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Home >> Chevrolet >> 1977 >> Vega Base, 2D Hatchback, Automatic Trans >> Repair and Diagnosis >> Heating, Ventilation & A/C (HVAC) >> HVAC Control Systems >> Automatic Temperature Control >> Testing & Diagnosis >> Temp. Control Problem Diagnosis >> System Operates Only at Max. Air Conditioning

System Operates Only at Max. Air Conditioning

This can be a vacuum or electrical problem. The problem can be separated into problems of external signal errors (to the programmer) or internal programmer malfunctions.

With the system operating, disconnect the in-car sensor electrical connector and observe programmer movement through slot in bottom of programmer cover. If programmer remains in maximum air conditioning position, remove multiple vacuum connector and check for supply vacuum at black hose. If no (or low) vacuum, check for leaks or disconnects in hose assembly. If vacuum is okay, programmer is malfunctioning. Remove cover and inspect for obvious disconnects. If necessary, use ATC Tester J-23678 to analyze and correct problem.

If programmer moved to maximum heater position when electrical connector was removed, problem is external to programmer:

Shorted in-car sensor. Shorted or miscalibrated temperature dial. Short in sensor circuit of-wiring harness. Disconnect or open in ground circuit (black wire) between control head and programmer. No electrical power to programmer (yellow wire). No ground at amplifier. Malfunction within programmer assembly.

NOTE: Use AK Tester J-23678 to analyze and correct the above problems.
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