2014 GMC Sierra Denali 1500 4WD Crew Cab Long-Term Update 6
Arctic Weather Prompts 4WD Actuator Trouble
This winter has been a brutal one in Detroit. The snow hasn't piled up to the extent it has on the East Coast, but the temperatures have been well below average. No problem. Mainstream automakers test their vehicles extensively at temperature extremes as low as 30 or 40 degrees below zero, both outdoors in exotic arctic locations and indoors in climate chambers. Well, all that effort evidently missed one potential flaw.
One frosty Friday evening, I briefly started our stone-cold long-term truck and moved it out of the driveway to the snowy curbside to provide access. The truck was in Auto-4WD mode. Saturday morning dawned equally chilly (single digits), and as I started up and drove away, a "Service 4WD System" warning appeared on the instrument cluster.
I immediately went to the dealership only to learn that this problem had been turning up on new GMT K2-series trucks in extremely cold weather and the company had not yet identified a solution. What?! The temporary workaround was for the dealer to reset the trouble code (transfer case code C0398 or C0326) and warn drivers to engage 4-high mode when needed but to return the truck to 2-high (RWD) mode before switching off the ignition. Not terribly convenient.
Well, apparently GM's best and brightest minds had already been hard at work on the issue because just a few days later I got a call from the dealer saying that a solution had been engineered and a new part was available. Here's what was apparently happening: When in Auto-4WD mode, a shift to Park causes the system to engage 4-high mode. Then when the ignition is switched off, it cycles from 4-high to 2-high and back to 4-high, recording the status and position of the shift shaft at the end. It compares this position with the position of the shaft at the next restart. If these positions don't agree, the warning light comes on, and apparently extreme cold weather was affecting the way a plunger that reports this position moved, though the transfer case still worked properly and was fully capable of engaging either mode. In hindsight, it was probably the fact that the stone-cold truck was started, moved a short distance, and shut off that prompted the fault in our case.
The part was redesigned and assigned a new number (23355888). Installing it is apparently an easy 20-minute job -- 4 minutes shorter than the time allotted to reprogram the shift-control module. Naturally our warranty covered the work. Because it's only affecting some vehicles operating in very low temperatures and it's not a safety concern, there will be no recall issued. All new production is receiving the new part. If you've experienced this problem, refer your dealer to technical service bulletin #PIP5250B.
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