2003 Sierra 2500 intermittent crank with key in run position

I have another thread on this truck, was totally dead and could not find anything wrong so replaced the ECM with one from a wreckyard. The truck started up instantly, whereas before it was a no crank / no start if jumping starter/ no inj. pulse/ spark/ rpm - dead although I could still communicate with the computer.

Upon doing the security relearn, on the second 10 minute cycle the truck started cranking, short bursts and would not actually start. If you turn it to crank the truck will start and run fine. Started checking and at the starter relay the ground enable from the ECM is showing intermittent more or less random ground signals. I checked at the crank fuse and was getting power there when it would crank in run position. I thought, simple - must be the switch to be getting power at the crank fuse. Wrong, it still does it even with the crank fuse removed. Note, that when it does this it never actually stays running, it kicks like it has spark but never stays running. Also note that very oddly, it will start up and run when I turn the key to crank EVEN with the crank fuse removed.

As in the diagram it shows power from the key switch in crank through crank fuse to the ECM to give the signal to crank, so I cannot figure how it is trying to start without that signal, and even more baffling is how it starts like normal in crank with the crank fuse removed. With the fuse removed I do get power to the crank fuse terminal as it should, but for some reason It is getting a signal to start without that one. I do have 90 warranty on the ECM, but want to eliminate other possibilities. The only similar cases I found on the net claimed a bad transmission switch was the problem. Any help appreciated.

Test the PCM side of the crank fuse to see if you're still getting power on it with the fuse pulled. If you are find out where that power is coming from. Look for aftermarket security or remote start first. Or other evidence of hacked wiring.

Problem found, corrosion in fuse box due to mice creating a short to power on that crank fuse. I removed both the feed from key switch and the crank feed to pcm and wired them into an external fuse temporarily and all works as it should. I have a replacement fuse box ordered. Thanks

Well, the problems just continue. Replaced the fuse box expecting to be done, but more issues. I did find the transmission to be slipping, but fluid was low due to a leak - no big deal. Now though after filling the fluid, there are more issues. The truck felt as though the TCC was locking up just in the yard, bad shutter and felt like it was shifting even though selector is in 1st. Scanner has code P0740, P0785, P1571, P1689. I graphed the TCC and it is duty cycling on. There are several reasons this should not be happening, it should be in high gear, should be warmed up, and should be going highway speed and none of these are met. The last 2 codes are related to traction control, so not sure what this is about,

It seems the ECM that I got, although the same number on the sticker came from a 2003 Yukon, which would be a 1500 based not 2500 and would not have the same transmission, and would likely have traction control whereas this truck does not. I am suspecting this is what is screwing things up. I may swap back the original fuse box first but suspecting the computer is not compatible even though the engine runs ok. Any ideas welcomed. Thanks

TCC duty cycling might be a response to the P0740. PCM continually testing the circuit or something??

P0740 and P0785 are both circuit codes so they could be caused by the N.E.W. fuse box. Or more rat induced wiring issues. I'd test all the transmission solenoids at the PCM connector with an amp clamp and grounded jumper before calling the PCM for those codes.

P1571 is missing hardwired torque request signal from the EBCM. So that's almost certainly caused by incorrect PCM programming.

Re-installed the old fuse box and it still acts the same. Re -installing the old pcm will tell me nothing as it needs to run to set the codes.

Really screwed at this point. The wreck yard says according to their listing, the pcm should be compatible with this truck. He said maybe it needs to be programmed at the dealer. Maybe, or maybe I pay a towing and a couple hours at the dealer to find the pcm is faulty. Plus, I don’t like or trust our local GM dealer at all, nothing but bad stories that I have first hand knowledge of.

My datastream is saying no short to ground or short to power, but is coming on - even in tow haul when it should be disabled.

Not warm enough, not fast enough, not in the right gear, there is no logical reason. I was thinking to disconnect the wires from the pcm to the tcc, but saw one place that said the transmission would go into limp mode. If I could get the tcc to not engage, I could possibly drive the truck to see how it goes otherwise.

Just compared 4L60E and 4L80E wiring diagrams. Guessing that's what the donor and your truck have. P0785 is circuit code for the 3-2 shift solenoid which your truck doesn't have. According to SI that code will force 3rd gear limp mode.

4L60E has TCC PWM and TCC engage solenoids. So constantly duty cycling the PWM valve may be normal for that application.

You could try putting appropriately sized test lights in place of the TCC PWM and 3-2 SS to make the PCM happy and stop it engaging TCC. No guarantee the transmission would shift correctly though and might set additional codes.

Thanks so much. I have an email sent to get a call back from a place that does programming and custom setups. I also stopped at another wreck yard to see if they have a PCM. I would have thought they could have atleast tried to get me one from a same model truck, instead of lumping them all in together.


I will try to find out if a reprogramming can fix this. Otherwise I would assume if I can get a PCM from a same model/ same drivetrain it would work.

Thanks again.