BCM Code F40: Air-Mix Door Problem: Notes

WARNING: This page is about a different car, the 1990 Cadillac Fleetwood and 1990 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: If no malfunction is uncovered using this procedure, it will indicate that DFI system is okay at this time.

Code F40 is set by BCM if it is unable to move air-mix door. The BCM requests door to move over data circuit (circuit No. 720) to programmer. The programmer then supplies voltage to DC motor which drives air-mix door. The BCM monitors feedback pot on DC motor which varies between zero and 5 volts, depending on its position.

If door is requested to move, but feedback voltage does not change, BCM checks to see if door has reached its mechanical limit of travel. If door is not at its mechanical limit of travel, BCM stores code to indicate that door is not responding to its commands.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) If Code F40 is stored and compressor clutch does not operate, then BCM's output requests are not being received by programmer over circuit No. 720, or programmer is unable to process the data. If clutch does not operate, then data is being received since both commands are sent by BCM over circuit No. 720.

Test Number 2) Operation of air-mix door can be evaluated in BCM data display by observing parameter P.2.3 while changing ECCP program number. As program number is changed from zero (maximum A/C) to 100 (maximum heat), door position should vary between its mechanical limits of travel (less than 20 percent and greater than 97 percent).

Readings of 0 percent or 100 percent indicate malfunction in feedback circuit (circuit No. 733) back to BCM, while fixed or restricted readings within range of travel indicate improper door movement.

Test Number 3) Checks circuits No. 707, 713 and 751 for a fault between BCM and programmer.

Test Number 4) Verifies programmer is receiving data from BCM. If programmer is receiving data from BCM commanding it to reposition air-mix door and does not do so, programmer is faulty. If it is not receiving data, circuit No. 720 is open or shorted, or BCM is faulty.