The two most common causes are a misadjusted brake light switch and a glitch in the throttle position sensor. When the problem is occurring, hold the brake pedal up with your toes. If the unlocking stops, suspect the switch.
Problems with the throttle position sensor is a little harder to pinpoint. A sensor that has totally failed, or an intermittent wire or connection, will set a diagnostic fault code and will be easy to find. It's when a small piece of carbon breaks free inside the sensor that it can report a wrong signal voltage that is still within the acceptable range. As long as the signal voltage stays between 0.5 and 4.5 volts, no fault code will be set, but a voltage that gets near 0.5 volts tells the computer it's time to unlock the torque converter in preparation for coming to a stop. A signal voltage that approaches more than about 4.2 volts tells the computer you're at wide-open-throttle and the torque converter should unlock to give you double the torque for passing that string of cars.
The next step is to connect a scanner to see if the Engine Computer is actually requesting the converter to unlock. If it is showing the torque converter clutch is being commanded on, but you feel it unlocking, the main suspect is the converter itself. If the solenoid, wiring, or connector terminals are open intermittently, that should be detected and set a fault code.
Look under "live data" too on the scanner to see what the coolant temperature sensor is reporting. Ford had a lot of trouble with their sensors in the early '90s. The torque converter will not lock up until the coolant has reached a certain temperature. If the temperature being reported is too low, the computer will turn the lock-up circuit off.
Wednesday, June 24th, 2015 AT 11:11 PM