Or you might try something else, which just this weekend worked for me. If your car has the heat sensor on the compressor, bypass it. And, install a relay in the circuit that energizes the clutch. These two things, on my '94 850 Turbo, bought me enough voltage increase at the clutch to make it engage properly. The initial problem was definitely that thermal sensor, but even with it bypassed, and when the engine was very hot, there was a voltage drop to the clutch. Now the system works fine, and if the clutch needs to be reshimmed, that will be the next logical, and apparently unavoidable, step.