Hard Trouble Code F40: Notes

Code 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 0 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.

1. If code 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 operate, then data is being received since both commands are sent by BCM over circuit No. 720.
2. Operation of air mix door can be evaluated in BCM data display by observing parameter "P.2.3" while changing ECC program number. As program number is changed from 0 (maximum A/C) to 100 (maximum heat), door position should vary between its mechanical limits of travel (below 20% and above 97%). Readings of 0% or 100% indicate malfunction in feedback circuit (circuit No. 733) back to BCM, while fixed or restricted readings within range of travel indicate improper door movement.