Fuel Heater Functional Diagnosis

Diagnostic Chart:

Schematic:

CIRCUIT DESCRIPTION
The fuel manager/filter assembly consists of the fuel heater, the water-in-fuel sensor, and a filter. The tiller contains the coalescer (the device that combines small droplets of water into larger ones) and the filter/separator.

A fuel lift pump delivers diesel fuel from the tank to the fuel filter. As fuel enters the filter, the fuel passes first though the fuel heater. The heater contains a thermostatic switch that opens or closes to turn the heater OFF or ON, depending on the temperature of the fuel.

The fuel then passes through the filter and the water coalescer, where the droplets of water in the fuel combine into larger drops that fall into the water reservoir in the filter. When fuel flows from the fuel manager/filter assembly to the injection pump, the fuel is clean and free of water.

The fuel heater is operated by a built-in thermostatic switch. The thermostatic switch completes the circuit for the fuel heater element when it senses a temperature below 8°C (46°F).

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table:
2. This step checks for a fuel heater that is stuck ON.
3. This steps checks for a thermostatic switch that completes the circuit.