Gunter,

Make sure the transmission harness is in good condition with no shorts or breaks. Switch the forward and reverse solenoids to try and locate the problem. If the lift only moves in forward when switched, solenoid is the issue. If it remains the same, either wiring or Forward Reverse switch. Only other condition that would cause this if the reverse clutch packs were locked together. Make sure fluid condition is good and not burnt. Good Luck.