need help with AC

i am trying to get my durango air conditioning to work and i am having a problem i cant figure out. the compressor went out a few years ago and i finally put a new one on last year. i oiled and charged it and i could get really nice cold air out... unfortunately the working AC exposed the flaws in the cooling system so i couldnt use it for long ... or at all on hot days without cooking the engine.

fast forward a year and i have addd a dual row radiator and the truck is happy as can be from a cooling system point of view. but i no longer can get cold air out of the AC. the compressor clutch engages... though seeming not for long and cuts in and out a lot so i thought i just might be low on refrigerant and this was blocking the clutch activation.

i tested the low side pressure and this is where i am confused. the pressure was at 45 psi (normal for that days temp) with the compressor running as suggested by the refrigerant can. the car was cool and in the shade so engine heat shouldnt have been messing with the condenser (which is clean). i cant seem to find a problem other than the excessive cycling of the clutch and a resultant low pressure side pressure reading that goes to nothing and back to 45 psi again with the activation and deactivation of the clutch. is this normal? and what should i be looking for to diagnose my problem?

any help would be appreciated! its starting to get hot here!

It's low on refrigerant and the low side pressure switch is cutting out the compressor. Low side pressure should stay in the 30-50 range when the compressor is running. If it's drawing down to nothing it's probably low. Dump a can in it and check it again.

Yup. Dump in a can with dye so you can find out where the leak is.

Is there any concern that dumping in another can would overfill it? The gauge has a lot of red at 55 psi or so.....

Not really. If the gauge is dropping to near zero with the compressor on, it's low on refrigerant 99% of the time.

If the gauge is in the 60-80 range with the compressor on then you may have a problem with being overcharged. The pressure reading with the compressor off don't mean anything. Static system pressure (compressor off) will be over the recommended low side pressure.

Do you have a high side gauge as well? If so it should be around the 200-250 range. If it's significantly higher you might have a stopped up orifice or filter. That will screw up the pressures on both sides of the system.

You also need to let the pressures settle down after starting the A/C. On Max cool with a hot car, low side pressures will read high. Let the system run several minutes before reading the pressures.