Overdrive has issues!

[91W350]

My truck has decided to be a rebel this week. The overdrive is slow to shift in. Normally it works just like clock work, kncking off a gear about every 10 miles per hour. Now, it might hold out until 50 or 60 before dropping into overdrive. On other days, it is back to normal. What actuates that over drive shift? Anybody been here before? Transmission seems to work just like always otherwise. I should add that this is my 1991 with 192,000 on it. Thanks, Glen

 

[cerberusiam]

The PCM calls for the OD shift depending on inputs from the VSS, TPS, and trans temp sensor. Transient changes in the OD shift have many sources, including hold your mouth just right.



Any pattern to the days the shift seems off? Warmer? Cooler? High humidity? Small fluffy dogs flying thru the air? (Sorry, PB started something. :-laf )



I would suspect 2 problem areas, the TPS and/or VSS. The TPS you can check for correct voltage and operation. The VSS without a scope is a parts swap.



Something you might try, the next time it starts shifting weird run by a car wash and pressure wash the engine and transmission especially on the drivers side. Drive it immediately after and see if it changes the shifting characteristics.



My . 02, good luck.