Have to hold pedal in on position for compactor to run

My Ge compactor will not run unless I keep the foot pedal in the up position. If you hold it in up position it will run through cycle and then thump at the top. If you let go of foot pedal at any time it will stop. Lift up again and it reverses travel but you again have to hold in actuated position. Is there a problem with a switch?? I searched the forums but could not find similar problem with this unit. Thanks in advance