GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Coolant ByPass Valve Solenoid Control

1

1

--

--

Coolant ByPass Valve Solenoid Ground

--

1

1

--

1. Auxiliary Air Temperature Malfunction

Circuit/System Description

The purpose of the auxiliary heating system is to supply heat to the rear interior of the vehicle. The heater auxiliary blower motor switch is located on the headliner and controls the auxiliary blower motor speeds for rear heater operation. Placing the heater auxiliary blower motor switch in any position other than OFF, turns on the solenoid allowing vacuum to open the water valve, this action allows coolant to flow through the auxiliary heater core. The location of the hot water shut off valve is under the right rear area of the cab. The only way to regulate the temperature of the air is to regulate the blower speed.

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

    •  Air Temperature Description and Operation
    •  Air Delivery Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Ignition ON, engine running, place the auxiliary blower switch in the LO, MED and HI speed positions. The auxiliary motor should activate and operate from low to medium to high when changing between the commanded states.

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

Circuit/System Testing

  1. Ignition OFF, disconnect the harness connector at the coolant bypass valve solenoid.
  2. Test for less than 1.0 ohm between the ground circuit terminal B and ground.
  3. If greater than the specified range, test the ground circuit for an open/high resistance.
  4. Ignition ON, verify that a test lamp illuminates between the control circuit terminal A and ground.
  5. If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit test normal, test or replace the auxiliary HVAC control assembly.
  6. If all circuits test normal, test or replace the coolant bypass valve solenoid.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Heater Water Shutoff Valve Replacement
    •  Heater Control Blower Switch Replacement