Engine/Transmission (1994 - 1998) OD dropout

[sstefanini]

Truck drops out of overdrive/OD OFF light comes on simultaniously at highway speeds. I'm sure everyone's seen this post enough already. Only difference is mines doing it once daily (160mi. round trip to work). I'm getting quicker at stabbing the the button but would like to fix it. After reading 10 pages of posts the only similar post I could find involved the output speed sensor. This sound familiar to anyone else? Other than this trans. shifts fine. I also should add I bought the truck last month w/ the od off button knocked back into the dash. Any help would be appreciated.

 

[bmoeller]

Could be a Throttle position sensor problem, or with the electronics on the valve body.

 

[CarlJensen]

The OD button is only a momentary switch telling the computer what you want to do, it does not supply a control voltage. Sounds like it could be a flakey TPS.

 

[Howard Durand]

When the OD button is pushed, it grounds a 9 volt circuit from the PCM causing the PCM to lock out overdrive and turn on the light on the switch. Any thing that could ground the circuit will do the same thing. It is possible the switch itself is defective or the wiring is damaged at some point. The first thing I would do is remove the switch completely and see what happens.

 

[sstefanini]

Thanks for the advice. I'll go through and check the wiring & clean /check all the connectors this weekend. As far as this being a tps issue I'll have to wait until it deteriorates as the rest of the transmission shifts great. I'm not sure if I could diagnose something that can't be forced to reproduce.

 

[Howard Durand]

As far as I know, a TPS problem would not cause the OD to lock out the same as pushing the button.

A dirty TPS or electrically noisy wiring can cause abnormal shifting in and out of OD and or TC lockup.

When my truck was new, it would occasionally lock out of OD for no apparent reason. Usually at around 60 mph. I Had to push the button before it would shift back in.

It hasn't done that for a long time now. I have no idea what caused it but suspect the PCM. If yours continues to do this after you have eliminated the possibility of a bad switch or shorted wires, you might have a dealer do a reflash of the PCM.