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Truck transmission seems to disengage when stopped - what’s wrong?

2010 GMC Sierra

General Motors

I have a 2010 GMC with a V-8 (82,000 kilometres) in which the transmission seems to totally disengage when stopped (say at a traffic light) and them slams harshly into gear when you accelerate. The dealer indicates this is not a driveline problem. Any similar experiences out there? – Rudy

This definitely does not sound normal.

Are you sure the transmission is disengaged when stopped? Could it be the brakes are seizing briefly?

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Conduct a little test. Find an empty patch on the level and bring the vehicle to a complete stop as if at a traffic light or stop sign. Take your foot off the brake. The vehicle should move forward on its own. If it does not move, the brakes may not be releasing properly. Tap the brake pedal a few times to see if this has any effect.

If it still does not move, shift the transmission into neutral or park and listen for any discernible change in engine revolutions – it will be easy to see if you have a tachometer. They should rise slightly, briefly, as you are manually disengaging the transmission.

You can attempt to free the brakes up by reversing while applying pressure on the brake pedal; do this several times. This might dislodge something stuck between linings or pads and the drums or rotors.

If the transmission has in fact disengaged, the engine will rev freely when you touch the throttle at a stop before the transmission engages. If it does not move for a second or it appears the transmission has in fact disengaged and there is a sudden "connection," take it to the dealer to ask the service adviser to accompany you while you show him the behaviour in their own parking lot.

Hot air

I left my car with a hotel valet park. When the young lad brought it back to me, it was blowing hot air through the ducts, even though it was a cool Vancouver morning and the heater was not on. It took more than 10 minutes of driving with the fan on high to cool down the airflow. My coolant is new and the coolant level is good. My temperature gauge works and the car was not overheated. No puddles, drips or leaks. This seems to have been a one-off, but I'm concerned about the cause. Any ideas? – Bob in Vancouver

My first thought was that the valet had gone for a little test drive or that the vehicle had been parked in close proximity to something warm like a heat duct, exhaust vent or some other heat source. If either one of those had been the case, the system should have cooled down within a minute of you driving off. I assume the temperature setting had not been altered by the valet or yourself.

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You do not say if the vehicle has a/c or not. If so and it is the automatic variety, there are a number of points in the system that could be at fault. If it does not have a/c, the trail is a little easier for a technician to follow.

Another factor could be the age and condition of the vehicle and system. Again, without this information, it is difficult to diagnose remotely. If you have not been able to repeat the scenario, keep an eye on things and chalk it up to one of those lovely intermittent events that plague vehicles on occasion.

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