Section III. ATC Temperature Control Diagnosis

1. Insufficient or No Heat Observe service vacuum with temperature lever atIf 3 in.hg. or lower, vacuum level is O.K. Next, check for hot heater hoses to and from core and that the servo positions the blend door in full heat position. If over 3 in.hg., disconnect line from servo and note discharge temperature. If heat is available, check sensor cable adjustment, refer to Shop Manual, Section 36-70. If O.K., change sensor. If no heat is available, correct problem in cooling system (coolant level, thermostat, refer to Group 27 in Shop Manual).

2. Insufficient or No Cooling Observe servo vacuum with temperature lever at 65. If over 12 in.hg. and no cooling, check refrigeration system pressures and outside/recirc door position to be certain it's in Recirc position. If under 12 in.hg., check sensor cable adjustment, refer to Shop Manual, Volume A, Section 36-70. If O.K., ensure vacuum to sensor (supply). If O.K., change sensor.

3. Discharge Temperature Rises on Acceleration

Observe servo vacuum under load. If it drops a faulty check valve or a vacuum leak exists. A severe leak will result in air distribution shift to Defrost.

4. Long Periods of Heat then Long Period of Cool or Vice Versa

Aspirator hose blocked or disconnected resulting in sensor not sampling interior air.

5. Discharge Temperature Too Hot in Cold Ambients and Too Cool in Warm Ambients

Sensor exhaust port blocked causing sensor to respond to ambient temperature only.

6. Sensor Buzzes

Sensor output line is blocked or pinched.

7. Heating and Cooling OK, but Settles Out Too Cool or Too Warm

a. Sensor cable misadjusted (refer to Shop Manual, Section 36-70)
b. Sensor inoperative
c. Sensor to Instrument panel seals (not sealing) or missing (most common on vehicles with ATC as optional equipment)