I had low freon pressure of 25psi and added a canister to bring it up to 40psi. The AC worked great for a month or so, but then stopped blowing cold. The pressure reading is now 100psi which indicates a possible mechanical problem. Can anyone advise me as to what might be happening and whether I need to take this in for professional repair?

 

I assume these are from the low side with the engine running but pressures change as the compressor cycles. Need to know what these numbers represent and is the compressor cycling still. What is the hi and low cycling pressures if there is any. Is the receiver/dryer cold.

Do you have a set of gauges, did you measure hi side?

 

Yeah the pressure reading was low side, and after cycling the compressor, the pressure reading is now back down to around 27. It now seems I have a freon leak somewhere. Just out of curiousity, is it normal to get a high reading of 100psi on the low side when the compressor is cycling on and off?

 

You were looking at the pressure while the compressor wasn't running. System pressure will equalize when the compressor is off - suction and discharge pressure will be approximately the same. Your discharge should have been reading about 100 psi too - was it?

 

That is correct. For example before you start the engine, if you connect the low pressure gauge it will read above 100, (I think mine reads more that 125 from memory but have not measured that in a while). When you start the engine it would pull down to around 22-25 pounds and then the compressor will cycle off. Then the pressure will climb to around 46 pound and the compressor will engage and cycle on. This process repeats and is controlled by the pressure cycling switch which controls the compressor clutch.

On a system that is low on freon will reach the 22 pounds to fast and compressor will cycle off to early causing no cooling. To much freon in system and you will not achieve the 22 pounds cycling point and compressor will just run, but will cool (an overcharge can cause compressor damage). A system that is slightly low on freon that is basically working, but has poor cooling, probably will not need a full can of freon. Put enough in so that is cycles correctly. There are specific numbers for your year of car. It is a somewhat of a judgment call that comes with skill.

 

Thanks. I have done some more reading and it sounds like the standard refill is about a 1/4 bottle (or 3oz) of freon .