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A/C goes flakey with "low pressure" cut-off

I'm not familiar with the 93 90 system specifically, but I'm going to assume
it is a TXV (Thermal Expansion Valve) system as opposed to the GM orifice
tube system used in some Audis. I say this because the switches you describe
located in the R/D are both high pressure side switches. The red connector
switch turns off the compressor if head pressure exceeds 409 psig. This
switch is there only to prevent the refrigerant being discharged by the
mechanical pressure relief valve if overpressure occurs. The pressure can
get this high if the system is overcharged, if there is air in the system or
if the engine cooling/condenser fan fails. This switch should never open if
the system is operating properly.

The yellow connector switch is a high pressure switch that closes at 190
psig and switches the engine cooling/condenser fan to high speed.

The low pressure switch is located on suction line just after it exits the
evaporator case.

I would be surprised that the system cycles at high load and not low load. I
would expect it to cycle at low ambient, low blower speed, high compressor
rpm on the low pressure switch. The reason for this is that when the
pressure goes below 28 psig (R134a) the temperature of saturated refrigerant
goes below 32 degrees f. Were this allowed to continue, the water condensed
on the evaporator would freeze eventually blocking air flow.

Are you sure it is cycling on the low pressure switch? The first thing to do
is borrow a gauge set, hook it to the system and run it at low ambient, low
blower speed and high idle. If the low side pressure goes below 23 to 25
psig, replace the low pressure switch.

If it is really the high side switch causing the problems, there is probably
a condenser fan problem.

Bob Cummings  87 Coupe GT 115,000 mi.  

>Date: Tue, 20 Oct 1998 15:50:18 -0400
>From: Chris Newbold <cnewbold@transarc.com>
>Subject: A/C goes flakey with "low pressure" cut-off
>For the last three or four months, the A/C in my 1993 90 has started to go
>a little flaky... Any insight from the resident A/C gurus would be much
>The problem is that the compressor cuts in and out sporadically. The A/C
>control head shows no DTCs, but (when the compressor cuts out) indicates that
>the "low pressure" switch is responsible. This typically only happens
>under sustained usage on the highway, with ambient temps. > ~80F. Around
>town, the compressor runs continuously regardless of the cooling load.
>When conditions are cooler, it will also run continuously in sustained highway
>use. When the compressor is running, the systems cools as strongly as ever.
>I would have suspected (another) leak in the system and a genuine low-pressure
>situation, except that it's been in this state for several months now. If
>there was a leak, I would have thought that enough refrigerant would 
>have been lost by now that the compressor would never even engage; or at least
>that cooling capacity would be severely impaired.
>Also, it seems like the conditions under which it cuts out would correspond
>with the highest system pressures, not the lowest. (At least that's my
>understanding of A/C systems: heavy cooling load = higher pressures.)
>I found what I believe to be the connectors for the high- and low-pressure
>switches: a red connector and a yellow connector, each attached to
>devices screwed into the filter/dryer. I cleaned the contacts in each 
>connector, which showed signs of arcing and burning. This doesn't seem
>to have improved matters, however.
>Any ideas?
>- -Chris
>1993 90CS 70k miles