you will have to give us more information, like what serial number and control panel is installed.
Usually the negative side of the contactor circuit is "made" with a solid state "driver" module or signal from the logics card, after the card sees all the correct signals in the correct order (seat-switch and ign.switch must close before accel switch etc...)
Please don't consider the negative side of a circuit on an electric forklift to be a "ground" circuit.
first things to check is that all the rest of the "static return to off" (SRO) circuits are in the correct positions and sending the correct signals to the logics circuits