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Rick Knoble
'85 300 CD
'87 190 DT
From [EMAIL PROTECTED] Fri Jun 09 19:17:13 2006
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Subject: Re: [MBZ] DIY AC repair in modern times
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> I was told to do this, The last time I had a gauge set on the AC
> system it
> had a lo side pressure of about 20 at idle and very low hi side. The
> high
> side would come up at about 3000 rpm the lo side would read vacuum 15-
> 20
> inches or so.
Sounds like a classic restriction in the system. There's a filter
screen inside the receiver/drier, and usually another one in the
expansion valve. If the system gets plugged you'll get a very low
pressure on the suction side (which could not happen with a 'weak'
compressor), because there's no flow and it's sucking a vacuum.
You will get very cold vent temperatures, but very little actual
cooling. I had exactly this in the Ebola Fishtank, where vent
temps hit 20 degrees and falling. It was a plugged screen in
the expansion valve, all I did was remove and clean it.
It's a very different experience when compared to the nightmare
that is the valve in a 126!
If you don't get 'normal' low-side pressures (in the 30 psi area) the
high side pressure doesn't mean much. You need flow, and watching
the sight glass can give you hints there. An undercharged system
(or a normal charge for a hydrocarbon refrigerant) should have _very_
visible flow through the glass. If there's no flow then the
pressures can lead you to why. High low-side with low high-side
and no flow indicates bad valves in the compressor. Low low-side
means that there is a restriction in the system.
-- Jim
