Auto Air Conditioning & Heater Repair: Aircon pressure good but no cool air, psi readings, vw jetta

Question
I have a 97 VW Jetta.  The aircon does not work.  The aircon is charged to the right pressure.  When you turn it on you can hear the compressor click on and the engine labor, but only for a couple of seconds - then a gap of 10 secs or so and then it clicks on again for a couple of seconds.  

Something more disturbing is that when the car is cold and I turn the aircon on, youy get what sounds like a small backfiring sound.  This can happen a few times. But stops when the car is warm.  As I said, this only happens when I turn the aircon on.

A mechanic friend of mine said there may be some block inside the aircon system - possibly at the condenser.  What can I do to find the fault first, and then I will worry about fixing it.  Thanks.

Answer
I need to get your high and low side pressure readings while they a/c is on before it cycles off. You need to get your hands on service gauges to do it. Ask your mechanic friend for some assistances..It`s the best way to check the performance psi readings.

good luck,davidt