Q: The A/C won't work on the 2002 GMC Sonoma I just bought. I charged the system with R134a from the parts store going by the instructions, it seems to hold refrigerant, and I don't see any visible leaks. The compressor and clutch both turn freely, but when the compressor engages, it clicks on for a couple seconds and then clicks off. It continues to cycle, but stays on for only a couple of seconds.
A: First, you have to be sure of a correct refrigerant charge before going any further in diagnosing air conditioning. You have to get it somewhere with a quality recover/charge station. Assuming the charge is correct, here are a couple of possible causes going by your description: It could be that an electrical problem is not keeping the A/C compressor clutch engaged. A fault with the clutch or coil, relay, pressure switch, PCM, etc. can produce an intermittent on and off. But it seems you're describing a uniform cycling of the compressor. Cycling is normal on a good system. But it should be mostly engaged and cycle off for a few seconds at a time -- not the opposite. There might be an internal system obstruction affecting system pressures. There are several scenarios of incorrect pressures and temperatures, dependent on the location of the obstruction.
For example, A/C systems have low pressure and high pressure service ports where you hook up the gauges. Normal operation should be around 150 psi on the high side and 25 psi on the low side. The expansion valve at the evaporator is what varies the pressure drop of the system. When this valve gets stuck in the closed position, the low side goes very low (even a vacuum) and high side pressure shoots way up. This can produce your symptoms. As soon as the compressor engages, the system pressure switch sees the low side pressure drop to nothing, and disengages the clutch immediately. After high and low side pressures equalize, it will repeat the cycle. Get it checked out by a technician with the proper equipment. It could be serious, and you don't want to throw parts at a fully contaminated system.
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