My 91 350SD is having problems getting A/C clutch to activate. My ACC module seems to be working OK on everything except when it comes to the compressor. I can't seem to get any any voltage to go out to the low pressure switch (and then on to the clutch relay module) Is there any way to bench test these modules as opposed to forking major $ for a rebuilt or new one? What sensors should I check? Thanks to anyone who can help.