Engine/Transmission (1998.5 - 2002) Shift problems when temp above 210 degrees

[rotohead]

Having transmission problems when transmission heats up above 210 degrees. Runs correctly when cold but won't downshift to first gear and sometimes won't upshift out of second gear when warm. Transmission shop that rebuilt three years ago has replaced about everything on the transmission itself and now are at a dead end. New batteries, new APPS, and no codes at this point. PCM? ECU? Starting to get real worried about this one. Any help would be appreciated.

 

[cerberusiam]

Was the temp sensor replaced? Gov solenoid and transducer?

 

[rotohead]

Yes, all parts in the transmission have been replaced twice. The new and improved versions from BD. Leaning toward the PCM.

 

[cerberusiam]

The temp sensor should be external on your truck, it gets over looked sometimes and can be the source of bad signals. Have you monitored actual gov pressure and TV pressure when the problem happens? Monitored demanded gov pressure? If it demands a certain pressure and it doesn't set to that mechanically then you have another direction to go. The PCM can definitely throw glitches and it gets worse as it gets warm. You can try replacing it or do a little more research on pressures to narrow it down.

 

[rotohead]

Thanks for the tips. All pressures seem normal. Can't find that external temp sensor part anywhere available to buy. The PCM would be a last resort but a possibility. Thanks again.

 

[cerberusiam]

You should have the external sensor and it should be a simple thermistor you can override by using a jumper wire. It will think it is cold but that never seemed to bother where if they went open when driving the PCM thought the trans was overheating. Might try that and see if it makes any difference.

Have you checked actual and demanded gov pressure when it won't upshift from 2nd gear? If that pressure is good based on wheel speed and the TV pressure is good, then it has to be a mechanical issue. Either a shift valve is hanging or the direct clutch circuit is loosing pressure so it won't apply the direct clutch pack.

 

[rotohead]

Turns out the problem was in the plugs (middle one) connecting to the PCM. Two wires in the middle of the plug had become 'loose'. And couple other suspect connections. This truck for a 17 year old rig is pretty clean. No rust (only driven in the west and very little in the winter) and no off road abuse. I guess this kind of thing can happen even if it was never driven. The transmission tech connected the Snap-on scanner and with the key on but truck not running would move the harness around and watches values jump all over the place. Did about two hours of clean splice work and wah-la. A smooth, clean shifting transmission again! Cost me less then $200 with new fluid, silinoid and transducer. Highly recommend Michael Dickinson TransCraft in Reno, NV.

edit: It also seems there was a diode in a harness wire my transmission guy had never seen before. It had no continuity so it was removed. It shifts great now.