buried in a field from sitting for 6 years. Original intent was to part the car but now I am changing the engine out for a winter tank.

So after getting the engine out, I assessed the wiring harness and found that many of the wires were chewed by field mice. I got them pretty much all fixed (cut out corroded sections, solder in sections to fix) and installed an SAFC and Zeitronix wideband. I also removed a VPC and Viper alarm/remote start. Now I have put a battery in the car with no engine or sensors and when I key the car on, I get no power to the SAFC, wideband, or ecu. I have power to pin 103 (constant 12v) but nothing to 102 (power source). I verified I am getting power to pins 10 and 3 on the MFI relay but I don't get power to pin 9. I'm also not hearing the fuel pump come on (I've heard different things about the pump not starting until the car cranks). Also if I key the car to the start position, I can hear the MFI click but then my radio cuts out.

My trouble here is that I can't find a schematic to trace the wires back from the ecu to ignition (assuming my issue is between these 2). I have accepted that I may be looking in the wrong direction and/or may not have a complete understanding of what I am doing. Any pointers to get me going in the right direction or better schematics than the FSM or Haynes manual would be greatly appreciated.

Thanks,


TJ

 

[steve]

Maybe this will help.

http://www.dsmtuners.com/forums/articles-tuning-ecu/435961-1g-basic-ecu-mpi-circuit-function.html

 

[wishihadatalon]

Thanks Steve,
I had found the MPI post in various other threads, but not the other portion. While I have tried multiple MPI relays, I will be testing the MPI when I get home to verify that I don't have multiple bad ones. It appears that I am not getting switched power despite having the power I need to pin 10. I will also check the ground connection pin 8 just to be sure.

Thanks again.

Alright Steve,
I had a chance to look at the car tonight. With the key to the on position, I get power to pin 110 (switched power) but if I check voltage across pin 10 on the mpi and 8, I get nothing. A continuity test shows no broken wire between 63 of ecu and 8 of mpi. I assume this narrows it down to the ecu. Could the fact that the car had a VPC before (has a keydiver chip for this) be part of the issue? I am going to check my spare parts for a spare ecu but I just wanna be prepared.

Thanks again for your help.

 

[steve]

Did you measure the voltage at pin 8 with the key off and again when it was on?
What happens if you ground pin 8?

 

[wishihadatalon]

Steve,
I get 0 volts across pin 8 and 10 with the key on and off. If I take a wire and run a ground to the back side of 8, I get the MPI to click over and my accessories/wideband/SAFC start (I didn't see if I got the ECU light on the dash because I was working on the other side of the car). So, I am pretty sure this narrows it down to the ecu, but I am not quite sure how the ecu switches to ground on that pin.

Thanks

 

[steve]

Did you measure the voltage at pin 8 with the key off and again when it was on?

I should have clarified, when someone asks for the voltage at a pin it's assumed they mean the voltage between the pin and ground.

Since there isn't any current flowing in the relay coil there should be any voltage different between on side (pin 10) and the other (pin 8) which you had already confirmed.

The ECU pulls pin 8 to ground to activate the relay like I said in the other thread. The how is by switching on a transistor on IC110 inside the ECU.

Sometimes people blow the transistor mis-wiring things but usually the connection between the transistor and the pin is damaged due to capacitor leakage eating the circuit trances. They can have the caps replaces and not clean up all the damage and over time the electrolyte eats the copper up.

 

[wishihadatalon]

Ok, I will check out the ecu for leaky caps and then check to make sure I didn't mess anything up while wiring in the SAFC and wideband. I will also check to see if there is voltage on pin 8 to ground, I made the assumption that because it was ground, I should find a pos 12v signal to verify it against. I think I understand why you wanted me to check now.

Ok, I checked the voltage on pin 8 of the MPI. I get 12v with the ignition on and off. Also, I don't see any caps damage on my ecu nor do I have any weird smells indicative of leaky capacitors.

 

[steve]

No need to check the voltage at pin 8 again, the result of grounding the pin told me what I needed to know and that was that the ECU wasn't able to activate the MPI relay but the replay works and that the MPI circuit get power.

You can try and see if the engine will start while your manually turn the power on by grounding pin 8.

 

[wishihadatalon]

I will give that a try after I get the engine back in (sometime this week I hope). Also after looking around and seeing that the two wires to switch to run a 91+ ecu in a 90 weren't important to the functions I was trying to diagnose, I took a spare ecu and put it in. With the other ecu, everything functions as intended. Should I start looking at repair options for the ic110 transistor, or was your thought behind me trying to start it, to see if the whole ecu is shot? If I need to start looking for repair options, do you have any good recommendations for a company or person to do the repairs?

Thanks,

 

[steve]

I send everyone to the guys at ECMTuning. Since you have time send yours in and they can tell you what's wrong.