A/C Diagnostic Aids

Here are a couple of tips to assist you when diagnosing Legacy A/C systems. Although confirmed and directed at Legacy vehicles using Zexel's optional A/C systems with a variable displacement compressor, the diagnostics should apply to Calsonic systems as well. Keep in mind, there are no absolute pressure readings when using a manifold gauge set due to temperature, humidity and load variations. Instead, key on what could be considered an acceptable range, as well as the relationship between the high and low gauge readings.

The gauge reading examples given in the Service Manual are based on 91~F.

Compressor Failure (fixed at minimum displacement) - Look for a much smaller than usual difference between the low side and high side pressures. In other words, the low side may appear slightly higher than normal and the high side slightly lower, as the two system sides move towards equalization.

The 1991 Service Manual illustration for this condition (4-7, Page 41, Fig. 82) shows the low side at about 15 and the high at about 85, when in reality a reading of about 50 and 75, low and high respectively, is more accurate.

Expansion Valve Failure - Excessive Restriction - Your manifold gauge readings will show the low side almost normal or slightly below normal and your high side will definitely be lower than normal.

A look at the Service Manual illustration for this condition (4-7, Page 39, Fig. 75) shows readings of about 20 and 175, while in reality they would probably be closer to 28 and 90, low/high respectively. In addition, if the expansion valve is clogged, stuck closed or inoperative, the expansion valve inlet area will exhibit heavy sweating or frosting, as well as only slight or decreasing cooling performance as the condition worsens.

Be aware that a reading of 25-35 low side is not necessarily abnormal and that the high side readings must always be considered as well as the low/high relationship.