DTC 40

CIRCUIT DESCRIPTION

The ECC Head sends through CKT 720 a program number calculated on various inputs such as in-car temperature, outside temperature, set temperature, etc. The programmer calculates from this number the air mix valve position number to achieve the set temperature. Code 40 will set if the air mix valve does not move or does not achieve its commanded value.

TEST DESCRIPTION

Numbers below refer to circles numbers on the diagnostic chart.

1. ECC parameter -23 displays actual air mix valve position. The normal range is between 20 and 170. ECC parameter -11 is for the program number.

2. This checks to see if the fault could be due to the 5 volt power and ground circuit or the feedback circuit.

3. This checks to see if the fault is due to the programmer or the feedback circuit.

4. This checks to see if the fault is due to the circuit or the ECC Head.

5. This checks to see if the open circuit reading is due to CKT 155A or CKT 705.

NOTES ON INTERMITTENTS

If an intermittent Code 40 is being set, manipulate the related wiring while observing actual air mix valve position, parameter -23, movement. If the failure is induced, the valve position will either stop or jump to an extreme value (0 or 255). This will help in isolating the location of the malfunction.

Exit diagnostics. Select 90 and 60 temperature settings using normal ECC controls, waiting a minimum of two minutes in each mode to see if code sets.