Yup, that's the converter unlocking. You can duplicate the occurrence by tapping the brake pedal while holding the gas pedal steady.
The converter also unlocks when you completely release the gas pedal such as when coasting down a hill. An intermittent bad spot in the throttle position sensor will affect the lockup function, but it should also turn on the "Check Engine" light when the problem is detected.
When the problem starts to occur again, try holding the brake pedal up with your toes. If the problem stops, the brake light switch needs to be adjusted. If the problem seems to always occur around the same speed, try cruising in the same area 5 mph faster or slower. If the problem stops, suspect the throttle position sensor.
Finally, lockup is not supposed to occur when the engine is cold or too hot. Your mechanic can drive the vehicle with a hand-held computer connected so he can watch the sensor readings in real time. I use my '88 Grand Caravan to pull an enclosed trailer that's bigger than the van! After about 35 miles, the converter will unlock due to the increased engine temperature even though it's not overheating.
Monday, January 18th, 2010 AT 5:34 AM