If the rpm is going up and down about 200 rpm, it likely is the torque converter unlocking intermittently. It will unlock when you press the brake pedal, in preparation for coming to a stop, when releasing the throttle, to allow you to coast longer when slowing down, and under hard acceleration to provide more torque. Lockup only occurs above about 35 to 45 mph and when the engine is nearly fully warmed up.
First, when the problem is occurring, hold the brake pedal up with your foot. If the problem stops, readjust the brake light switch or be sure the pedal is returning fully. You can also verify the torque converter is unlocking by holding the accelerator pedal perfectly steady at highway speed, then very lightly holding the brake pedal down 1/2" with your left foot. The rpm should stay up at the higher speed and never drop back down. Don't use the brake lights to determine if the brake light switch is causing the problem. There's two or three different switches built into one assembly and the torque converter clutch can be on a different switch than the brake lights and cruise control.
The next thing is to connect a scanner to view live data and watch if the Engine Computer is commanding the torque converter to lock and unlock intermittently. If it continues to display lockup "on" while the problem is occurring, the cause is inside the transmission or the wiring to it. Normally a wiring problem will set a diagnostic fault code. Look at the coolant temperature sensor. If you have a temperature gauge on the dash, that uses a different sensor, so that is not relevant to this story. Ford had a lot of trouble with their Engine Computer coolant temperature sensors in the early '90s. They would make the apparent temperature bounce around all over the place and the computer would respond accordingly. That's very uncommon on most other cars, but not unheard of. On your car I would be more likely to expect to find a defective thermostat that is keeping the engine too cold, and it's right on the borderline of where torque converter lockup is allowed.
Don't overlook low fluid level too.
Saturday, October 18th, 2014 AT 12:53 AM