The HVAC control assembly uses vacuum to control the mode doors. Vehicles equipped with diesel engines use a vacuum pump to create vacuum. The vacuum pump has ignition voltage circuit and a ground circuit to operate. The pump turns on when vacuum is low and turns off when vacuum reaches 21 Hg.

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Manual HVAC Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, mode set to the VENT position, place the blower switch in the 1, 2, 3 and 4 speed positions. The blower motor should activate and operate from 1 through 4 when changing between the commanded states.

⇒	If the blower motor speed does not change or fails to operate at a selected speed, refer to Blower Motor Malfunction.

2. Rotate the mode control switch from the OFF position to the VENT, BI-LEVEL, FLOOR, FLOOR/DEFROST and DEFROST positions. The air flow should follow the commanded states and blow through the vent, floor and defrost outlets.

⇒	If the air flow fails to blow through the selected outlets, refer to Vacuum Control System Diagnostic.

1. Ignition OFF, disconnect the harness connector at the vacuum pump.
2. Test for less than 1.0 ohm between the ground circuit terminal A and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify that a test lamp illuminates between the ignition circuit terminal B and ground.

⇒	If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance.

4. If all circuits test normal, test or replace the vacuum pump assembly.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Vacuum Pump Assembly Replacement