FBH Unit

FBH Unit:

FBH Unit Connector Pin Details:

The FBH unit is installed on the bulkhead in the engine compartment, on the side opposite the brake servo, and is connected in series in the coolant supply to the heater assembly. Two electrical connectors on the top of the FBH unit connect to the vehicle wiring.

Sectioned View Of FBH Unit:

The FBH unit consists of:
^ A circulation pump.
^ A combustion air fan.
^ A burner housing.
^ An ECU/heat exchanger.
^ An air inlet hose.
^ An exhaust pipe.

Circulation pump: The circulation pump is installed at the coolant inlet to the FBH unit to assist the coolant flow through the FBH unit and the heater assembly. The pump runs continuously while the FBH unit is in standby or active operating modes. While the FBH unit is inactive, coolant flow is reliant on the engine coolant pump.

Combustion air fan: The combustion air fan regulates the flow of air into the unit to support combustion of the fuel supplied by the FBH pump. It also supplies the air required to purge and cool the FBH unit. Ambient air is supplied to the combustion air fan through an air inlet hose containing a sound deadening foam ring.

Burner housing: The burner housing contains the burner insert and also incorporates connections for the exhaust pipe, the coolant inlet from the circulation pump and the coolant outlet to the heater assembly. The exhaust pipe directs exhaust combustion gases to atmosphere at the bottom of the engine compartment.

The burner insert incorporates the fuel combustion chamber, an evaporator and a glow plug/flame sensor. Fuel from the FBH fuel pump is supplied to the evaporator, where it evaporates and enters the combustion chamber to mix with air from the combustion air fan. The glow plug/flame sensor provides the ignition source of the fuel:air mixture and, once combustion is established, monitors the flame.

ECU/heat exchanger: The ECU controls and monitors operation of the FBH system. Ventilation of the ECU is provided by an internal flow of air from the combustion air fan. The heat exchanger transfers heat generated by combustion to the coolant. A sensor in the heat exchanger provides the ECU with an input of heat exchanger casing temperature, which the ECU relates to coolant temperature and uses to control system operation. The temperature settings in the ECU are calibrated to compensate for the difference between coolant temperature and the heat exchanger casing temperature detected by the sensor. Typically: as the coolant temperature increases, the coolant will be approximately 7° C (12.6° F) hotter than the temperature detected by the sensor; as the coolant temperature decreases, the coolant will be approximately 2° C (3.6° F) cooler than the temperature detected by the sensor.