MU4 MU5 Transmission Characteristic Upshift or Downshift on Grades - kw brake bind calibration decelerate delay excessive fourth intermittent performance shift third transmission

The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.

Customers may comment the transmission has a delayed or no 3-4 upshift after cresting a hill or when driving in hilly terrain. Customers may also comment the transmission downshifts without any apparent driver input when descending a grade.

Engineering has confirmed this is an operating characteristic of the uphill/downhill mode calibrations. The ECM calculates the throttle opening angle and the acceleration rate to determine whether the vehicle is in the uphill or downhill state. The actual acceleration calculated from the speed sensor signal is compared with the reference acceleration stored in the ECM to judge uphill or downhill driving. While driving uphill on a winding road with ups and downs and the ECM judges an uphill condition, the 4th upshift is restricted after a 4-3 shift has occurred to ensure a smooth drive. Also, if a brake application is detected while the ECM judges a downhill travel in 4th, the transmission automatically downshifts to 3rd in order to provide an appropriate engine brake.

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Please follow this diagnosis 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. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.

(2003-2004 Pontiac Vibe)

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