RPM's increasing by 300-500 while driving on flat road

pickupwbb

Hello, I have a 1994 Ram 2500 Cummins diesel engine. Great vehicle with minor problems. My questions is while driving on the highway on a flat surface with cruise control on or off at a constant speed the vehicle increases RPM by 300 to 500 and then settles back down. So 74 mph RPM = 2000 every couple of minutes it will increase to 2300 for about 15 seconds then drop back to 2000. Dealer says they can not find a problem. Well this happens every time I drive on the highway w/o a load so they are hoping I go away. Does anyone have any thoughts on what could be wrong?

Thanks

mullins87

Do you have an auto tranny? My first guess is the TC unlocking. As long as your not taking your foot off the accelerator or pushing down on it or touching the brake pedal, it should not unlock. I had a 1/2 ton once that would do something similar. That one had a bad TC lockup solenoid that would overheat electrically and kick out, then re-engage a few seconds later after it had cooled down.

pickupwbb

Thanks for the reply and yes I have an Auto Tranny. Questions is what is the TC? And yes this occurs when I am maintaining a constant speed either in Cruise control or not no touching the brake and constant pressure on the fuel pedal.

Thanks

mullins87

The TC is the torque convertor. When it locks it creates a direct, or at least nearly direct, hook-up between the engine and tranny, similar to the clutch on a manual tranny. The TC will unlock if you take your foot off the accelerator and the truck slows below 35-40 mph or if you touch the brake. It will also unlock if you press down on the accelerator to quickly speed up.

lariat1

Another thing to check is the TPS. (throttle position sensor) My 01 had the same problem so I adjusted the TPS and it cured it.