Checking A/C the right way?

[J-Ri]

Help me settle a disagreement. A customer comes in and says "My A/C isn't working, can you take a look at it?". We pull the car in and see that the compressor clutch isn't engaging. We hook up the A/C machine and see that there's about 75 PSI in the system on an ~80° day.

What's the next logical step? Check the electrical side and see why the compressor isn't coming on, or evacuate the system to see how much refrigerant is in it? (or something else?) A brief explanation of the answer would be appreciated.

Thanks,
Jason

[gmtech1]

I would recover and measure, then recharge the system. That way you KNOW that it has the proper charge. If it still does not work, the continue with the diag. What are you working on?

[J-Ri]

It's a '98 Park Ave, I got it diagnosed but my business partner was giving me crap for not doing it his way. If the static pressure is good and the compressor doesn't come on at all, I always go straight to electrical. If it's a quick and easy fix like a fuse, relay, or wire I fix it and check the performance and the high/low pressures, if it's cooling good and the pressures are good, the charge is good as far as I'm concerned.

This one started working after removing and reinstalling the fuse for the compressor (the clutch is all mangled but works, somehow, I think it was a coincidence it started working). Once the compressor kicked on, the low side was 30 PSI and the high side was 100 PSI, so obviously undercharged (.37 lb in 2 lb system).

It's just that if the system has pressure in it, the compressor should at least short-cycle (or in the case of variable stroke compressors, have a very low high side pressure). It's just so irritating to watch him spend 20-25 min recovering, vacuuming, and recharging the system before he even starts diagnosing it. I had that one diagnosed in 20 min, it would have taken him at least 40.

I pulled it in, looked at the compressor, saw it wasn't turning, hooked up the A/C machine, saw the pressure was enough the compressor should be turning, checked the fuse, checked for power and ground at the fuse with the fuse removed, reinstalled the fuse, heard the clutch click, saw the gauges were off, shut off the engine, hit recover on the machine, looked for dye while it was recovering (saw some one the compressor housing), started pulling a vacuum on the system, went in the office and made an estimate, went back out and charged the system, saw it worked, and started recovering it while I went back into the office. 20 min start to finish. His way just adds 15 min waiting on the machine to do its thing rather than making an estimate.

[gmtech1]

On your '98, that was a good way of doing that. I would have thought same with the 75psi. On the new stuff, where refrigerant charge is measured in grams, not pounds, the systems are very sensitive to over and under charging. A guy really needs to know what the charge is.