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1993 c3500 electrical issues

M92PV4U

M92PV4U

Falling Down
Joined
May 19, 2020
Member Number
110
Messages
4,477
Loc
D.p. Ca.
The truck was running and driving fine. It rained, the truck started running rough and got a check engine light. After a couple of miles the transmission got stuck in second. I shut the truck off, after everything was nice and warm (inside the cab and engine) the truck ran good the rest of the day and shifted fine. It sat a day in the rain. I started it this morning it was running rough and had a check engine light. I hooked my snapon mt2500 to it but it couldn't connect to the ecm.
The truck has:
Spark
Fuel
Fuel pump runs momentarily when key is on

The truck does not have:
Injector pulse
Power to injectors
Connection between ALDL Connector and ecm

I assume that the scanner not linking to the ecm and no injector pulse are related, I also assume that the ALDL wiring harness stays under the dash and doesn't go out into the engine bay?

I don't see any signs of water intrusion under / inside of the dash :confused:.

Any recommendations? A Book... of matches?
 
Paper clip to terminal A to B on the aldl will give you a code. 1 2 will be first, three times. Then the code (flash pause flash will be the 2 number).

But sounding like an ignition module or coil (pick up or ignition).
 
Rttoys said:
Paper clip to terminal A to B on the aldl will give you a code. 1 2 will be first, three times. Then the code (flash pause flash will be the 2 number).

But sounding like an ignition module or coil (pick up or ignition).
Click to expand...
It has spark, I fired the injector with a 9v battery and it started momentarily. I'm trying to check grounds now.
 
That junction block on the pass bulkhead can burn and a lot of connections come together at the starter main terminal. Could be tight but the terminal is loose. If that makes sense
 
Rttoys said:
That junction block on the pass bulkhead can burn and a lot of connections come together at the starter main terminal. Could be tight but the terminal is loose. If that makes sense
Click to expand...
It looks like there is a pcm ground weld, crimp near there. That is where I plan to start. I didn't realize that was a bulkhead connection and thought that the wires just passed through the firewall there :homer:.
 
You should be able to probe the injector and tap to ground and it should fire each time you touch

If it wont you arent getting 12v to the injector/s

Iirc you need power through the dist and the oil psi to power fuel pump. Fuel pump relay MIGHT have a hot tap dangling from it. Jump 12 with a heavy gauge wire and it will run, then tap the injectors to ground.
If you get spray you might have lost signal in the ecm.
 
total newb said:
You should be able to probe the injector and tap to ground and it should fire each time you touch

If it wont you arent getting 12v to the injector/s
Click to expand...
Right now I don't think that the ecm/ pcm is grounded.
total newb said:
Iirc you need power through the dist and the oil psi to power fuel pump. Fuel pump relay MIGHT have a hot tap dangling from it. Jump 12 with a heavy gauge wire and it will run, then tap the injectors to ground.
If you get spray you might have lost signal in the ecm.
Click to expand...
The fuel pump runs for 10s or whatever after turning the key on, with a 9v battery on the injector the truck will start.
 
The ignition control module in the distributor controls spark and injector pulse. Since you said it happened only when wet, I’d pull the cap and see if there’s any moisture in there.
 
does the ecm ground the injectors to fire? If so replace the relay that gives the injectors power when you turn the key on.
 
Rttoys said:
The ignition control module in the distributor controls spark and injector pulse. Since you said it happened only when wet, I’d pull the cap and see if there’s any moisture in there.
Click to expand...
I am going to do that first thing when I go back to work.

I could be getting hung up trying to figure out multiple problems in the wrong order, just seems weird that I couldn't read the ecm with the meter today, and two days ago the transmission quit shifting for a while.
 
Shot in the dark here, but I had/have a 1992 C3500 and had both transmission shifting issues in the past, and one time got caught in the rain and it flat shut down.
The transmission shift computer is near the glove compartment (don't remember if that's the overall engine computer too, but it's the transmission one) There is also an upgrade that was done to the wiring to the transmission so if it's ever had tranny redone in the past, you might have a hack fuck set of splices near the transmission like there is on my truck. It literally looks like a dog chewed the shit apart and it was put back together by a first grader. That's something I still need to work on to see if the transmission will finally tell the TC to lockup.
Point being, unless you've got shit splices near the tranny that got wet, the firewall connection likely got wet if you have shifting issues and ignition too.
Another time I was driving the truck to work and drove through a pretty big puddle and splashed alot of water and it shut down totally. I let it dry a while and it started and resumed the trip to work. All was fine until I got to a stop sign and apparently some water pooled on top of the air cleaner from the big puddle splash hit the distributor again and shut down again for a while.
The engine computer is also under the dash near the glove box but I don't remember now if it's the same one as the transmission computer. So yeah, if you're having issues with both I'd look at firewall connection first.
 
It was either the pickup coil or the ignition module. Replaced both after checking the ground crimp and it fired up. Thanks for the imput guys, I'll update again after I see if the code reader links to the ecm/ pcm.
 
Well that didn't last.
1000048429.jpg
 
The last stored codes that I pulled, then the ecm reset, it was running good, started running rough cle came on then lost communication.
1000048430.jpg
 
blakes said:
ya get another one

looks like it was already a reman
Click to expand...
I replaced the ecm not long ago when I was having problems, but reused the prom. Do you think that the prom is reman too? I'm going to replace it, I don't know what else to do.
 
This is what a ecm / pcm firewall pass through looks like taken apart, so no connections in there to check.
1000048439.jpg
 
Emailed Brian Harris at TBI chips.com he said more than likely the ecm, not the prom. So I ordered the ecm.
 
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