F40 Part 1 of 2

Code F40 (Part 1 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.

1 . If code F40 is stored and the compressor clutch does not operate, then the BCM's output requests are not being received by the programmer over CKT 720 or the programmer is unable to process the data. If the clutch does operate then data is being received since both commands are sent by the BCM over CKT 720.

2. Operation of the air mix door can be evaluated in BCM data display by observing "actual" air mix door position (parameter P.2.3) while changing the ECC program number. As the program number is changed from 0 (max A/C) to 100 (max heat), the door position should vary between its mechanical limits of travel (below 20% and above 97%). Readings of 0% or 100% indicate a malfunction in the feedback circuit (CKT 733) to the BCM while fixed or restricted readings within the range of travel indicate improper door movement.

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.