2008 GM No Crank, No Start

2008 Hummer H2 6.2L

No Crank, No Start. Seems like this should be easy, right?

No Codes. At All.

PRNDL display appears in cluster, but no gear is underlined/boxed. ECM pid shows correct gear as you move the shift lever though.

Battery & connections clean & tight. No visible corrosion or green in the underhood fuse box

ECM pid shows "crank request" when key is turned, but "Starter Relay" status never changes.

Unplug the TAC, and vehicle will start right up & stall, BUT the first time you try to crank it, nothing happens. After that first key turn of nothing, it starts up every time unless you remove the key and put it back in. Then the cycle repeats. During this time, the PRNDL still doesn't indicate the gear.

No antitheft light (ECM is getting the crank signal from the BCM, so that means the key was verified)

Any GM Dealer techs ever run into this?

Only other thing out of the ordinary is that there is low voltage at the trailer light fuse (3v). Unplugging the fuse makes no difference.

This is one that I am helping another tech with from across the country. I feel like I'm close, but my brain is starting to hurt :pinch: . I do not understand why the TAC being plugged in would cause a no-start, being that there are no TAC codes when it is plugged in.

That is a weird one. No codes in any modules? When you say starter relay status doesn't change, that is a PID on the scantool? So it sounds like the BCM is sending the request but the ECM is not trying to energize the relay, correct?

What happens when you bridge the starter relay with the TAC module plugged in?

bruce.oliver wrote: That is a weird one. No codes in any modules? When you say starter relay status doesn't change, that is a PID on the scantool? So it sounds like the BCM is sending the request but the ECM is not trying to energize the relay, correct?

This is correct. According to the SI, Once the ECM recieves the crank signal from the BCM, the PCM checks for Park or Neutral position, then commands the starter relay. Despite the cluster not showing which gear I am in, I am getting the correct gear selection in the ECM pid, though.

I mean no arrogance by this, but I very rarely get stumped, especially when it comes to electrical problems. This one is really “humbling” me though.

juergen.scholl wrote: What happens when you bridge the starter relay with the TAC module plugged in?

When he jumps 30 and 87, the starter will engage, regardless of whether the TAC is plugged in or not.

Early on, I had him confirm we are working with a good relay circuit components, both on the control and load side. The control side even has a 12v bias voltage/monitored circuit, which made circuit testing pretty easy. We just have no command unless the TAC is unplugged. And like I said, that “first” key cycle there is absolutely nothing even with TAC unplugged. But all subsequent key cycles we get a start..... until of course you plug the TAC back in. There are so many theories that I have had, but then dismissed because the darn thing won’t set a code (unless you unplug things of course)

It just feels like it should be a bad ground, resistance or short somewhere because unplugging the TAC changed things. But looking at the TAC diagram, I just do not see Where there may be a potential issue that could cause this but not set a code. As we all know, those GM throttle assy.’s are very well-monitored circuits that will just about set a code if you look at it the wrong way, lol

I guess I actually wanted to know wether the engine cranks, starts and stalls or starts and runs while the relay is jumpered with the TAC connected.....I take it that it just cranks....

If there are no voltage drops/ high resistences present, all power feeds and grounds are fine, all input signals present as you describe and there are no fault codes then I would seriously regarding the pcm beeing at fault.

Does the datastream show meaningful pids for the pedal. Does the engine eventually crank with the APP disconnected instead of the TAC?

Do all the wires of the TAC/APP, especiacilly the signal wires, run to the proper terminals at the pcm, any chances they are mixed up? If they were this could cause a no code failure.

Not having the car in front of you to put your hands on makes it even more difficult,for sure.

juergen.scholl wrote: I guess I actually wanted to know wether the engine cranks, starts and stalls or starts and runs while the relay is jumpered with the TAC connected.....I take it that it just cranks....

If there are no voltage drops/ high resistences present, all power feeds and grounds are fine, all input signals present as you describe and there are no fault codes then I would seriously regarding the pcm beeing at fault.

Does the datastream show meaningful pids for the pedal. Does the engine eventually crank with the APP disconnected instead of the TAC?

Do all the wires of the TAC/APP, especiacilly the signal wires, run to the proper terminals at the pcm, any chances they are mixed up? If they were this could cause a no code failure.

Not having the car in front of you to put your hands on makes it even more difficult,for sure.

I don’t know whether is fired up when jumping the relay, but I don’t see any value in going back to check that at this point in the game. Feel free to correct me if I’m wrong there.

This is supposedly a regular at the shop that doesn’t take their vehicle anywhere else. The customer said they didn’t have it elsewhere, so I guess the likelihood of the pins being mixed up hinges on how much you trust what a customer says, lol. (I would insert a “tongue in cheek” emoji here if I had one).

As of Friday afternoon, I instructed him to inspect the TAC harness and engine grounds on the back of the heads. If he responds that they are good, I was going to be having him test the pcm grounds and then swap out the Throttle. You make a great point about looking at the ACC-TPS pids though, I am now curious to at least see what they are reading with TAC plugged in and unplugged. I will have him do that before he swaps the throttle.

I hate swapping parts, but at this point I have probable cause that it could be the throttle, and have had him nearly test everything I can think of. I am super cautious about calling for ECM replacement, and I really still have an issue with that conclusion, because the fact remains that the computer WILL send that starter request if the right conditions exist (TAC unplugged).

Any progress?

Unfortunately, the customer had it towed to the dealer before we could do anymore testing. The vehicle had been at the shop for quite some time with no results before they asked for my help. By the time I got involved, the customer was already at the end of his rope on patience. They just waited too long to ask for help. On the bright side, I learned a few things about how that particular system works and the tech learned some new testing methods throughout the process that I guided him through. I hate not getting a chance to finish the diag, but not a total loss I suppose.