The test for the serpentine belt saving circuitry on a 60x diesel engine is to spray the fan belt with water with the AC running. That WILL disable the AC compressor clutch if the circuitry is working as it should work! The AC compressor should not restart until the engine is shut off and restarted. If the belt tension is inadequate (spring weak) OR the belt load is excessive (power steering pump load or alternator excessive OR the AC compressor marginal) or the AC compressor clutch slips very much (oil or grease on it or the gap too wide) the compressor clutch WILL shut down whenever the compressor speed and the exgine speed deviate more than the allowed amount (about 25% as I recall). If you've removed the sound shields covering the bottom of the engine compartment you greatly increase the chance of the compressor shutting off in wet weather.