Auxiliary Heater

WARNING: This page is about a different car, the 2013 Ford F-550 Super Duty, 2013 Ford F-450 Super Duty, 2013 Ford F-350 Super Duty, and 2013 Ford F-250 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.

An electric auxiliary heater is available as optional equipment on diesel vehicles. The auxiliary heater assembly contains the Auxiliary Heater Control Module (AHCM) and element, and is located in the airflow immediately downstream of the heater core.

Power is supplied to the electric auxiliary heater element through a fused cable connected directly to the positive battery terminal with an in-line connection at the dash panel heater tube opening. Ground is supplied to the electric auxiliary heater element through a ground cable connected directly to the negative battery terminal with an in-line connection at the dash panel heater tube opening. The electric auxiliary heater is activated by a medium speed Controller Area Network (CAN) message from the HVAC module. The Medium Speed Controller Area Network (MS-CAN) message is converted to a High Speed Controller Area Network (HS-CAN) message by the instrument cluster module and is then detected by the electric AHCM.

The auxiliary heater will be activated if all of the following conditions are met:

The ignition switch is in the RUN/START position.
The engine is operating at greater than 500 RPM.
Any ON mode is selected on the HVAC module.
The temperature setpoint is greater than 80% of the highest setting on vehicles equipped with EMTC. Dual-zone EATC vehicles use a software algorithm based on interior temperature and setpoint.
The engine coolant temperature is less than approximately 75°C (167°F).
No alternator faults have been recorded and the alternator is operating at less than 95% capacity.

The auxiliary heater will be deactivated if any of the following conditions exist:

The ignition switch is in the OFF or ACC position.
The engine coolant temperature is greater than approximately 85°C (185°F).
An alternator fault has been detected or the alternator is operating at or above 95% capacity due to external electrical loads.
The engine is operating at less than 500 RPM.
The auxiliary heat request is NO from the HVAC module. If the vehicle is operating with a high alternator load, the AHCM will stabilize auxiliary heater operation at 85%. The AHCM will deactivate fully if the alternator load reaches 95% capacity.