Question: I'm back again with more issues on my 2009 GMC Sierra SLE. After finally getting all the electronics repaired a few months after my new purchase, another issue has risen that the dealer has been unable to diagnose or repair. The "Four-Wheel Drive" service message came on. I brought the truck back to the dealership, where they applied special grease to all the connections, stating that GM had issued a bulletin on this repair due to moisture entering the connection outlets. Within two weeks, the service message came on again. This time, the dealership replaced the 4wd module on the transmission. Within four weeks, the same message came on again -- so they replaced the 4wd module and the 4wd switch. After nine months with no issues (25,910 miles on the truck), the same service message came on again. The dealership applied special grease to all the connections, referring back to the GM service bulletin. After two weeks, the message came on again, and again the module was replaced. One week later, the message is back on, the truck has now been at the dealer for 10 days, and still no word.
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Answer: I remember your previous intermittent problem, which was finally diagnosed and repaired by a GM technician sent down from Detroit. That time, it boiled down to high resistance at a single electrical terminal.
Every time the four-wheel-drive message comes up, there's a Diagnostic Trouble Code (DTC) stored in memory. Did they tell you what the original DTC was? Did the DTC change with any of the previously attempted repairs, and, most important, what is the most recent DTC? There are several Service Bulletins referring to a 4wd message. One of them is a software update, but we have to know the code. TSB #08-07-30-021E is a scary one. It gives a rundown of weak areas causing intermittent electrical conditions, including water intrusion, loose terminals, and chafed wiring -- indicating general poor quality harnesses, routing, and insulation. One portion of this TSB shows five known wiring harness contact points, meaning spots where wire insulation gets rubbed though by a sharp metal object causing a short to ground or another circuit. Ask the GMC district service representative to send another specialized technician (hopefully the same guy) to help figure it out -- it worked last time. I don't recommend this too often, but it might be time to sell. Seeing this much aggravation at such low mileage, I can only imagine what's waiting down the road.
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