Intermittent Unwanted Downshift, No Upshift 5L40E/5L50E - keywords apply brake shift throttle

This PI is being superseded to update concern and model years. Please discard PIP3268A.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Intermittent unwanted downshift while operating the vehicle in a coast (i.e. 5-4, 4-3, 3-2) with no driver input brake apply. When the condition occurs, the transmission will not upshift until the vehicle comes to a complete stop and then the transmission will shift normally until the next time the condition is duplicated. There will not be any trouble codes associated with this condition.

Note: If the concern happens when coasting on a down hill grade and the brake is applied by the driver the transmission will downshift and not upshift again until enough throttle is applied to overcome the acceleration of the down hill pull, this is a operating characteristic and no repairs should be attempted.

The cause is a weak internal vacuum brake booster return spring resulting in the brake pedal position sensor not returning to the off position. This unwanted brake pedal signal (depressed) is sent to the EBCM then communicated from the EBCM to the PCM via class 2. The PCM then communicates this message to the TCM resulting in a downshift command during this type of driving maneuver.

Note: The brake pedal position sensor data is not available for monitoring with the Tech II when viewing TCM data. Brake pedal position data can only be monitored with the Tech II when viewing PCM or EBCM data.

Replace the vacuum brake booster to repair this concern.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.