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Q: Engine revved in drive - 2006 Chevrolet Avalanche 2500

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My truck’s engine revved freely when the transmission was in Drive, as if it were in Neutral. On the shoulder of the road, I coasted to a stop, then shifted into Park, and released the brake pedal. At that point, my truck started rolling forward, downhill, I assume because the transfer case shifted into neutral. The indicator was not on. I turned off the engine. I had a code reader, but it did not show any codes at all.

I restarted the engine and shifted into Drive, at which time I felt the transmission engage (the transfer case must have engaged from neutral). Gradually I accelerated and then drove down the highway for ¼ mile. When I released the accelerator pedal, the transfer case shifted into neutral again, engine revving when it was still in Drive. After I coasted to a stop on the shoulder, I pressed the 4WD high and then felt the transfer case engage, pressed 2WD hi and felt the transfer case disengaged; however, now it was in 2WD instead of neutral.

Gradually accelerating I drove down the highway and when I released the gas pedal the transfer case shifted into neutral again while the engine was revving with the truck in Drive. Coasted to a stop again and repeated the sequence. Shifted into 3rd, gradually accelerated, and drove some more and the transfer case remained in 2WD high. On the shoulder I stopped slowly, turned off the truck, restarted, and shifted into Drive, gradually accelerated and drove some more - released pedal and transfer case shifted once again into neutral. In your assessment, what is causing this problem and what do I do to correct it?

This sounds like the transfer case isn’t completely engaging selected gears (High/Low, 2/4wheel drive). There is a GM Service Bulletin on this issue - TSB Reference #3424. Check with your dealer on this one.

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