Heating Ventilation Air Conditioning (HVAC)

CIRCUIT DESCRIPTION
The electric actuator is driven by the H-A/C control to operate the temperature valve. The position of the temperature valve is adjusted to maintain the set temperature. A feedback potentiometer is used in the electric actuator so that the H-A/C control will know the position of the temperature valve. The feedback potentiometer is part of a flat-pack design motor which is mounted on the rear-facing surface of the heater and air conditioning evaporator module. A low voltage at CKT 1218 (about 1.0 volt) represents a hot air valve position and a high voltage at CKT 1218 (about 4.0 volts) a cold air valve position. The H-A/C control drives the electric actuator motor through CKT 1199 by applying 0-2.5-5 volts through the circuit. Applying 0.0V on CKT 1199 to the logic portion in the electric actuator causes the temperature valve to move to the full heat position. Applying 5.0V on CKT 1199 will cause the temperature value to move to the full cold position. Applying 2.5 volts causes the electric actuator to stop moving.

DTC WILL SET WHEN
With ignition in "RUN," the H A/C control will detect this fault anytime that CKTs 1217and 1218 see voltage near or at 0.0V.

ACTION TAKEN:
DTC 11 will set in the H-A/C control. The ACAC system will use a default value of 128 until the fault is cleared.

DTC WILL CLEAR WHEN
This fault will clear as soon as the fault has been repaired. The ACAC system will then return to normal operation.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This test checks for a short to ground in CKTs 1217 and 1218.
2. This test checks for feedback voltage at terminal "C8" of the H-A/C control.

DIAGNOSTIC AIDS
Check for a binding temperature valve while replacing the actuator. Check or loose connection or bent pins at the electric actuator connector. Operation of the electric actuator may be observed by opening the instrument panel compartment and then changing the H-A/C control temperature setting from full hot to full cold.

If the H-A/C control voltage supply was disconnected, a new actuator was installed or the temperature valve was disturbed. The H-A/C control will have to be recalibrated. This will prevent a DTC from being set because of a detected difference between the former and present temperature valve positions. To recalibrate the H-A/C control, remove I/P Fuse 33, install Fuse 33, and turn ignition switch to the "RUN" position.