The high pressure switch is a dual function four wire switch which protects the A/C system. The high pressure switch turns OFF the A/C compressor and engages the condenser fan motor when line pressure exceeds 2965 kPa (430 psi).

The evaporator temperature sensor protects the A/C system. The sensor located by the evaporator core provides the HVAC control module with the surface temperature of the evaporator core. If the evaporator temperature sensor reads a temperature of 0°C (32°F), the HVAC control module will turn OFF the A/C compressor clutch until the evaporator temperatures reach 2°C (36°F).

The A/C high pressure switch is located between the A/C compressor clutch relay and the A/C compressor clutch. When A/C is requested, the relay closes and applies voltage to the A/C high pressure switch. If the A/C high pressure switch is closed, the voltage will then be applied to the A/C compressor and the compressor will engage. If A/C high side pressure is not at an acceptable level, the A/C high pressure switch will open, preventing the A/C compressor from being engaged.

The following conditions must be met in order for the A/C compressor clutch to engage:

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

Engine running, observe the A/C compressor operation while pressing the A/C switch ON and OFF, the A/C compressor clutch should engage and disengage when changing between the commanded states.

1. Ignition OFF, disconnect the A/C compressor clutch relay.
2. Verify that a test lamp illuminates between the relay coil ignition circuit terminal and ground.

⇒	If the test lamp does not illuminate, test the signal circuit for a short to ground or an open/high resistance. If the signal circuit tests normal, refer to Blower Motor Malfunction.

3. Verify that a test lamp illuminates between the relay switch B+ circuit terminal and ground.

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

4. Ignition OFF, disconnect the harness connector at the A/C high pressure switch.
5. Ignition ON, connect a 20A fused jumper wire between the relay switch B+ circuit terminal and the relay controlled output circuit terminal. Verify that a test lamp illuminates between the control circuit terminal 3 and ground.

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

6. Ignition OFF, connect the harness connector at the A/C high pressure switch. Disconnect the harness connector at the A/C compressor clutch.
7. 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.

8. Ignition ON, connect a 20A fused jumper wire between the relay switch B+ circuit terminal and the relay controlled output circuit terminal. Verify that a test lamp illuminates between the control circuit terminal B and ground.

⇒	If a test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the A/C high pressure switch.

9. Ignition OFF, connect the harness connector the A/C compressor clutch.
10. Ignition ON, connect a 20A fused jumper wire between the relay switch B+ circuit terminal and the relay controlled output circuit terminal. Verify the A/C compressor clutch engages.

⇒	If the A/C compressor clutch does not engage, test or replace the A/C compressor clutch.

11. Ignition OFF, connect the A/C compressor clutch relay. Disconnect harness connector at the evaporator temperature sensor.
12. Ignition ON, verify that a test lamp illuminates between the control circuit terminal C and ground.

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

13. Test for ignition voltage between the control circuit terminal A and ground.

⇒	If less than the specified value, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the A/C compressor clutch relay.

14. Ignition OFF, connect the harness connector at the evaporator temperature sensor. Disconnect the harness connector X2 at the HVAC control assembly.
15. Ignition ON, test for ignition voltage between the control circuit terminal 3 and ground.

⇒	If the test lamp does not illuminate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the evaporator temperature sensor.

16. If all circuits test normal, replace the HVAC control assembly.

Relay Test

1. Ignition OFF, disconnect the A/C compressor clutch relay.
2. Test for 60-80 ohms between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

3 and 85

•

85 and 87

⇒	If not the specified value, replace the relay.

4. Install a 30-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

A/C Compressor Clutch Test

1. Ignition OFF, disconnect the harness connector at the A/C compressor clutch.
2. Install a 20-amp fused jumper wire between the control circuit terminal B and 12 volts. Install a jumper wire between the ground circuit terminal A and ground. Verify the A/C compressor clutch engages.

⇒	If the A/C compressor clutch does not engage, replace the A/C compressor clutch.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.