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Pressure Chart Question - Readings with Compressor On or Off

Question:  I am chargeing my 1992 ford auto air conditioning.  Looking at the pressure chart it has ambient temp, low side p.s.i and high side p.s.i.  Is high side reading set with the compressor runing or off?  The compressor comes on and off.  Is the high side set at on or off?

Pressure derived with compressor turned off is refered to as static pressure.  There is little we can do with static pressure readings.  The compressor needs to be in operation to determine the next course of action.  As a general rule, with R12, your final high side reading should be about 100 psi greater than the outside temperature.  Low side pressure should be between 28 and 40 psi, depending on conditions.  If your system has an orifice tube and a cycling clutch switch, your system cycles based on evaporator (low side) pressure.  The cycle on point should be around 45, and the cycle off point will be around 24 psi.  So, you'll need to have at least 24 psi or greater on the low side to maintain compressor operation to get a stable reading.  If the system drops below 24 psi, the low side switch will cycle the compressor off.  It will not turn back on until system pressure on the low side starts to equalize and reach 45 psi.  That's when the switch decides it's safe to turn the compressor back on. 

To speed up the charging process, many technicians will jumper the cycling clutch switch to maintain steady compressor operation while adding refrigerant.  Just don't run the system jumpered for too long.  When low side pressures are too low, the evaporator will start to freeze, and the compressor will be starved of the oil it needs to stay lubricated.

If your interested, here's a good link for basic recharge procedures:

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