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A/C High Side Reading >400psi

Feb 15, 2006
MS Delta

System started to not cool.....
The low side read only 25psi....
Added 134a and the compressor started "Short Cycling"
Finally got a guage and with approx. 40psi low side the High side is quickly jumping to >450psi before the high pressure switch custs the compressor off. This only takes about a second or so of compressor run-time for this to occur. Obviously somethiing is stopped up. I am thinking the the expansion valve.

FWIW... in the morning... After sitting up all night the compressor would run for a minute or two and the system would produce cold air.

Am I on the right track? Any other ideas from the A/C experts?

Also, what do you think of this synthetic "sub-zero" 134a? Is it worth using when I recharge, or should I just the standard stuff?


Last edited:
Sep 15, 2008
53402, Racine, WI and 92111 San Diego, CA
This may help:

Doing it right: A/C system inspection | Motor | Find Articles at BNET

High side should be roughly 2x the psi of the ambient temperature. So if it's 70 out, it should be about 150psi at normal operation.

Something is obviously clogged or stuck.

I would not use anything other than straight 134A. If you replace components then replace the amount of oil the FSM specifies. If you're going to be taking things apart I would also recommend doing all the orings and drier. About $30 for an oring kit and new drier from NAPA.
Nov 7, 2006
Tucson, AZ
A couple possibilities:

1) The system is overcharged.

2) There's an obstruction, as stated above, could be the drier has come apart and plugged the liquid line or the expansion block. Remotely possible that the condenser is plugged, but usually that takes a compressor coming apart, which is usually audible and the compressor no longer pumps. Yours obviously does if it pegs the high side.

x2 on the straight R134

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