ECM troubleshooting (1 Viewer)

[HazMatCat]

I'm troubleshooting an issue with my '87 4WD pickup. The "Check Engine" light does not come on when you turn the key, but it will turn over (no start). I checked and cleaned the molex plugs and inside fuse box & relays & cleaned the grounds for good measure, and yes, I replaced all of the dash lamps while I had it torn down (it had a few out). Following the troubleshooting guide, the engine light comes on when I ground pin W of the ECM, so that part of the circuit is OK. I discovered while working on it that if I disconnect the battery ground for an hour or so, the light will come back on and the engine starts & runs, but upon turning it off, the symptom repeats. The trouble shooting guide points to the ECM at this point, so I sent the unit out for repair/ rebuild. The repair guy said that he found a leaking capacitor, and after replacing it, it checked out. Just got it back today - and with great excitement, I reinstalled it, but no dice. Same problem, same recovery after removing battery ground. I'm about to send it back to the rebuilder, but I wanted to check in to see if anyone has heard of a bad sensor showing up as this kind of problem. Sound familiar to anyone?
Thanks

 

[gnob]

yes can happen.

 

[pappy]

I wonder if it's the Circuit Open Relay. It controls the fuel pump. Under the hood, next to the battery is a DIAG box. Short FP and B+. This will bypass the COR (and the AFM). See if you can hear the pump running at the tank with the key ON but engine not running. Try and start the truck if you hear the fuel pump.

 

[gnob]

I wonder if it's the Circuit Open Relay. It controls the fuel pump. Under the hood, next to the battery is a DIAG box. Short FP and B+. This will bypass the COR (and the AFM). See if you can hear the pump running at the tank with the key ON but engine not running. Try and start the truck if you hear the fuel pump.

along these lines, i have had failed efi relays too. especially 89+

 

[HazMatCat]

I wonder if it's the Circuit Open Relay. It controls the fuel pump. Under the hood, next to the battery is a DIAG box. Short FP and B+. This will bypass the COR (and the AFM). See if you can hear the pump running at the tank with the key ON but engine not running. Try and start the truck if you hear the fuel pump.

I get fuel pump when the jumper is in and the engine light is on, then later when the engine light is out (and no start), the jumper has no effect. Dang, I wish I had a schematic of the ECM and the '87 22RE wiring diagram. I used to be an electronic technician and still tinker about repairing things - still have my test gear. I'm going to try some freeze spray on individual components next - if it's a temperature-sensitive transistor or voltage regulator on the board, that would be a piece of cake. 37 year old ICs? That might be tough to lay my hands on. It just scares me to pay mystery guys $350 for used ones on ebay, especially as ratty as some of them look. But I'm dyin' for my truck!!!!

 

[pappy]

37 year old ICs? That might be tough to lay my hands on.

Ya, well, I have one. Hit me offline if you are interested.

 

[HazMatCat]

Ya, well, I have one. Hit me offline if you are interested.

View attachment 3599688

That's for an '88, do they interchange?

 

[pappy]

Good question. I think I ran this on my 1985 for awhile until I found a 1985.

 

[gnob]

here's an 86. should get you there