11 Yukon XL Denali Electrical Issue P0621 SOLVED

[SteelBaller3]

Voltage not necessarily too high, the charging system allows that when it thinks it's needed.
If your modules show low voltage due to a bad ground, it would be cranking up the alternator voltage.

Since you have a scanner, try and look for 'control module voltage' or something like that.

I will go back out there and do that soon my wife had to leave for a moment so watching my kids. But I also noticed when I tried to graph the battery it’s just blank with no readings. When I go to component testing to try to control certain components it says module not supported or something like that.

 

[Fless]

Just want to put this here... my money is on the L-terminal circuit -- based on the code the CONTROL circuit is shorted to ground or to voltage.

 

[SteelBaller3]

Graph like you did with pedal position might show better if its fluctuating, with engine running.

I will. Wife had to run errand and I’m watching the kids. I also noticed when I tried to graph the battery it didn’t show anything. Just blank. When I try component testing it says module not supported

 

[SteelBaller3]

Sorry didn’t mean to double send that. But I will check that too as soon as I have more than 5 minutes to troubleshoot. L terminal circuit is what plugs into alternator from ecm correct?

 

[SteelBaller3]

The big fluctuation is when I started it

 

[SteelBaller3]

Zoomed in

 

[mikez71]

The big fluctuation is when I started it
View attachment 480377

normal, try unplugging alt connector

My pedal position logs like you would expect, flat at idle, D and E signals mimic each other but at different scales. Pedal connects straight to ECM, D signal goes to 84% at WOT, E signal to 41% (4.2V and 2.06V)
18.8% and 9.4% at idle. (.95V and .45V) respectively..

and maybe you should tell us the other codes

 

[Foggy]

My guess is battery isn't good.. Need to LOAD TEST It !!
Ohmmeter voltage is barely useful when checking condition of a battery.
IF the battery is weak, That would explain why the alternator is charging
at 15+ volts - to recharge after a start with a weak battery

 

[mikez71]

Would a bad battery still cause these issues after starting the engine, and the alternator outputs steady voltage and computer modules all have stable voltage?

I was thinking the control circuit on alternator initially also, but again he seems to have steady voltage, which can still be controlled to ouput 15V - 15.5V.

Looking at his pedal datalog makes me think pedal is toast and causing the rough running. (or bad ECM, because pedal connects straight to ECM, and D and E both ground to the same ECM pin F)

Voltage issues maybe a separate issue..

Those other codes might help, but he wants to keep them a secret for some reason!

 

[Fless]

Well, if the somewhat simple diagnostic path tests were done for the listed code, we'd have better info. There's a reason that code is showing.

 

[SteelBaller3]

okay sorry been super busy. Didn’t have time to take alternator off and test at autozone but did battery today and they said it’s perfect. Doing alternator test tomorrow. I tried running the simple test with the wires. I had 4.5 at alternator plug but couldn’t get the ecm connection off. Ended up snapping the red pull out. My personal opinion is it’s the ecm. The other codes I can’t post a video and that’s all I have of them but I’ll screenshot them.

 

[SteelBaller3]

okay sorry been super busy. Didn’t have time to take alternator off and test at autozone but did battery today and they said it’s perfect. Doing alternator test tomorrow. I tried running the simple test with the wires. I had 4.5 at alternator plug but couldn’t get the ecm connection off. Ended up snapping the red pull out. My personal opinion is it’s the ecm. The other codes I can’t post a video and that’s all I have of them but I’ll screenshot them.

Also idk what these codes even are because they are not in history and current. They are in additional. Whatever that means

 

[SteelBaller3]

 

[SteelBaller3]

 

[SteelBaller3]

 

[SteelBaller3]

 

[SteelBaller3]

 

[SteelBaller3]

 

[SteelBaller3]

 

[SteelBaller3]