Shudder Or Slip In Reverse Under Hard Acceleration - keywords chatter

This PI was superseded to update models and add tire inspection notes. Please discard PIP4022A.

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

Shudder or slip in reverse on hard acceleration only. Most easily duplicated by shifting to reverse and immediately accelerating very aggressively on level ground, less aggressively up a hill or with a heavy load.

Note: Ensure the vehicle is equipped with the OEM tire size prior to attempting the below diagnosis. Also, make sure that the rear tires are matching side-to-side in make, size and depth of tread (wear). Mismatched tires can contribute to axle hop that can feel like shudder.

Before attempting to duplicate this concern make sure to ask the customer if the condition occurs when the vehicle is loaded or has a trailer attached or if the vehicle is unloaded. This condition can also be caused by rear axle hop, especially when backing a trailer up a hill. When attempting to duplicate the condition have another person watch the rear tires to verify a possible hop condition. Also, when attempting to duplicate the condition allow one second after moving the shift lever to the Reverse position before depressing the accelerator pedal. Increasing throttle before one second may result in slip or shudder even though there is not a problem with the transmission as the clutches are not fully filled and applied.

If slip or shudder occurs verify that reverse boost can be obtained. Use a line pressure gauge to monitor line pressure and a Tech 2 to monitor PSC amperage during the concern and document in a TAC case. Engage D4, raise engine RPM to 1,000, allow three seconds for line pressure to stabilize and record line pressure. Engage Reverse, raise engine RPM to 1,000, allow three seconds for line pressure to stabilize and record line pressure. Reverse pressure should be at least 20 psi higher than Drive pressure.

Find an identical vehicle and duplicate the same driving maneuver. If it does exhibit the same symptom, this may be an unreasonable driving maneuver and repairs may not be necessary. If another vehicle does not exhibit this concern, inspect the reverse input and low/reverse clutches for damage and root cause (i.e. circuit leak).

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.