Hello! I happened across this forum while trying to find an answer to my troubles and hope someone can be of help. Looks like this issue has been covered quite a bit on here but after reading though multiple posts I'm still looking for a solution. I have a 1994 Bonneville SE that I puchased last year with the issue that it would drop out of overdrive after driving at highway speed. So after taking it to a local transmission shop they recommended a rebuild as it was showing a torque converter code. So I had them rebuild the whole transmission...only to not have the issue solved. I've had it back twice since then, the first time they said was because the converter they installed was bad, so it was replaced and now the issue happens infrequently but still occurs. The second time they tested it for about a week and decided that it may be temperature related so they installed an external trans cooler. I just took the car on a long trip and sad to say it is still dropping out of overdrive. When I first start driving everything works great until everything is warmed up, or as on the trip I'd have it in cruise until I would need to slow down or turn the cruise off and I'd loose overdrive. If I stop somewhere and let the car sit, it will shift back into overdrive until eventually it drops out again. It'* the difference between 2k and 3k so I know when it'* not working. What else could the issue be? Trans parts should be new, it'* got a cooler, fluid is new. Please if anyone has any thoughts they would be appreciated. I'll be taking the car back but it'* just getting frustating. Thanks for looking!

There are a lot of things that will cause the TC to unlock like that. Misfires, incorrect engine temp, incorrect trans temp and a few others.

Have you checked the car for codes yet? It can be done via the paperclip method.

Well I stopped by two parts stores and neither showed any codes present in the car. No check engine light or anything like that either. As for coolant temp, it usually sits right below the 200 and I've never had an issue with overheating. So I guess this doesn't really shed any light on the issue. Hopefully you can still point me in the right direction as to what to do. Thanks!

" drop out of overdrive after driving at highway speed."

Does this occur primarily under load/ while driving up a incline? If so, as Mike stated, you want to look at a misfire (coil, plug wire, spark plug).

Your going to need to get a scanner you can monitor the PCM data while this problem occurres. If the PCM isn't commanding it out of lock up and OD you have a wiring or mechanical issue with the transmission.

Doesn't seem to matter where it is, I've been on flat highway when it is happening.

What kind of scanner would you recommend?

Sadly this is the only issue plaguing me with this car and it drives great otherwise so it'll be great to get this figured out so I can really enjoy the car. It'* only got 114000 on it and it came from the original owner so I'm looking to keep it around for the long haul.

Thanks for the input so far!

If your car is OBD2 there are many scanner options to choose from. My 94 SSEI was obd2 so I'm hoping your Bonnie is also. Just make sure it can monitor live data and isn't just a code reader. I have a tech2 which is what the dealer uses but they are kind of spendy. I believe harbor freight sells a rather inexpensive scanner. More than likely it is a bad TCC solenoid but I would want to make completely sure before I tore into the transmission.

All the ODB II scanners I read about say they read 96 and newer so I'm hesitant to purchase one. I know the service manual says to use the tech 1 scanner and doesn't make any mention beyond that I can find. I was hoping to have any idea about what was going on but in the next few days I'll just take it back to the trans shop that rebuilt the transmission and have them take another go at it. This will make the 4th week so maybe that will be the charm. I just can't believe this is this ellusive of a problem that it has been this difficult to discover.

Yes the 94-95 Bonnie'* were kind of an odd duck because GM was trying to transition into the OBD2 system for 96 so unless you have a high end scanner with the proper software it'* kind of hard to talk to them.