A lot of the ECM circuits use 5VDC. The compressor clutch relay is 12VDC, the ECM provides a ground for the relay, Relay has the 12 VDC positive for the coil waiting for a ground. The compressor clutch requires strong 12V, thus the reason for the relay.

EDIT See previous post #19, Thanks Ls1nova

 

Ls1nova71 thank you! From what I hearing from you my 2007 express van pcm should work just fine for me.
In my original system I only have one switch a binary witch . I don't think I have a low pressure switch . Does this mean that I have to plum and install a low pressure switch and give that signal to my pcm ?

 

That switch in your accumulator in your pic is the low pressure switch. The high pressure switch should be in the back of the compressor. Are you using the 2007 van compressor?

 

No I'm using a sandem compressor