a/c diagnosis help

'91 volvo 940

its originally an r12 system converted to r134 at some point in it's life

a/c isn't blowing cold... we just moved to TX, right before we did I recharged the system and got her blowing nice and cold, 40-50* based on the the little thermometer that is still in the dash vent... about 2-3 weeks and it's now blowing cold... pulled out the a/c manifold gauge set to do some real diagnostics to it

no surprise it was very low pressure, tossed in a can of dye/coolant mix so I could get the pressure up enough for the compressor to kick on, and find equal pressure when the car is off (low pressure as the system is obviously still low), crank the car and put a/c on high and the compressor clutch kicks on, but the a/c stays the same on both high/low...

when I rev the engine the PSI drops on both (still equally)

so you a/c gurus who understand this stuff better... what am I looking to replace?... i've got a good spare compressor but don't want to just jump into swapping it if it's something else.

thanks guys mark

To me it sounds like you need to truly vacuum the system down to the proper 28" of vacuum and hold it. I suspect you have a small leak, but not enough to register for a good long while. Now through all the added refridgerant, it is overcharged.

My A/C book says that if both are high, it's either blocked condenser fins, air, or overcharged.

Dont need AC in Texas!

First, you can't have equal pressures if the compressor is running. You likely had both valves open, which feeds the high side pressure to the low side in the gauge set. You need to have the high side valve closed to get accurate pressures. This is also why they both went down when you rev'd the engine. You were really just reading the high side. Second, just to be accurate, you have to evacuate a system to a minimum of 29.2" of mercury in order to boil moisture out of the system. 28" will take the air out, but not the moisture.

er didn't add that... I did pull a vac on the system before adding r134 the first time...

based on pressure it is NOT overcharged...

i'll do another test with the manifold shut off on each side... def a bit of an oversight lol... thanks.

bravenrace wrote: First, you can't have equal pressures if the compressor is running. You likely had both valves open, which feeds the high side pressure to the low side in the gauge set.

That part confused me. Ah. Well deduced.