98 Voyager Cooling Fans ac need help

What activates the radiator fans specifically related to the a/c. My head press goes over 400 psi and the fans never come on. Fuse is fine. I found the relay, but I am not sure how to troubleshoot. Is the high press. switch supposed to activate the fan relay, or should the fans come on any time tha a/c is switched on?
First off you should never see 400 for a high pressure!! Compressor should shut down before that as told to by the high pressure switch.

Many vehicles will run the fan with any A/C request but it may rely on head pressure to kick on. Relay is likely here. You could try tapping it and it may kick on or just replace it. Also check that the fan isn't frozen up. Should spin (don't use your hand) with the van off freely. You could check to see if it gets power or jump power to it to test it out.

Does this vehicle cool at all? Air flow isn't real good in those just by driving along so it needs the fan

I suspect the head press switch is bad, because the clutch burned up because of this problem. I replaced the clutch and thats when I noticed the fans not comming on and the head press getting to high. What press should this switch cut the clutch out? Is there a shrader valve in the port to where I can just unscrew the press switch and put a new one on?
 

I'm not sure about the shrader for the high switch but if you just look at a new one you could tell that it has a pin to push it in. If that doesn't show you hit me at home email or try for Nacho at www.acSource.com as he does A/C year round in Mexico and would know off the top of his head, Good luck,