I have a 02 Grand Caravan that the AC compressor locked up about a year ago so I decided to replace it along with the filter drier and the expansion valve on the system. Now when I start charging the system the head pressure start getting high. It takes a few minutes for it to get there but the radiator cooling fans will not come on. When the engine gets warm the fan will come on and the head pressure will drop from around 300lbs to 100lbs but when the fans go off it starts going back up again.

I have changed the fan relay and the pressure switch on the AC with no luck. I have the correct voltage going to the fan relay and the pressure switch and when the head pressure is at 300 the signal from the pressue switch is about 2.5 volts

Anyone have any ideas on what could be wrong here

 

Have you ck'ed the fuses.

 

Yes the fuses are all ok

 

Only thing left is the compressor and H-valve?

 

The compressor seems to be working fine.

What is the H-valve???

 

Follow the two a/c lines to the bulk head or firewall, you see a box the lines are going into. Thats the H-Valve. It could have a blockage.

 

Oh I have already replaced that when the compressor was replaced