Fuel Delivery and Air Induction: Description and Operation

The Diesel Fuel Filter System combines five functions into a single package.
1. It heats diesel fuel to prevent wax from plugging the filter;
2. It combines very small droplets of water in the fuel into larger drops and separates the water from the fuel;
3. It filters the diesel fuel;
4. It detects the presence of excess water in the fuel; and
5. It provides a means to drain the water it has separated.

The Fuel Filter Assembly consists of the Fuel Heater, the Water-in-Fuel Sensor and a Filter. The Filter contains the "coalescer" (the device that combines small droplets of water into larger ones) and the filter/separator.
An electric fuel pump delivers diesel fuel from the tank to the fuel filter. As fuel enters the filter, it passes first through the Fuel Heater. The heater contains a thermostatic switch. The switch opens or closes to turn the heater off or on, depending on the temperature of the fuel.
The fuel then passes through the Primary and Secondary filters. Next the fuel flows through the water coalescer. Here the droplets of water in the fuel are combined into larger drops, the drops fall to the water reservoir in the filter. When fuel flows from the Fuel Filter Assembly to the injection pump, it is clean and free of water.
The Solid-State Water-in-Fuel Sensor applies voltage to a Probe. When the Probe touches water, the Module closes a switch. This completes a circuit to ground to light the SERVICE FUEL FILTER Indicator.
A time delay circuit in the Water-in-Fuel Module grounds the Indicator bulb briefly to test the bulb each time the system is turned on.
The Fuel Heater is operated by a built-in thermostatic switch. When the switch is closed, battery voltage is supplied to the heater from the GAGES Fuse.