92 Volvo 740...A/C recharge not so good

The high side pressure is what you really need to be looking at, there's not much you can tell from just the low side. The 740 uses a simple orifice expansion device so the charge is somewhat critical and it's easiest to start out from scratch and weigh it in but in a pinch you can watch the pressures and check the outside ambient and interior return air temps to get an idea of where you are. I would expect a drop of more like 40 below ambient with the car moving but it sounds like the system is at least working.

Did you draw a good vacuum before charging? It's important to suck it down as low as possible first to get all the air and moisture out of the system. You should replace the reciever/dryer too any time you open the system, it's the silver can on the firewall above the exhaust downpipe. I normally connect the pump and let it suck for about an hour to give plenty of time for all the moisture to evaporate. You want at least 29 in/hg vacuum but more is better.

I haven't dealt with DuraCool, but my results with R-134a have not been good. The one tip I would offer is to verify the condensor fan is blowing the whole time. Broadly, though, it is very hard to tell what is going on with R-134a (and DuraCool, I suspect) with just pressures and temperatures.

When everything is clicking just right, the evaporator is correctly "flooded" - full of liquid refrigerant. The compressor is drawing off vapor as the heat from the car warms the refrigerant and a thermostatic limiter keeps the temperature from dropping to 0 C. If the charge is too low, the difference in vent temperature and evaporator rises because the cooling takes place in only part of the evaporator. If the charge is too high, liquid spills out of the evaporator into the suction line, where it can make it as far as the compressor. When that happens, the compressor tries to compress liquid - it is called "slugging" because slugs of liquid are hitting the compressor - and is very destructive to the compressor.

I no longer even try it myself, but take non-R12 systems to a pro. They tell me R-134a can only be filled by evacuation and filling with the measured amount. I'm a believer now.

Mike

Yea, I evacuated the system for over an hour, and had around 29-30 in hg. Well, if the compressor is being slugged with liquid, would this make a tell-tale noise I can detect?

Thanks, Ger>>

In my son's Acura (the one 134a system I tried) I began hearing an occasional stumbling sound, more in the engine than in the compressor, that went away when we turned off the compressor. I told him to leave the A/C off until the charge was fixed, and the stumbling sound was never heard again. We were lucky, mainly. Slugging is very bad news.

Mike

The systems have a large reciever so I think you'd need a LOT of extra refrigerant to get liquid slugging. If it does happen though you'll get very ugly noises until the inside of the compressor breaks.

Well, the RPM drops a little when the compressor kicks in, but it doesn't seem to be loading the engine down excessively, and there is no abnormal noise, so I am OK I guess. Man, I have looked over all the high pressure line from the firewall to the compressor, but i didn't spot any high side fitting. Thanks, Geronimo

There should be one right on the back of the compressor.