PCM chassis grounding locations?

[Matthew Jeschke]

I have a pinout for the PCM but curious if anybody knows which chassis grounds are closes the PCM / tied into the PCM.

As well as the Aux power.

Backstory: I tuned my truck. Wired up wideband and ran it off aux power with Cigarette power adapter. I noticed when the watching the wideband w/o the data logger hooked up my AFR was wrong 13.0:1 (rich). I noticed when I plugged the data logger in the issue went away (ties PCM ground and Aux power ground together). I assumed the ground on my Aux power was floating high causing the discrepancy. Well I noticed my gas mileage absolutely SUCKED at the last fill up. I suspect my theory was backwards. My PCM ground must be a bit high causing narrow bands to read wrong AFR and richen up fuel. I was going to check the ground spots for the PCM. I've gone through damn near every ground and cleaned it since buying the truck but is worth another check, just not sure which chassis grounds are closest / tied into the PCM.

 

[Fless]

See attached zoomable PDF. Primarily Ground Zone 4, upper-lefthand in the diagram.

 

[Matthew Jeschke]

That diagram is AWESOME where did you pull it from. I have the FSM but dont recall seeing that. Thanks!

 

[Fless]

From the GM Upfitter site. https://www.gmupfitter.com/light-duty-vehicles/light-duty-prior-to-2013/

It's a page from the Electrical document for the year range.

 

[Matthew Jeschke]

Awesome, I know the one on the driver side, under the exhaust manifolds / on crankcase.

I don't know of one of the passenger side of the block... do you know are they referring to the back of the lifter valley behind the cover / near firewall?

 

[Fless]

I don't know. Might be on a bell housing bolt for all I know.

 

[Matthew Jeschke]

Thanks for help Yeah, I'll have to dive deeper into that.