Air Conditioning Troubleshooting and Repair
Explains automotive A/C theory of operation for expansion-valve and orifice-tube systems, provides a comprehensive symptom/cause/repair troubleshooting table, and lists normal A/C system gauge pressure readings by ambient temperature.
Introduction
Working on the 944 A/C system is no different from working on any other automotive A/C system — same components, same principles. When a problem arises, you have two approaches: replace components and hope you guess right the first time, or use A/C gauges to properly narrow down the cause. The second approach is strongly recommended.
A/C System Theory of Operation
There are two basic types of automotive A/C systems, distinguished by the device that controls refrigerant expansion:
- Expansion valve system — used on 944s
- Orifice tube system — typical of GM vehicles; described here for completeness
Expansion Valve System (944)
Compressor: takes low-temperature refrigerant gas and compresses it to a high-pressure, high-temperature gas.
Condenser: sits in front of the radiator; removes heat from the refrigerant, causing it to change phase from hot gas to warm liquid.
Receiver-drier: a small canister (usually with a sight glass on top) located near the driver's side headlight on left-hand-drive 944s. Removes moisture from the refrigerant via a desiccant. No phase change occurs here.
Expansion valve: located in the passenger's footwell (LHD cars) near the centre console, next to the firewall. Senses pressure at the compressor suction and modulates to maintain that pressure. The expansion of warm liquid through the valve causes a pressure drop and a large temperature drop, producing a low-temperature liquid refrigerant.
Evaporator: air from the passenger compartment passes across the outside of the coils; low-temperature liquid refrigerant passes through the inside. The refrigerant removes heat from the cabin air and returns to the compressor as a low-temperature gas.
Orifice Tube System (for reference)
The orifice tube is a fixed expansion device. System pressure at the compressor suction is maintained by cycling the compressor clutch on and off rather than by modulating the expansion device. An accumulator between the evaporator and compressor suction collects any un-evaporated refrigerant and contains desiccant — analogous to the expansion-valve system's receiver-drier.
The diagrams below show both system types.
Tools
- A/C gauges and hose set
A/C System Troubleshooting
The following is a general troubleshooting guide. Many items may not apply to the 944 specifically.
| Symptom / Possible Cause | Repair |
|---|---|
| Low Compressor Discharge Pressure | |
| 1. Leak in system | 1. Repair leak |
| 2. Defective expansion valve | 2. Replace valve |
| 3. Suction valve closed | 3. Open valve |
| 4. Refrigerant shortage | 4. Add refrigerant |
| 5. Plugged receiver-drier | 5. Replace drier |
| 6. Compressor suction valve leaking | 6. Replace valve |
| 7. Bad reed valves in compressor | 7. Replace reed valves |
| High Compressor Discharge Pressure | |
| 1. Air in system | 1. Evacuate and recharge system |
| 2. Clogged condenser | 2. Clean condenser |
| 3. Discharge valve closed | 3. Open valve |
| 4. Overcharged system | 4. Remove some refrigerant |
| 5. Insufficient condenser airflow | 5. Check fan operation |
| 6. Loose fan belt | 6. Tighten fan belt |
| 7. Condenser not centred on fan or too far from radiator | 7. Centre and check distance |
| Low Suction Pressure | |
| 1. Refrigerant shortage | 1. Add refrigerant |
| 2. Worn compressor piston | 2. Replace compressor |
| 3. Compressor head gasket leaking | 3. Replace head gasket |
| 4. Kinked or flattened hose | 4. Replace hose |
| 5. Compressor suction valve leaking | 5. Replace valve plate |
| 6. Moisture in system | 6. Replace drier |
| 7. Debris in expansion valve or screen | 7. Replace drier |
| High Suction Pressure | |
| 1. Loose expansion valve | 1. Tighten valve |
| 2. Overcharged system | 2. Remove some refrigerant |
| 3. Expansion valve stuck open | 3. Replace expansion valve |
| 4. Compressor reed valves faulty | 4. Replace reed valves |
| 5. Leaking head gasket on compressor | 5. Replace head gasket |
| Compressor Not Working | |
| 1. Broken belt | 1. Replace belt |
| 2. Broken clutch wire or no 12V power | 2. Repair wire or check for power |
| 3. Broken compressor piston | 3. Replace compressor |
| 4. Bad thermostat | 4. Replace thermostat |
| 5. Bad clutch coil | 5. Replace clutch coil |
| 6. Low refrigerant — low-pressure switch has cut off clutch power | 6. Add refrigerant |
| Evaporator Not Cooling | |
| 1. Frozen coil, switch set too high | 1. Turn thermostat switch back |
| 2. Drive belt slipping | 2. Tighten belt |
| 3. Hot air leaks into car | 3. Check for holes or open vents |
| 4. Plugged receiver-drier | 4. Replace drier |
| 5. Capillary tube broken | 5. Replace expansion valve |
| 6. Shortage of refrigerant | 6. Add refrigerant |
| 7–9. High head or abnormal suction pressure | 7–9. See corresponding pressure fault above |
| 10. Defective expansion valve | 10. Replace expansion valve |
| 11. Frozen expansion valve | 11. Evacuate and replace drier |
| Frozen Evaporator Coil | |
| 1. Faulty thermostat | 1. Replace thermostat |
| 2. Thermostat not set correctly | 2. Adjust thermostat setting |
| 3. Insufficient evaporator airflow | 3. Check for excessive duct length, kink, or bend |
A/C System Gauge Readings
The following are general pressure and vent temperature guidelines based on ambient temperature. Actual readings will vary with humidity and system condition.
| Ambient Temperature | Low Side Pressure | High Side Pressure | Centre Vent Temperature |
|---|---|---|---|
| 60 °F (16 °C) | 28–38 psi | 130–190 psi | 44–46 °F |
| 70 °F (21 °C) | 30–40 psi | 190–220 psi | 44–48 °F |
| 80 °F (27 °C) | 30–40 psi | 190–220 psi | 43–48 °F |
| 90 °F (32 °C) | 35–40 psi | 190–225 psi | 44–50 °F |
| 100 °F (38 °C) | 40–50 psi | 200–250 psi | 52–60 °F |
| 110 °F (43 °C) | 50–60 psi | 250–300 psi | 68–74 °F |
| 120 °F (49 °C) | 55–65 psi | 320–350 psi | 70–75 °F |