Electrical Issues after LML swap

Ianborg · November 1, 2021

My father-in-law recently had his engine go in his 2012 3500 duramax while hauling a heavy load down in the states (from Canada). He ended up getting a low mileage LML out of a 2015 swapped in while he was down there. Everything was smooth sailing on the way home but it has since developed strange electrical problems. All the lights will randomly dim or go out, the gauge cluster will completely lose power, the beeps become all distorted. It sounded like a grounding issue to me... maybe a ground rattled loose after the swap. I spent the weekend searching for all the ground and sure enough, one of the grounds on the engine block was halfway loose and the connected was bouncing around. I cleaned up the connection and torqued it tight, thinking it would solve the problem. No dice. The problem persists.

I am wondering if anyone has any insight. It could be an additional ground that is loose that I couldn't find, or possibly a plug that had a loose or pinched cable. There are no issues while the truck is idling but it can be easily reproduced by putting it into reverse, specifically when the rear parking sensors are triggered. It also happens intermittently when in drive. It will throw the following service warnings randomly as well, "Theft deterrant, Parking Sensor, Trailer Brake, and 4x4".

Let me know if you have any ideas! Going to give it one more college try before sending into the shop for diagnosis. Thank you!