Heres the situation. While driving my 1990 Bonneville LE, the RPM at a given speed has recently increased over what it formerly was. ie, at 55mph, it used to run about 1600 RPM, and now seems to run about 2000 RPM instead. A couple of nights ago, when driving home from work, i was driving with the cruise control set, and the RPM jumped up while driving on a flat piece of road, and it never came back down. It doesn't seem as if its shifting down, so am I looking at a TC problem. This problem has brought decreased gas mileage, which is costing more all the time it seems, and I would very much like to get my 5-8mpg back.

Get to a steady speed around 55. Without taking your foot off the gas, tap the brake pedal lightly. You should see a slight jump in RPMS. If not, try pulling the pedal back with your foot.
Two likely causes: The brake switch, which disenguages the TCC when you apply the brakes. OR, the TCC solenoid. The first is an easier fix, the second requires removing the side of the tranny case and axle. It'* actually more common that they lock up, especially on the TH125s.

-T

Ok, I got up to about 60, hit the brake with my foot still on the gas and the RPMS didn't climb, jsut slowly fell with mph. I gave it more gas, and it eventually downshifted. What does this tell me?

if you are going 60 mph and it TCC lockup, Pressing the brake will unlock the TCC, thus causing the RPM'* to increase

You shouldn't hit the brakes hard enough to engage them, but it'* sounds like the TCC is not locking. The first thing to check is the switch on the brake pedal.

Yes, the cruise control works properly. Is the brake pedal switch going to be under the pedal in the car? Hopefully thats all it is and not the solenoid.

This sounds like a stuck open thermostat. I mean, you were driving on a flat road, with lots of air flow going through the rad, cooling the coolant down, then because the thermostat is stuck open, all that cold coolant rushes in and out of the engine before it can even warm up. This gives the ECM a low temp reading, which definitly keeps the TCC from engaging. I remember last september when I had to change my thermostat, because the TCC would not engage.

To tell for sure, hows the dash temp reading?

If the brake switch was bad, then wouldn't the cruise not operate, as well as the stop lights?

I am going to say its the TCC solenoid, and not the thermostat. Only reason why is that I have the same problem, and the car was running cool, so I put in a 195 thermostat. But I still have the TCC problem. It works fine if it'* cold outside, but if it warms up outside, and the car has been running 15-20 mins, the car drops out of OD and the TCC unlocks. Too bad, because I really dont want to get into the TCC solenoid, but I have no choice. While I am at it, I will probably change the spring that smooths out the 1-2 shift. It gets kind of clunky shifting when the OD/TCC problem occurs. :(

Good Luck with yours!
John

I can pretty well garuantee that it isn't a thermostat. The car heats up quite well, as I still have the winter cardboard in front of radiator. The temp gauge usually gets about 1/4-1/2 the way, never farther than that.