low A/C voltage
 

I'm trying to fix the A/C on a '87 S4. I have 12 volts at the low pressure switch and 12 volts on both poles of the freeze switch. With the compressor clutch engaged the voltage dropped to 0. I replaced the blue relay in the control head but still have the same problem. Does anyone know where else I could be losing voltage with the compressor engaged? Thanks.