Resolved 91 1G - Troubleshooting 2 ECMs' codes - zero then steady 6 or 9v. No pulses

[Yardfudge]

Hello,

Newbie here. 1991 Plymouth Laser, 1G SOHC non-turbo, Dayton Ohio. Ran great .... then suddenly it just turns over but doesn't start.

Injectors not firing, no spark, fuel from fuel pump into rail. Tried taking ECM trouble code but it had no voltage. (inside of ECM looked OK.) Replaced the ECM ($40 eBay, its insides look good too), and it now reads a steady 9 volts, try to start it, then it reads steady 7 volts. Car battery is 12.5, fully charged. All fuses fine. Per Haynes manual this means both ECMs are bad ... but given its 32yo and DSM's reputation for poor wiring, I wonder if it's something else. Or do I try another ECM?

I searched but didn't come across this problem in the forum. Ideas? Is there a 'Master 1G ECM Troubleshooting Guide' somewhere? Can I check the wiring going into the ECM? Thanks.

 

[Vegas Smith]

Where are you checking voltage? Post a pic of the ecus so maybe somebody can spot an issue. If it's not a bad ecu then I wonder if it's a bad MPI relay as that is involved in ecu power supply.

 

[steve]

So this is a 1.8L 4G37 we are discussing? MD159561 ECU?

If you're not getting a heart beat (on/off repeated) or an fault code it doesn't sound good for the state of the ECU.

 

[Yardfudge]

Where are you checking voltage? Post a pic of the ecus so maybe somebody can spot an issue. If it's not a bad ecu then I wonder if it's a bad MPI relay as that is involved in ecu power supply.

Thank you for the 'MPI relay' hint which lead me to https://www.dsmtuners.com/threads/1g-basic-ecu-mpi-relay-circuit-function.435961/ where I was able to confirm both my original ECU (no CEL, that showed steady 0 VDC fault code, no heart beat) and a used-ECU-from-eBay (no CEL, that showed a steady voltage when taking fault code, no heat beat) were both bad. Voltages and grounds on the ECU and MPI Relay wiring were good as per that guide.

Oddly, both ECUs looked clean on the inside, no messy capacitors (but I'm no electronics technician, I'm comparing only to what I see on https://www.ecmtuning.com/wiki/ecurepairhome).

I'm going to try another used ECU from eBay since that seems cheaper than finding / paying a shop to troubleshoot everything further.

I am slightly concerned something else in the car is creating ECU problems, as https://www.dsmtuners.com/threads/troubleshooting-fried-ecu-components.116585/ suggests but since I'm getting two different ECU effects and not seeing damaged drivers, I'm thinking its not from "from wiring problems (shorts,etc) or faulty actuators/solenoids"

So this is a 1.8L 4G37 we are discussing? MD159561 ECU?

If you're not getting a heart beat (on/off repeated) or an fault code it doesn't sound good for the state of the ECU.

Yes, MD159561 ECU.
I think I did as much troubleshooting as I can to confirm the original (to me) and a used one from eBay are both bad but in different ways. (see other reply for details). Unless you've another idea I'll just try a 3rd ECU.

 

[steve]

Yes, MD159561 ECU.
I think I did as much troubleshooting as I can to confirm the original (to me) and a used one from eBay are both bad but in different ways. (see other reply for details). Unless you've another idea I'll just try a 3rd ECU.

Without seeing them I have nothing else to go on. From your description it's not clear if the original was failing to turn the MPI on or not. Sounded like the second one did but had some other issues.

I think I have a good one in the basement. I'll check when I have a moment. (I do, PM if your interested)

 

[Yardfudge]

Without seeing them I have nothing else to go on. From your description it's not clear if the original was failing to turn the MPI on or not. Sounded like the second one did but had some other issues.

I think I have a good one in the basement. I'll check when I have a moment. (I do, PM if your interested)

I installed a remanufactured ECU from eBay and it started right up. Kudos to https://www.ebay.com/str/flowersautowreckers.

Thanks for the replies.

Now to work on the dozen non-critical issues....