Weak ground on Run / Crank fuse 2007.5 GMT900 Sierra Denali

BruceW · March 13

My 6.2 Sierra Denali became hard to start which continued after replacing the battery. Then one day it would not turn over at all. I replaced the starter but still no crank. Also learned it is much easier to replace the starter through the wheel well than from below.

Jumping the switched pins of the starter relay activated the new starter turning over the engine but not starting even with the key switch on.

Before replacing the starter I remember that the run / crank relay was clicking and opening the throttle when the key was on. It was not clicking now so I made some test leads and found that the ground to the run / crank relay was weak. I found no continuity to ground through the ground pin of the relay socket. Checking voltage from the trigger pin of the relay socket to the ground pin of the socket I found 9.76 volts while testing from the trigger pin to the firewall ground I had 12 volts.

I used a compressor to blow out the fuse box and that socket with no change. I disassembled the fuse box and could not find continuity from the ground pin of the socket to the pins on the bottom of fuse box. The black wire in the bottom connector closest to the run / crank relay did not have continuity to ground. The black wires in the other connector bundles did have continuity to ground and allowed for 12+ volts to pass. If I remember correctly that was connector X2.

I found that shorting the switched pins of the run / crank relay opened the throttle allowing the truck to start and run when the key was turned. I also learned that the key switch would not turn off the truck with the run / crank relay socket shorted. Removing the wire across that socket would stop engine.

As a short term solution I ran a ground wire from the firewall ground to the ground pin of the run / crank relay. This allows the truck to start and stop normally via the key switch.

Can anyone tell me where the far end of the ground for the run / crank relay / X2 connector goes to? I suspect I have a ground issue there.

Additional issues that have developed over the last year are that the oxygen sensors and fuel sender are not reporting. Fuse 8 oxygen sensor burns up when replaced.

Thanks in advance for any advice.

Also thanks to my brother who spent an afternoon troubleshooting with me.

dna9656 · March 14

First, if you're a amateur that was some fine trouble shooting! Except the replacing the starter part... Back in the day a starter was $30.00, not such a lot of money even then; but today replacing good parts (especially an electric part as in most cases you can't return it) is more serious.

I am in agreement with you about a weak ground. This family of vehicles is known for corroded grounds. On my '06 there was a ground point used by several circuits underneath the truck essentially below the driver's feet. I cleaned it all up and put some dielectric grease on all surfaces. When I'm under the vehicle for any reason I try to remember to check that ground.

GM has data showing the locations of grounds somewhere in the repair manuals. A word on GM manuals; not all systems are covered in 1 manual, there are (or were at one time) several manuals, there was a shop manual, a transmission manual, and (I think) an electrical manual so don't look in the shop manual, look for an electrical manual. I'm sure the great dealer mechanics we are privileged to have with us on this board can and will shine some light on your issue as well as explain about the different manuals thing.

Again, that was some great trouble shooting you and your brother did! Congrats on getting so close to the issue!

BruceW · March 18

On 3/14/2023 at 10:53 AM, dna9656 said:

Except the replacing the starter part...

It was a $70 starter off of Amazon and I believe the 225K mile starter was bad. Early on in my troubleshooting the positive lead that powers the truck, not the one that powers the starter was pulling 60 amps when the key was turned. Around 10 mps was being pulled when the key was turned when I discovered the ground issue. I do remember the Run/Crank fuse clicking early in ny troubleshooting. The battery had a stronger charge during the earlier testing which might explain the difference.