No OD lockup when hot

'88TC. When the car gets up to temp, the overdrive lock-up stops locking up. It goes into overdrive fine, but no lockup. Is there a temp sensor or something in the trans.? Or better yet, somewhere outside the trans.? It only stops working when it gets hot. Help! Thanks... Chuck

No internal temperature sensor. The manual lists these possible causes of the clutch not engaging:
1. Clutch not being electrically energized:
Wires to solenoid shorted or open circuit.
Transmission case connector not seated.
Open or short circuit inside solenoid.
Malfunctioning engine coolant temperature sensor.
Malfunctioning throttle position sensor.
Malfunctioning manifold absolute presseure sensor.
2. Brake switch malfunction.
3. Malfunctioning EEC-IV processor.
4. Solenoid energized but valve stuck open.
5. Converter clutch shuttle valve stuck in unlock position.
6. Converter clutch shift valve stuck in downshift position.
7. Torque converter internal malfunction preventing lock-up piston application.

Because the lock-up and shifting depends on hydraulic pressure scheduled by the valve body in the transmission the solenoid acts to inhibit converter clutch operation and is not energized as follows:
1. Engine coolant below 128* or above 240*.
2. Brake application.
3. Closed throttle.
4. Heavy or WOT acceleration.
5. Quick tip ins or tip outs.
6. When actual engine speed is below a certain value at lower vacuums.

From the above the first thing I would suspect after being sure the fluid was in good condition and at the right level, would be the ECT.

It only doesn't work when it gets hot. I beleive the ECT is ok as the fan comes on and goes off at the right temps. I had cooling issues for a while, but I put in a new t-stat and replaced the air deflector under the car and it's much better now. When the car gets up to about 195*, it's all over with the lock-up. I also checked the level and it's good. Tranny was rebuilt not long ago, but they had the car FOREVER and I really don't want to bring it back there again. Had to bring it back 4 times in about 4 weeks right about a year ago. Live and learn. The car NEVER hit 240*. I have autometer gauges and like I said, the fan comes on at the right temp. according to the gauge. The possible causes listed seem like it would NEVER work, that's not the case. Example: I live about 20 mins. from the expressway and my office is about 2 mins. off the expressway. When I get on going to work, no lock-up. Going home, works fine until car gets up to temp. If I get on and it goes into lock-up, it will stay there solid until I slow down or whatever un-locks it, then it's all over... again. It's better since I put the deflector on, so I don't know if it's the engine temp or tranny temp as I'm sure they are both running a little cooler. Maybe a better tranny cooler? Still have the stock one in there.

I have the opposite problem .... no lock-up or over-drive when I press, slightly, on the accelerator. As soon as I back-off, it goes into overdrive/lockup. I have replaced most of the sensors with Motorcraft parts .... maybe, the throttle sensor is not properly adjusted. This started to happen about a year after the tranny was rebuilt and warranty expired.

Carl

Well, the way the manual describes it, the transmission will work fine without any electronic control as the solenoids only have effect on the clutch and the 3-4 and 4-3 shifts by modifying the hydraulic programming. The lock up is inhibited and the car downshifts when you push the accelerator depending on the load on the engine as determined by the computer based on the information supplied by the sensors listed above so the the torque converter can supply more torque so the car won't bog. The solenoid can be energized both in lock up and unlocked conditions depending upon what the valve body program is doing. The manual also states that the transmission is a hybrid design so you have to consider both electrical and hydraulic systems so you have a difficult problem. You have to know that the computer is sending the right signals and that the signals are getting through. Many electrical and electronic components work perfectly when cool but fail when hot, including wiring and connectors. So they have to be checked out and you have to know that the ECT is within specs at the temperature the lock up fails at. Numbers 1, 2, and 3 above are electrical problems that can be checked easily. Numbers 4, 5, 6, and 7 are mechanical and not so easy to check. 4, 5, and 6 and can be accessed by dropping the pan. Number 7 requires replacing the torque converter. I thank the transmission gods that I havn't had a problem since the Cruisamatic in my 63 Tbird lost reverse in 1967.