B0408 - HVAC Systems - Automatic

DTC B0408

CIRCUIT DESCRIPTION
The following DTCs are for the HVAC door actuators:
- B0263 is for the mode actuator.
- B0268 is for the recirculation actuator.
- B0408 is for the left air temperature actuator.
- B0418 is for the right air temperature actuator.

The HVAC control module controls the HVAC door actuators to regulate the airflow through the HVAC system. Each actuator consists of a stepper motor, a logic circuit, and a potentiometer. The potentiometer inside the door actuator allows the module to monitor the current position of the actuator drive shaft. The logic circuit inside the actuators receives control signals from the HVAC control module and controls the internal stepper motor. When a door positional change is required, the HVAC control module calculates a commanded door position. The module compares the commanded door position to the actual door position and determines the needed direction of motor rotation. The module applies a signal voltage to the door control circuit that is an input to the internal logic circuit of the door actuator. A 5-volt signal increases the door position. A 0-volt signal decreases the door position. When the commanded door position is equal to the actual door position, the HVAC control module sends a 2.5-volt signal to the door actuator and motor rotation stops. The ignition 3-voltage circuit provides source voltage to the logic circuit. The module provides ground to the actuator logic circuit through the low reference circuit.

DTC DESCRIPTORS
This diagnostic procedure supports the following DTCs:
- DTC B0263 Air Flow Control 6 Circuit
- DTC B0268 Air Flow Control 7 Circuit
- DTC B0408 Temperature Control 1 Circuit
- DTC B0418 Temperature Control 2 Circuit

CONDITIONS FOR RUNNING THE DTC
- Source voltage is 8.7-16.5 volts.
- The ignition is ON.
- The HVAC control module commands the actuator to move.

CONDITIONS FOR SETTING THE DTC
The actual door position is not near the commanded door position.

ACTION TAKEN WHEN THE DTC SETS
The HVAC control module does not command the actuator to move for the remainder of the ignition cycle.

CONDITIONS FOR CLEARING THE DTC
- The DTC becomes history during the next ignition cycle in which the HVAC control module no longer detects a stall condition.
- The history DTC will clear after 100 fault-free ignition cycles.
- The DTC can be cleared with a scan tool.

DIAGNOSTIC AIDS
- If the condition is not present refer to Testing for Intermittent Conditions and Poor Connections in Diagnostic Aids.
- Inspect the appropriate door and door actuator for the following conditions:
- A misaligned door actuator
- Binding linkages
- A binding door
- An obstruction that prevents the door actuator from operating within the full range of motion

TEST DESCRIPTION

Steps 1-9:

Steps 10-12:

The numbers below refer to the step numbers on the diagnostic table.
2. This step determines if the HVAC door actuator is stalled.
3. This step applies control circuit voltage to the input of the HVAC door actuator.