Cabin Heater: Operation: Activation

When the ignition is in Run, the PCM monitors the PCI bus for the Cabin Heater Activation request. The Automatic or Manual Temperature Control initiates this request only when all conditions for Cabin Heater activation are favorable (see below). The request carries the status bit that the FCM requires to activate its Cabin Heater Assist Control Output. This output is a low side driver (coming from FCM pin 15) which supplies a ground signal to the Cabin Heater (pin 5). When the Cabin Heater receives this ground signal input, it interprets this as an activation signal. The FCM low side driver is also capable of diagnostic sensing. The driver will sense an open circuit when the driver is off, and will sense a short to voltage when the driver is on. The FCM will set DTCs for both of these types of faults.

For vehicle's with a Manual Temperature Control system, the DCHA will activate only:

• When The Engine Is Running.
• When The Coolant Temperature Is Below 66°C (151°F).
• When The Fuel Tank Has Greater Than 1/8 Of A Tank Of Fuel.
• When The Power Switch On The A/C - Heater Control Module Is On.
• When The Blend Control On The A/C - Heater Control Module Is Set Above 90% Reheat (Within 2 Detents Of The Full Heat Position).
• Once per ignition cycle, when the ambient temperature is below 9°C (49°F), and the vehicle speed is above 25 km/h (15.5 mph) for two minutes, and the Blend control on the A/C - Heater Control Module is set anywhere from 80% to 90% reheat (3 to 4 detents from the full heat position). Under this circumstance, the DCHA will remain active for five minutes unless additional input is supplied to the DCHA.
• When the Front Control Module (FCM) sees the Cabin Heater Activation request that is bussed from the A/C - Heater Control Module.

For vehicle's with a Automatic Temperature Control system, the DCHA will activate only when the:

• VIN indicates that the vehicle has a diesel engine
• Vehicle's odometer reads more than 5 miles.
• Engine speed is above 500 RPM.
• Coolant temperature is below 66°C (151°F).
• Fuel tank has greater than 1/8 of a tank of fuel.
• Power switch on the Automatic Temperature Control is on.
• Driver Temperature Control on the Automatic Temperature Control is set above 22°C (72°F).
• Front Control Module (FCM) sees the Cabin Heater Activation request that is bussed from the Automatic Temperature Control.

When the DCHA starting sequence begins, the glow plug and the combustion air fan are activated. After 30 seconds, the fuel dosing pump begins operating and the combustion air fan operation is suspended for 3 seconds. Subsequently, the combustion air fan speed is increased in two ramps within 56 seconds to nearly full load operation. After a stabilization phase of 15 seconds, the combustion air fan speed is again increased in a ramp within 50 seconds to nearly full load. After reaching full load fuel delivery, the glow plug is deactivated and the combustion air fan operation is increased to full load. During the subsequent 45 seconds, as well as in normal operation, the glow plug functions as a flame sensor to monitor the flame condition. After all these events, the automatically controlled heating operation starts.

In case of a no flame or a flame out condition, a restart is automatically initiated. If the no flame condition persists, fuel delivery is stopped and the heater enters an error lockout mode with a rundown of the combustion air fan. This will set one or more DTCs in the DCHA Control's memory. If six continuous attempts to start the heater fail due to one or more faults in the DCHA system, the heater enters a heater lockout mode. This will set DTC B1813 along with any other fault(s) that the DCHA Control identified.