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

Question for the AC experts (I don't have much experience with this at
all)

I filled the AC system using DuraCool. Well, it is cooling, but not
so great. 40% of the R-134 charge is what DuraCool recommends, which
came out to 14 oz. That is about what I put in... the DuraCool cans
are 6 oz ea., so I put in two full cans and guessed that I had about
1/3 of the third one in. Its a bit hard to tell...it could be
1/2.....cannot be sure. Guess I should have a little weighing scale
for doing this.

Today is cool weather, outside temp 72. The air temp at the discharge
registers was (a not so good) 40 deg. I did not find any fitting for
the high side, so I could only read low side pressure...here is what
it is: Idle: 55 psi. At abt 2400 rpm, it drops to 35 psi. With
the car shut down, the low side pressure went to 87 psi (equalized).

DuraCool, they say, runs at a lower head pressure than R-134a. HOw
much lower....who knows?

Any idea how to tell if it is overserviced? I suppose you can't know
that unless I can see what the high side press. is running? In 95 deg
summer weather, 40 deg register temp is not going to cut it, right?
Actually, I was getting a drop of abt 30 deg below ambient temp, so
does that mean that when ambient is 95, I would get a 65 deg discharge
air temp? Thanks, geronimo

 

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, Geronimo

 

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.