No spark from coil

BADOTS28

MEMBER
1999 GMC SONOMA
4.3L
V6
2WD
AUTOMATIC
225,000 MILES

I am at a loss here, I shut the truck off one day and came back out approximately 10 minutes later and it would not start. Engine turns over at ease but I quickly found that it would not fire. I traced wires and checked everything out. I put an ignition control module, coil, crankshaft position sensor, and distributor on it to try and get an idea on what it could be. I ended up towing the truck home and ruled it to be the ECU/PCM after finding that the signal wire doing into the Ignition control Module was not getting a 5v pulse when cranking the engine. I have checked all wires to and from the ICM, Coil, and Crankshaft position sensor and this signal wire was the only issue found.

I ordered a new ECU/PCM flashed and programmed to the VIN. When it came in I plugged it in. Started the ignition code relearn sequence and the truck started ran and idled great for about 30 minutes. I shut the truck down and figured it was a done deal. Went out the next day started the truck right up drove down to the garage behind the house and after exiting the truck for about 45 seconds it shut off on its own. Went back to start it up and back to square one no signal from the signal wire going to the coil. I need help!

FYI, I checked all fuses and checked for codes(none) no lights. Security light does as it should. Nothing is standing out to me but something is tripping this pcm or something. I appreciate any help at all! I feel as if I have hit a wall and don’t know which route to go now.

1 Reply

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Hi,

Lets see what we can figure out. Please bear with me. I need to get a handle on exactly what is happening. You indicated fuses are good, so I suspect the ECM 1 fuse is good and has power both to and from it.

Is there power at the pink wire on the ignition coil when the key is in the run position? Have you confirmed ground for the ignition control module which is at the left rear of the engine (black wire/white tracer)?

If you go back to the PCM and check pin 9 EST output signal (at the PCM) are you getting a signal? If you aren't getting a signal right at the PCM, it sounds like the PCM is bad even though you just got it.

When you checked the crankshaft position sensor, was there power at the pink wire (key in run position)? Did you check the purple wire for ground and the yellow for a signal?

One last question. Note that the ICM and VCM both ground at the same location, behind the engine (black wire/white tracer). It isn't uncommon for the splices to corrode and provide a weak signal or no signal. Also, the terminals that are bolted to the engine break. Make sure that wire isn't bad. Check for continuity to ground and wiggle the wires around to see if you lose continuity. I have a feeling this is the issue simply because of the symptoms. However, that is only a guess at this point. You may even want to run a temporary ground for both components to see if that changes things. I feel it got moved around when you replaced the VCM (PCM) and made a temporary connection.

Sorry for so many questions. Just trying to narrow things down.

I attached the entire wiring schematic below for your reference. I highlighted the wiring portions that I felt are relevant to the problem. Note that there are 4 pages of wiring for the powertrain control system. I had to cut each in half to make them readable for you. I did overlap them so you can follow from one to the next.

Let me know.

Take care,

Joe

See pics below.

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