Is there any other way to test the PBU and or the ground as to where it grounds the A/C compressor clutch?

__________________
---------------------------------
87 300SDL
87 190D 2.5NA 5sp
90 300GD with 606 turbo!
08 GL320 CDI
12 ML350 Bluetec
---------------------------------
Past rides:
82 300SD - the one that started it all!
81 240D
87 300SDL - the first one.
92 300D 2.5 Turbo
99 E300 Turbodiesel
07 E320 Bluetec
12 S350 Bluetec