412-02B Auxiliary Heating: Description And Operation (page 7 of 8)

 
Shutdown: To stop the FFBH, the control module de-energizes the auxiliary fuel pump to stop combustion, but continues 
operation of the combustion air fan and the circulation pump for a time, to cool down the FFBH. The cool down time is 
100 seconds if the FFBH was operating at part load combustion and 175 seconds if the FFBH was operating at full load 
combustion. 
 

Supplementary Heating 

Supplementary heating is requested by the ATCM while the engine is running if the ambient temperature is less than 5 °C 
(41 °F) and the engine coolant temperature is less than 75 °C (167 °F). 
 
The ATCM cancels supplementary heating if the ambient temperature increases to 5 °C (41 °F), the engine coolant 
temperature increases to 75 °C (167 °F), or the engine stops. 
 

Diagnostics 

The control module monitors the FFBH system for faults. Any faults detected are stored in a volatile memory in the control 
module, which can be interrogated by T4. A maximum of three faults and associated freeze frame data can be stored at 
any one time. If a further fault is detected, the oldest fault is overwritten by the new fault. 
 
The control module also incorporates an error lockout mode of operation that inhibits operation to prevent serious faults 
from causing further damage to the system. In the error lockout mode, the control module immediately stops the auxiliary 
fuel pump, and stops the combustion air fan and circulation pump after a cool down time of approximately 2 minutes. 
Error lockout occurs for start sequence failures, combustion flameouts, heat exchanger casing overheat and if battery 
voltage is out of limits. The error lockout mode can be cleared using T4, or by disconnecting the battery power supply 
(connector C0926) for a minimum of 10 seconds. 
 
Start Failure and Flameout: If a start sequence fails to establish combustion, or a flameout occurs after combustion is 
established, the control module immediately initiates another start sequence. The start failure or flameout is also recorded 
by an event timer in the control module. The event timer is increased by one after each start failure or flameout, and 
decreased by one if a subsequent start is successful. If the event timer increases to three (over any number of drive 
cycles), the control module enters the error lockout mode. 
 
Heat Exchanger Casing Overheat: To protect the system from excessive temperatures, the control module enters the 
error lockout mode if the heat exchanger coolant temperature exceeds 125 °C (257 °F). 
 
Battery Voltage Limits: 10.25 - 15.5 volts. 
 

AUXILIARY HEATING CONTROL DIAGRAM 

NOTE :  

A = Hardwired connections; D = High speed CAN bus; F = RF transmission; N = Medium speed CAN bus; P = 
MOST bus