having intermittent problems

[natedogco2007]

So here is my problem.... sometimes when it's really cold outside ( here in sc....20's ) my explorer will sometimes kick out of overdrive and then when you stop and put it in park the motor will rev to like 2000 revs. but when you turn it off and restart it will be fine. it has done it when it was hot outside but it tends to only do it when it's really cold out. i have checked vaccum lines and cleaned the iac out completly. i am not getting a cel either so i don't have a clue what's up. ooh yeah i also cleaned the mass air flow sensor. can anyone help?

 

[MattPersman]

look at the TCC sounds like that may be your ticket

 

[MrShorty]

I've seen many people who confuse the torque converter lockup clutch with 4th gear (aka OD)* Which is it? Next time you feel the unexpected shift, move the transmission selector from OD to D and see if the transmission downshifts again. If it does, then the unexpected shift is the TCC lockup clutch disengaging.

If it is the TCC lockup clutch, then, from your brief description, I would suspect a fault in the engine coolant temparature (ECT) sensor/circuit. When the ECT indicates that the engine is too hot (~230 F or higher I think), the PCM will disengage the TCC to increase engine RPM to allow the fan to draw more air through the radiator. Idle speed also is increased.

*Educational note. Overdrive refers to any gear ratio less than 1. Otherwise stated, the output shaft spins faster than the input shaft. Automatics aren't the only transmissions with "overdrive". A manual transmission can also have one or more overdrives.

 

[natedogco2007]

mr shorty, one thing that i forgot to mention.... i replaced the coolant temp sensor last year when it did it the first time and i got a cel. the only thing that really confuses me is that i don't always have to get up to higher speeds ( 45 mph ) for this problem to happen. also the tranny and converter were replaced less than a year ago. i don't think that i have put on more than 9 k miles on it since then. i don't think that it could have gone bad that soon. i really think that it is an electrical problem since i can shut off the car and then it's fine. i read somewhere on here that someone said that it could be either a tps or a bad battery terminal? anyone else heard of this?

 

[natedogco2007]

ttt

 

[MrShorty]

Just because you replaced the ECT sensor doesn't mean that the wiring between the ECT and the PCM is intact.
TPS could also be causing this problem. Note that the TPS and the ECT share Vref, if there were a fault in Vref, that could cause both sensors to give faulty data.
I might also add VSS to the pot.
Bad battery cable is also a possibility. Most sensor data is in the form of a voltage or voltage drop. In order for the PCM to get an accurate reading, it needs a very good ground. If there is a significant voltage drop across the PCM's ground (say from a corroded battery cable), the PCM can't read the other voltages accurately.
And note that I'm talking circuits, not sensors. Circuits include the sensor, the wiring from the PCM to the sensor, and the portion of the PCM that reads/controls that circuit, and the PCM ground.
Have fun, sounds like it could be a challenging diagnosis, especially on a '91 where you can't just hook up a scanner and have the computer feed you sensor data.

 

[natedogco2007]

thanks mr shorty. i replaced the battery terminals the other day and found that the positive lead came off really easy and the leads were all messed up. i think that might have been part of the problem. i still need to check out the tps voltage. i really hope that it isn't the tcc. i would be really mad if i paid that much for a tranny and something is bad. but i guess that i will have to check it out.

 

[natedogco2007]

any other suggestions?