Hi John

Twin motors? If so lock out each motor and see if it makes any difference to the speeds. Check contactor tips for direction contactors and clean if required in case you are not getting a good connection through the tips.

Have you tried another foot pedal? Also is the bypass not energising? It may be the foot pedal is not reaching maximum output in order to energise the bypass contactor.

On some of the earlier model cats/daewoos the park brake switch could sometimes reduce the speed to the traction motors, even though I think on this truck it cuts out traction its always worth checking.

I am guessing no fault codes? If there was a component failure I would have expected some error codes or even if the thermal switch circuit was faulty it should also give some type of error indication.

If you dont have a spare foot controller to try this can be diagnosed through the diagnostic procedures.