2003 GMC Sierra 2500 HD, 6.0, no crank no start

Turbopete wrote:

( Not sure if I mentioned that at one point I was measuring 12 volts on a couple of PCM grounds, but later it was back to ground. Thought I was crazy, but measured voltage on them twice, and also at one point checking with test light, the engine started to crank)

You need to spend some time investigating this. It's getting late, and I'm going to bed, but I will look at the wiring diagrams, tomorrow. Do you remember which PCM grounds were carrying voltage?

I don’t have my papers with me but one of the grounds was the one right beside the starter relay enable wire that would ground the relay from pcm, the other ground was right across from it on the next row same connector.

Turbopete wrote:

one of the grounds was the one right beside the starter relay enable wire that would ground the relay from pcm, the other ground was right across from it on the next row same connector.

The starter relay enable wire is on pin 39. Pin 40 should be grounded from G103, Right Rear of Engine. With the negative lead of your volt meter connected to battery negative, Check the voltage on Pin 40 with the key in the START/CRANK position. Pins 24, 27, 40, 64, and 67 should all be GROUND.

Well, after exhausting other possibilities I bit the bullet and got an ECU from the wreck yard. Plugged it in today, and what do you know, it cranked. Next step was to relearn the security by doing the 3 ten minutes key cycles. All was fine until half way through the second cycle, the truck started to crank on its own. I had remarked earlier that it seemed to crank once or twice without being fully in crank position. I pulled the relay and was able to complete the security relearn.

The truck starts and runs fine now, but however will intermittently crank in the on position, as in you turn the key and it cranks a spurt before being in crank, or if you leave it in the on position it may after several seconds crank briefly, it will never crank long enough to actually start. I probed the relay terminal and the ground feed from the ECM seems to be sending random brief ground signals. Will need to do some more testing but wondering if its a problem with this ECM, but will need to rule out the switch etc.first. At any rate, its encouraging that the truck is running after all this time.

Its not the relay, checked that.