F40 Part 2 of 2

Code F40 (Part 2 of 2):

Air Mix Door Circuit:

DESCRIPTION:

If no malfunction is uncovered using this procedure, it will indicate that the system is OK at this time. For an intermittent code refer to NOTE ON INTERMITTENTS.

TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.

Code F40 is set by the BCM if it is unable to move the air mix door. The BCM requests the door to move over the data circuit (CKT 720) to the programmer. The programmer then supplies voltage to a D.C. motor, which drives the air mix door. The BCM monitors a feedback pot on the D.C. motor, which varies between 0 and 5 volts depending on its position. If the door is requested to move but the feedback voltage does not change, the BCM checks to see if the door has reached its mechanical limit of travel. If the door is not at its mechanical limit, the BCM stores code F40 to indicate that the door is not responding to its command.

3. Checks CKTs 705, 713, and 751 for a fault between the BCM and the programmer.

4. Verifies that the programmer is receiving data from the BCM. If the programmer is receiving data from the BCM commanding it to reposition the air mix door and does not do so, the programmer is faulty. If it is not receiving data, CKT 720 is open or shorted to ground or the BCM is faulty.

NOTE ON INTERMITTENTS:

If an intermittent code F40 is being set, manipulate the related wiring while observing "actual" air mix door position (parameter P.2.3) movement. If the failure is induced, the door position will either stop or jump to an extreme value (O% or 100%). This will help in isolating the location of the malfunction.