94 explorer comes out of overdrive on highway

[Jcrews]

I've got a 94 explorer xlt with about 122k and have recently encountered a problem with the truck coming out of overdrive after about 50 or so miles on the highway. Check engine light comes on at times with relatively heavy throttle (above 2500 rpm or so) and goes off when power is reduced. If I park it for a while and let it cool down a bit it will go into overdrive no problem at around 55 mph, but the problem comes back once I get back on the highway for an extended period of time. The "slip" out of overdrive occurs more rapidly if I'm cruising at more than 65 mph.

From reading other threads I'm getting that it may be a TPS issue and should have the code read. If the check engine light is not on is there still a code to be read?

Any ideas of what other problems I may have are appreciated. Thanks!

 

[MrShorty]

If the check engine light is not on is there still a code to be read?

Yes. the computer should remember the code. And that is probably the best place to start.

One additional thing I might suggest - when you say "overdrive" be clear if you are talking about 4th gear or about the torque converter lock up clutch (or both). I hear "overdrive" used to refer to both, and it could be useful to know for sure which "shift" is dropping out.

 

[Jcrews]

One additional thing I might suggest - when you say "overdrive" be clear if you are talking about 4th gear or about the torque converter lock up clutch (or both). I hear "overdrive" used to refer to both, and it could be useful to know for sure which "shift" is dropping out.

I'll count the shifts tomorrow, but I'm sure it comes out of the last shift as it runs fine for a while. Would that be the torque converter lock up clutch?

 

[MrShorty]

Could be. TCC can lock up in either 3rd or 4th gear. Next time it "drops out," note how much the rpms change. TCC unlocking will usually change it less than 500 rpm. 4th to 3rd gear will change it more. Tap on the brake while watching the tach. If the TCC is working, the engine speed will temporarily jump a few hundred, them drop back down as the TCC locks back up. Press on the throttle and note engine speed. If the TCC is locked, the engine speed will only be able to speed up as the vehicle speed increases. If the TCC is unlocked, then the engine speed will be able to increase without a corresponding increase in vehicle speed.

 

[Jcrews]

It's definitely the tcc that is coming unlocked. After counting the shifts and noting the rpm change after it unlocks, that seems to be the issue.

I put it in neutral going downhill the other day a,d when I put it back into drive the rpm surges up and down, like it is confused about which gear it should be in.

 

[MrShorty]

If it is TCC only that is unlocking (4th gear seems to work fine), then I would inspect the TCC solenoid circuit carefully. Look for loose connections. Using a voltmeter and a wiring diagram, hook up a voltmeter across the solenoid. Using the output state test (see my notes on pulling EEC-IV codes) you can see if the PCM is able to "switch" the solenoid. Then take it for a test drive while watching the voltmeter (maybe use a helper). See if the the PCM is switching the TCC off when it shouldn't. If the TCC seems to be responding the PCM's commands, then we can look further into the electronic controls to see why the PCM is telling it to unlock. If the transmission is shifting without the PCM command, then the fault is probably hydraulic/mechanical.