Air Conditioning Troubleshooting and Repair

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

This is a general troubleshooting guide. Connect a set of A/C gauges and read both the high (discharge) and low (suction) sides, then match what you see to the sections below to narrow down the cause. Many items are common to all automotive A/C systems and may not all apply to the 944 specifically.

Low compressor discharge pressure

A low high-side reading usually points to a charge or sealing problem on the discharge side:

Leak in the system — find and repair the leak.
Defective expansion valve — replace the valve.
Suction valve closed — open the valve.
Refrigerant shortage — add refrigerant.
Plugged receiver-drier — replace the drier.
Compressor suction valve leaking — replace the valve.
Bad reed valves in the compressor — replace the reed valves.

High compressor discharge pressure

A high high-side reading means heat is not being rejected, or the system is over-full:

Air in the system — evacuate and recharge the system.
Clogged condenser — clean the condenser.
Discharge valve closed — open the valve.
Overcharged system — remove some refrigerant.
Insufficient condenser airflow — install a larger (higher-flow) fan.
Loose fan belt — tighten the fan belt.
Condenser not centred on the fan, or too far from the radiator — centre it and check the distance.

Low suction pressure

A low low-side reading means too little refrigerant is reaching the evaporator:

Refrigerant shortage — add refrigerant.
Worn compressor piston — replace the compressor.
Compressor head gasket leaking — replace the head gasket.
Kinked or flattened hose — replace the hose.
Compressor suction valve leaking — replace the valve plate.
Moisture in the system — replace the drier.
Debris in the expansion valve or screen — replace the drier.

High suction pressure

A high low-side reading means the expansion device is letting too much through, or the system is over-full:

Loose expansion valve — tighten the valve.
Overcharged system — remove some refrigerant.
Expansion valve stuck open — replace the expansion valve.
Compressor reed valves faulty — replace the reed valves.
Leaking head gasket on the compressor — replace the head gasket.

Compressor not working

If the compressor clutch never engages or the compressor does not turn:

Broken belt — replace the belt.
Broken clutch wire or no 12 V power — repair the wire or check for power at the clutch.
Broken compressor piston — replace the compressor.
Bad thermostat — replace the thermostat.
Bad clutch coil — replace the clutch coil.
Low refrigerant (the low-pressure switch has cut off clutch power) — add refrigerant.

Evaporator not cooling

The compressor runs but little or no cold air reaches the cabin:

Frozen coil, thermostat switch set too high — turn the thermostat switch back.
Drive belt slipping — tighten the belt.
Hot air leaking into the car — check for holes or open vents.
Plugged receiver-drier — replace the drier.
Broken capillary tube — replace the expansion valve.
Refrigerant shortage — add refrigerant.
High head pressure or abnormal suction pressure — diagnose using the corresponding pressure section above.
Defective expansion valve — replace the expansion valve.
Frozen expansion valve — evacuate the system and replace the drier.

Frozen evaporator coil

Ice builds up on the evaporator and blocks airflow:

Faulty thermostat — replace the thermostat.
Thermostat not set correctly — adjust the thermostat setting.
Insufficient evaporator airflow — check for excessive duct length, a kink, or a 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 °C61 °F)	28–38 psi	130–190 psi	44–46 °F
70 °F (21 °C70 °F)	30–40 psi	190–220 psi	44–48 °F
80 °F (27 °C81 °F)	30–40 psi	190–220 psi	43–48 °F
90 °F (32 °C90 °F)	35–40 psi	190–225 psi	44–50 °F
100 °F (38 °C100 °F)	40–50 psi	200–250 psi	52–60 °F
110 °F (43 °C109 °F)	50–60 psi	250–300 psi	68–74 °F
120 °F (49 °C120 °F)	55–65 psi	320–350 psi	70–75 °F