Section IV. ATC Component Diagnosis

1. ATC Sensor and Harness Assembly

The vacuum output of the sensor is the modulated vacuum signal to the servo. The two typical failures

associated with sensors are improper calibration and leakage. Prior to changing a suspect sensor, inspect for the following:

a. Cable adjustment (refer to Shop Manual, Section 36-70)
b. Vacuum supply to sensor (15 in. or more), the check valve holds the vacuum
c. Aspirator hose not blocked and connected at both ends
d. Dash seals intact

VIEW 2 - ATC PERFORMANCE CHECK AT 75~ +/- 5~F INTERIOR TEMPERATURE:

Observe the sensor vacuum output as described in Section II, View 2. This will check the calibration setting. Refer to "All Function Selector Valve Positions Leak" diagnostics chart in the Shop Manual, Section 36-70 for leak checking.

VIEW 1 - BLOWER TROUBLESHOOTING:

2. ATC Servo, Valve and Switch Assembly

The three typical failures associated with the servo valve, and switch assembly are: vacuum leakage at diverter valve, electrical (blower) failures, and loose attachments. Refer to "All Function Selector Valve Positions Leak" diagnostics chart in the Shop Manual, Section 36-70 for vacuum leak diagnosis, View 1, "Blower Troubleshooting", and observe blend door movement simultaneous to vacuum input change to the servo assembly.
3. Thermal Blower Lockout (TBL) Switch

The three typical failures associated with the TBL switch are: electrical, vacuum leak, or coolant leaks. Coolant leaks (internal or at hose connections) can be observed visually. Refer to "All Function Selector Valve Positions Leak" diagnostic chart in Shop Manual, Section 36-70 for vacuum leaks. Electrical functions can be checked as follows:

a. Blower Inoperative in Floor Position Engine temperature must be over 120~F. Unplug electrical connector from TBL switch. Short across the two terminals of the harness connector. If blower comes on, replace TBL. If blower does not come on, there is an open circuit between the selector switch and the TBL (see View 1, "Blower Troubleshooting", Item #2).
b. Blower On At All Times in Floor Position

If the blower is on with engine coolant under 110~F, unplug electrical connector from TBL switch. If blower shuts off, replace TBL. If blower continues to run, there is a short in the circuit between the selector switch and the TBL (see View 1, "Blower Troubleshooting", Item #4.)

OTHER APPLICABLE ARTICLES: None
WARRANTY STATUS: "INFORMATION ONLY"