Heat Switch Inoperative 1998
Circuit Description
The HEAT switch completes a circuit to pin C4 of the HTCM when closed. This input is used by the HTCM to determine whether to operate the Refrigerant/Heat Pump (R/HP) compressor as a heat pump or an A/C compressor, or to activate the Auxiliary Fuel-Fired Heater. If the ambient temperature is between 2°C (36.4°F) and 18°C (64.4°F) and this switch is closed, the HTCM will operate the R/HP compressor as a heat pump. If the temperature is below 2°C (36.4°F), the HTCM will inhibit the R/HP compressor operation and activate the Auxiliary Fuel-Fired Heater. Above 18°C (64.4°F) turning this switch on will inhibit A/C operation. Run0 voltage (approximately 13.8 volts) is provided to the HVAC Mode Select switch through the HVAC 20A fuse when the ignition is ON. The HVAC Mode Switch supplies voltage to the HEAT switch at pin 2 in any position except OFF. If the HVAC fuse is open, the blower motor, and the A/C Request to the HTCM will be inoperative as well. When the HEAT switch is closed, voltage is supplied to the LED in the switch which is grounded at pin 4 and to the HTCM through pin 3.
Diagnostic Aids
A potentiometer can be connected in place of the Ambient Air Temperature (AAT) sensor to simulate the temperature ranges. It is recommended, however, that the system be tested in the environment for which the symptom exists.
Do not use this test if the HVAC fuse is open. Refer to HVAC System Check to identify the correct diagnostic to execute.
Inserting a fuse into the Auxiliary Fuel-Fired Heater Service Port will allow operation of the Auxiliary Fuel-Fired Heater at any ambient temperature.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
DEFINITION: This test diagnoses the HVAC HEAT Switch operation. It is assumed that a short to ground in CKT 910 (GRY) wire would be identified by a blower motor symptom due to the HVAC fuse being open. | ||||
1 | Did you perform the Auxiliary Heater System Check? | -- | Go to Step 2 | |
Press and release the HEAT switch and observe the HEAT switch LED. Does the LED turn on and off? | -- | Go to Step 8 | Go to Step 3 | |
3 |
Is the voltage near the specified value? | Battery Voltage | Go to Step 5 | Go to Step 4 |
4 | Locate and repair open CKT 65 (BRN) wire. Is the repair complete? | -- | -- | |
5 | Connect a DMM between CKT 1050 (BLK) wire cavity 4 and ground. Is the resistance less than the specified value? | 5ohms | Go to Step 6 | Go to Step 7 |
6 | Replace HEAT Switch. Did you complete the repair? | -- | Go to Step 2 | -- |
7 | Locate and repair open CKT 1050 (BLK) wire. Did you complete the wire repair? | -- | Go to Step 2 | -- |
8 |
Is the voltage near the specified value? | Battery Voltage | Go to Step 9 | Go to Step 10 |
9 |
Did you complete the repair? | -- | -- | |
10 | Replace HEAT Switch. Did you complete the repair? | -- | -- | |
