Unknown sensor (?) wire

[Rocket Man]

HOW can I verify a bad PCM ??

I have NO codes. Nothing, though the code reader seems to be scanning for codes and I get other gauge indications on the scanner (voltage, speed, G's, etc)

You should buy a subscription to www.alldatadiy.com which gives you access to the digital version of the shop manuals, including wiring diagrams. It’s about $30 for a year I believe. It’s well worth it if you’re going to be working on your truck.

As far as testing for a bad PCM, there’s no simple way to do that. You will have to troubleshoot and eliminate every else basically.

One thing I just thought of is the Vehicle Anti Theft System (VATS) aka Passlock system. Is the little yellow security symbol in the instrument cluster on solid when you’re cranking it? It might just have to be relearned. Here’s a YouTube video on how to do it. Some trucks will still start if it’s not learned, some won’t. When I swapped out a PCM in my 02 Denali, it wouldn’t start until I did the relearn. Every time you start the engine, the Passlock unit in the steering column needs to communicate with the PCM and say it’s ok to start. You might try this procedure regardless of if the symbol is lit in the cluster.

 

[Fless]

You should buy a subscription to www.alldatadiy.com which gives you access to the digital version of the shop manuals, including wiring diagrams. It’s about $30 for a year I believe. It’s well worth it if you’re going to be working on your truck.

As far as testing for a bad PCM, there’s no simple way to do that. You will have to troubleshoot and eliminate every else basically.

One thing I just thought of is the Vehicle Anti Theft System (VATS) aka Passlock system. Is the little yellow security symbol in the instrument cluster on solid when you’re cranking it? It might just have to be relearned. Here’s a YouTube video on how to do it. Some trucks will still start if it’s not learned, some won’t. When I swapped out a PCM in my 02 Denali, it wouldn’t start until I did the relearn. Every time you start the engine, the Passlock unit in the steering column needs to communicate with the PCM and say it’s ok to start. You might try this procedure regardless of if the symbol is lit in the cluster.

AlldataDIY's prices went up a while back. Here's a sample. YMMV, but I doubt it.

 

[Rocket Man]

AlldataDIY's prices went up a while back. Here's a sample. YMMV, but I doubt it.

View attachment 365683

Yeah it looks like it went way up. Still worth it though. I will admit that I’m not in a hurry to renew my 08 Silverado one now after seeing that increase. Most of that truck isn’t stock anymore anyway lol. I see they won’t even let me renew it anyway, I have to start over using their new website. I used to be able to just hit renew and it was about half the current price.

 

[MassHoe04]

I don't know about your scanner, but my middle-grade Zurich scanner from Harbor Freight had an extra menu in the system that does dig a little deeper. It was called "GM OEM Enhanced" somewhere in the menu tree of the scanner. Maybe yours has something similar? You might be able to see what pending code or error exists (the one lighting up the MIL on your cluster).

Before jumping to the worst-case conclusion of having a fried PCM, see if there is any way to see if there is any specific code pending or error that can point you to what that was might should be connected to.

You might be able to find out where it goes, plug it in and be back in business.

Worth a shot!

Good luck!

 

[KVacek]

Thanks for the tip on AllData. Every time I looked at it in the past all I saw was a zillion dollars a month, but that must have been the all-vehicle subscriptions. I bent over for the 3-year plan, less than 1/4 the current Helm manual cost.

Pretty sure it's a ground that goes to G102. Supposed to be one that looks like what I have on the loom but without the custom bullet connector and red wire. There's an empty stub, attached to nothing, with electrical tape around it, presumably to a terminal attached to the ground point there under the negative battery wire. I can't do much with it where it is, below the left exhaust manifold and above the oil cooler lines. All I can to to barely touch it and look with a mirror. I'm certainly not going to pull the oil cooler lines again to look closer or reattach there.

I assume that genius job was done for the original owner - it wasn't me and I've done everything except trans work since I've had the truck. Maybe the (usually EXCELLENT, for decades) trans guys did it since the front diff has been out at least once and that would be accessible with the diff out.

Going to look at the harness further and then if it still looks that way I'll confirm with a fuse to ground first.

 

[KVacek]

BINGO
Somebody did a half-assed repair of a damaged ground wire, using a color that led me away from thinking of a ground.
Jumped it to ground and it runs just fine.

Had the battery disconnected again all night in case it was something that needed to be reset, so it'll have to re-learn some trims I guess, but after a few moments it settled to a good idle.

THANKS A BUNCH to everyone who urged me to get AllData. It's not as nice as a book (I'm OLD and like paper), but once I figure out their web interface it'll do all I need.

14 years and aside from an emission code that ran me through both evap valves and a fuel cap before I stopped it, I've mostly only needed torques.

 

[MassHoe04]

BINGO
Somebody did a half-assed repair of a damaged ground wire, using a color that led me away from thinking of a ground.
Jumped it to ground and it runs just fine.

Had the battery disconnected again all night in case it was something that needed to be reset, so it'll have to re-learn some trims I guess, but after a few moments it settled to a good idle.

THANKS A BUNCH to everyone who urged me to get AllData. It's not as nice as a book (I'm OLD and like paper), but once I figure out their web interface it'll do all I need.

14 years and aside from an emission code that ran me through both evap valves and a fuel cap before I stopped it, I've mostly only needed torques.

Awesome!!

The evap valve issues might have been another opportunity for a Tech 2 scanner to verify if the valves/pumps were actually still functioning or dead. There is a chance something got replaced that might have still been working. Sorry! I guess I might have drunk too much of the Tech 2 Kool-aid... But I can see now!!

 

[Rocket Man]

BINGO
Somebody did a half-assed repair of a damaged ground wire, using a color that led me away from thinking of a ground.
Jumped it to ground and it runs just fine.

Had the battery disconnected again all night in case it was something that needed to be reset, so it'll have to re-learn some trims I guess, but after a few moments it settled to a good idle.

THANKS A BUNCH to everyone who urged me to get AllData. It's not as nice as a book (I'm OLD and like paper), but once I figure out their web interface it'll do all I need.

14 years and aside from an emission code that ran me through both evap valves and a fuel cap before I stopped it, I've mostly only needed torques.

That’s awesome! Glad it was simple. Hope that wiring diagram on alldatadiy helped you figure this out. It does take a minute to figure out how to navigate but just remember to keep looking to the right side as well as the left, there’s a complete other set of menus there.

 

[Sam Harris]

I'd guess this was part of an aftermarket alarm, or remote start. Looks to me like something was installed, and removed before you purchased it. Especially given the no-start condition, and 0V / 12V scenario. Perhaps you can look in the fuse boxes to see if there are any additional wires / looms added there?

Late to the party. Glad you got it figured out with the excellent guidance of the pros on this forum.

 

[Fless]

So the wire that had 12v on it was the one you grounded?