Fuel Pump Control Unit: Description and Operation

Removing 4X4 Control Unit:

The 4X4 control unit, located under the drivers seat, operates the fuel transfer pump, along with its other 4WD control functions. The control unit incorporates a timer and a relay. The timer delays the operation of the transfer pump for ten seconds after the ignition is turned "ON".

A magnetic float switch in the fuel tank (part of the fuel transfer pump/gauge assembly) signals the fuel level to the control unit. As long as the fuel level is sufficient, the float switch is "OPEN", and the control unit powers the transfer pump. When the tank is just about empty, the float switch closes, grounding a terminal of the control unit. The control unit then interrupts power to the transfer pump, turning the pump "OFF". This is to ensure the pump does not run "dry".