Check return voltage from pedal potentiometer under floor.
Should be arround 4.9 - 5.0 Volt in neutral position, and 8.0 - 9.0 if fully pressed. There are 3 wirings: positive 10V(supply), negative, and return voltage which has to be whitin correct range. You can adjust it by unscrew the pot and twist slightli until reach correct settings. Be aware, as if the truck is in the gear and you twist it too much, it will drive away!!! One more things: Sometimes potentiometer is faulty and loosing parameters on end range. Best to connect multimeter and check return voltage whitin full range by pressing slowly drive pedal to the floor and observe indications.