Failure-proof A/C diagnostics

New technologies make A/C diagnostics quicker and more accurate than ever.


Air conditioning is not the most difficult field of repair, but it often requires patience and the right tools.

Most A/C problems roll into the shop coupled with the same exact customer complaint: “The air conditioning doesn’t work!” Generally, there are two reasons: the refrigerant is low (either because of a tiny, long-term leak, or a leak that needs to be repaired) or the system itself is not operating properly. In this case, a bad compressor, internal restriction or something of the like may be the cause.

In Fig. 1, we can see a fairly standard plan of attack for the first five minutes of any A/C diagnosis. Even though it is a GM TSB, it would be good for pretty much any vehicle, but with a couple additions that are covered below.

Foolproof Approach to A/C

First, an A/C system always needs to be full on refrigerant. If for whatever reason it is low, this will complicate diagnosis because the compressor itself often will automatically be shut off, which will throw low, side-gauge pressures high. Furthermore, dye will often not properly circulate and find its way out of the leak point without proper system pressure acting to force it out.

So, in plain English, every vehicle should be sold on a recharge and dye injection for the sake of a better diagnosis (See Fig. 2). The customer should be informed there may be a big or small leak, and the only way to find it is to charge the vehicle up and let the vehicle run with refrigerant in it. Most A/C machines when they do a leak test will only find a gross leak. It is not uncommon for an A/C system to pass a leak test, get charged with refrigerant and then have no A/C again only a day or two later.

With a full charge, at 70 degrees F ambient temperature, pressures should be about 35 to 40 psi on the low side and 145 to 160 psi on the high side. For every 5 degrees F it is warmer out, pressures should be 5 psi higher and vice versa.

If low side pressures are low and high side pressures high, there might be an internal restriction in the A/C system. Expansion valves and receiver driers are the common culprits, due to water vapor in the system that freezes and blocks the flow of refrigerant. However, system contamination, such as sealant, can also cause blockages in strange places. Oftentimes the blockage causes an extreme temperature difference on an A/C component, such as in Fig. 3.

If low side pressures are high and high side pressures low, the compressor is not running. The quick way to see if the compressor works is to command it on with a Power Probe or jumping its relay, if the compressor has one (See Fig. 4). If the compressor still does not turn on, the vehicle needs one.

But how about more complicated issues?

Atypical A/C Problems

On a 2001 Buick Century 3.1L, the A/C blew cold on one side and warm on the other. These vehicles are known to have blend door problems, but before we begin doing an electrical diagnostic instead of an A/C diagnostic, a quick search on Identifix shows us that a common problem that creates this symtptom is low refrigerant (See Fig. 5). Otherwise, with a multimeter we can test if the blend door actuator is getting 5V, but is not moving. If this is the case, a system reset can be done by removing the system's fuse for a minute. A recalibration will take place in many cases, but if it does not, the actuator needs to be replaced.

Some customers might come to you “knowing” their vehicle has an A/C leak, such as with a 1997 Toyota 4Runner that came into one shop. If they don’t want to wait for the refrigerant to leak out, or if the price of refrigerant skyrockets again and you don’t want to lose the stuff to the atmosphere, you can try using “flash dye.” Cliplight’s Flash High Resolution Dye, No. 980, injects dye as an aerosol into the A/C system when it is running, as seen in Fig. 6. This allows the dye to instantly circulate in the system. Here, dye was found on an A/C line before the vehicle was sent out (See Figs. 7 and 8).

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