Initial Inspection and Diagnostic Overview

While a detailed diagnostic procedure for all air conditioning systems would be impractical due to the many variations in construction and operation, there are three fundamental components of a total diagnosis:

1. Refrigerant systems must have an adequate, but not excessive charge.
2. Determination must be made whether the refrigerant system is governed by a cycling clutch compressor or by valves which control evaporator pressure.
3. The air distribution system (blower motor, switches, vacuum lines and air ducts must be operational before checking the refrigerant system.

Check the blower; if inoperative, examine switches, fuses, connections, wiring and the blower motor. If blower is operating but the air output is low, check for loose wire connections or shorts, undercharged battery, dirty or loose switch contacts, or a faulty blower motor. Inspect the air distribution system for obstructions and ensure proper door operation.

If the blower is circulating the air but there is no cooling, check the compressor drive belt; ensure it is not broken or slipping. If the pulley is turning but the compressor shaft is not, check the magnetic clutch. On models equipped with a cycling clutch, the following hand check method will determine whether the problem lies in the refrigerant system or further testing of the distribution system is required.

1. With engine warmed up and at a normal idle, set selector lever to Norm, temperature lever to Cold and blower on Hi.
2. Place one hand on the evaporator inlet pipe and the other on the accumulator surface with the compressor engaged.
3. If both surfaces are the same temperature and colder than ambient temperature, refrigerant system is normal.
4. If the inlet pipe is cooler than the accumulator surface, refrigerant system is low on charge. Add small amounts of refrigerant until both feel the same temperature. Then add 14 ounces (one can) of additional refrigerant.
5. If inlet pipe is frosted over and accumulator surface is warmer, proceed as in step 4.

The individual components of the refrigerant system will often give clear signs of their malfunctioning. Use the following general descriptions to pinpoint faulty components.

COMPRESSOR
A faulty compressor will display one or more of the following symptoms: noise, seizure, leakage or low inlet and discharge pressure. A steady, resonant noise from the compressor is not necessarily an indication of a problem, but irregular metallic rattling may indicate broken parts and should be investigated. A thumping noise from the compressor and a cool, sweating suction line into it may indicate an overcharged system. Check seizure by disengaging the magnetic clutch and rotating the driven plate. If the compressor is seized, the driven plate will not rotate.
False compressor seizure may occur after an extended period of disuse or storage. Lubricating oil drains away from the polished surfaces of ball seats and axial plate and the compressor appears to be seized. Use a clutch hub holding tool to turn the compressor in the opposite direction of rotation at least three revolutions. Check for false compressor seizure if compressor has not been used in a month or longer.
If compressor is not seized but will not rotate, check for current at magnetic coil. Low discharge pressure may be caused by faulty seals within the compressor, a restriction in the compressor or elsewhere, or by a low refrigerant charge. The compressor must have the correct amount of the proper viscosity oil. Excess oil will restrict refrigerant circulation and reduce compressor outlet pressure.

CONDENSER
The condenser may malfunction either due to leakage or restriction. If restricted, compressor discharge pressure will be excessive. Icy or frosty spots on the condenser will indicate a partial restriction within the condenser. Ensure all foreign matter is removed from the front of the condenser. Similarly, bent cooling fins will block air flow through the condenser and result in high discharge pressure.
EVAPORATOR
A faulty evaporator will provide insufficient cooling to the vehicle. The core may be restricted with dirt, the case may be cracked, or a seal may be leaking sufficiently to prevent cooling. If evaporator restriction is due to icing, the expansion valve, capillary tube or suction throttling valve, if equipped, may be at fault and should be investigated.
Since there is a constant condensation of atmospheric moisture on the outside of the evaporator coils, ensure that the draining system is unobstructed and clean. Some vehicles have an auxiliary evaporator in the trunk or between the headliner and the roof.

RECEIVER-DEHYDRATOR
A restriction inside the receiver-dehydrator will result in high head pressures if the restriction is on the inlet side of the unit. A restriction at the outlet side will cause low head pressures and little or no cooling. An exceedingly cold receiver-dehydrator may be restricted. If the system has been in service for a considerable amount of time, the desicant element may have lost its moisture absorbing ability. This condition is indicated by the constant presence of small bubbles in the sight glass if equipped and a wide difference in temperature between the inlet and outlet receiver-dehydrator lines.

THERMOSTATIC EXPANSION VALVE
Faulty expansion valves will be indicated by low suction and discharge pressures on the manifold gauge set. In most cases the power element fails and the valve closes. Occasionally the inlet screen becomes clogged with contamination or desicant beads loose in the system.

REFRIGERANT LINE RESTRICTIONS
1. A restricted suction line is indicated by low suction pressure at the compressor, low discharge pressure and little or no cooling.
2. A restricted discharge line will usually cause the pressure relief valve to open.
3. A restricted liquid line will cause low suction and discharge pressures and little or no cooling.