1993 K30 6.5 turbo. some times the A/C compressor will not engage the clutch.. it has full charge. when i pull the plug on compressor and measure voltage I get 13.5... when I plug it back in I get 0 .I check ohms of coil I get 3.5 ohms. I jumped coil with 12 volts from plus battery terminal.. compressor started working and stayed working .. coil voltage is 11.5 volts with a 14.1 battery voltage when running. I think I am getting a voltage drop some where , in series , in the circuit.. is there a compressor relay ? or is the power to the clutch coil go thru the A/C control in the dash?