Scan Reveals What’s Causing Chevy’s Speed/Shifting Problems

Posted 11/14/2011
By Bill Petersen

The 2008 Chevy Silverado has speed/shifting problems. While the Silverado is towing a trailer at highway speeds with “Tow/Haul” selected on the shifter, its transmission downshifts from sixth to fifth, to fourth, to third gear, and then upshifts to fourth, then to fifth gear at 55 to 60 mph. When the vehicle is accelerated slightly, the transmission will upshift to sixth gear. Relaxing the throttle to maintain vehicle speed causes the downshift/upshift sequence to happen again. With Tow/Haul off, the problem disappears, the transmission shifts normally and stays in sixth gear at the same operating conditions. 

The Tow/Haul switch receives 5 volts from the body control module (BCM) and ground from a dash ground. This is a momentary switch, closed only when it’s depressed. The BCM receives the Tow/Haul input and sends the information as data to the transmission control module (TCM) and the engine control module (ECM).

When the TCM receives the Tow/Haul input from the BCM, it alters the transmission shift strategy for towing or hauling a heavy load. The Tow/Haul mode maintains lower gears longer and increases shift pressures. Automa­tic downshifting (powertrain engine braking) will activate when Tow/Haul mode is active and the brake pedal is depressed. After a powertrain braking situation ends, upshifts will typically be commanded when the accelerator pedal is pushed down. (Note: See GM Technical Service Bulletin 08-07-30-030 for related information.)

Tow/Haul is designed to be most effective when the combined weight of the vehicle and trailer is at least 75 percent of the vehicle’s gross combined weight rating  (GCWR). Operation of Tow/Haul in a lightly loaded or nonloaded vehicle will not cause damage. There is no benefit to the selection of Tow/Haul when the vehicle is unloaded. Using the Tow/Haul mode during unloaded driving conditions will decrease Tow/Haul fuel economy and may generate shift feel concerns. Testing for Tow/Haul problems when not towing or hauling a load may give misleading or inaccurate test results.

I asked the technician to scan the TCM, ECM and BCM when the problem occurred. The transmission was being commanded to upshift and downshift. This indicated a possible false input to the TCM. The result of the scan data review was the TCM brake switch status indicated the brake was applied when it was not. While driving during the condition, the technician lifted up on the brake pedal and the brake status changed on the scan data. The transmission shifted into sixth gear and operated normally from that point while the pedal was being held up. When the pedal was not held up, the condition returned. The brake lights were not on when the brake status showed on the scanner as incorrect.

The brake switch assembly has two brake switches and one switch is normally open. When the brake pedal is depressed, the switch connects battery voltage to the ECM, TCM and BCM. The BCM also gets an input from the other normally open brake switch and sends a current-limited battery voltage and a ground to the switch. When the brake pedal is released, the BCM sees battery voltage. When the brake pedal is depressed, the BCM sees 0 volts. The BCM uses this switch to control the brake lights, center high mount stop lamp, and cruise control. The brake lights would not be affected by the brake switch to the ECM, TCM, and BCM.  When the brake switch was replaced, the problem was gone.

Bill Petersen is an Identifix GM specialist. He is a certified Chevrolet master, Buick master, Pontiac Service Guild and ASE master with L1 certification.

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