Re: 164 AC system.

[bearbvd@domain.elided (Greg Hermann)]

>I have never been happy with the AC output on my 1995 164 Super. On hot
>days (100+) or when the car has been out in the sun, you can barely feel
>any cooling effect out of the system. I recently had the car checked by
>an AC "technician" who promptly hooked up some pressure gauges to the
>appropriate places on the system and declared the pressure to be too
>high, i.e. the system had been overcharged by the PO. He proceeded to
>release some of the gas, and guess what?-- air temperature from the AC
>vents increased. He then recharged the system with new gas until he got
>similar pressures as those he started off with. After this short
>exercise in futility, I decided to do some research on my own. I found a
>technical bulletin (No. 2022) from VACP in England (http://www.vacp.com)
>where it states that checking an AC system that uses R134a for proper
>charge using traditional methods does not work. That the best way to do
>this is to charge the unit by weight!
>In my case, I had already eliminated the following sources of the
>problem: Compressor, cooling fan, drier, thermostatic switch, clogged
>evaporator, dirty condenser, and AC filter. Everyone of these seems to
>be operating properly. I have also run the AC computer diagnostic tests
>with zero faults. Should my next step be to discharge the entire system
>and recharge by weight as recommended?
>Any ideas appreciated.
>Jose Castro
>San Salvador

If no bubbles of gas are visible in the little sight glass (at the filter
dryer) when it is running, it has an adequate charge. If there are any
bubbles visible in the glass, it does not have enough charge. Regardless of
refrigerant used. (Needs to be all liquid at this point.) And no gauges are
needed to check this. The symptom of overcharge is excessively high high
side pressure--the excess liquid backs up into the condenser, reducing
affective condensing area, thus causing a higher condensing
pressure/temperature than one would normally observe. Overcharge produces
SOME reduction in capacity (because the higher high side pressure reduces
the compressor's volumetric efficiency somewhat, but it is VERY doubtful
that there would be enough effect to cause the symptoms you describe
without a grotesque amount of overcharge.

There is usually a little filter screen in the liquid line where it
connects to the thermal expansion valve, just prior to the evaporator.. I
have seen these screens get plugged up with either little or big pieces of
"stuff"--things like little pieces of the inside lining of the hoses--and
the restriction presented to liquid flow into the TX valve and evaporator
by this screen being partially plugged is easily able to produce the
symptoms you have described.

To get at this screen, you would need to discharge the system, undo the
line, and check/clean the screen, then reassemble, evacuate, and recharge
the system.

Is your heater valve shutting off tightly?

Regards, Greg

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