I've got a 96 e-150 conversion van with a mysterious A/C issue and I can't figure it out. As soon as you turn on the A/C switch, it blows the fuse. I've torn apart the dash, and tested the connector that goes into the A/C switch. Two of the four poles are hot (See diagram). Now, I don't know how this particular switch works, but logic would dictate that one of the poles should be hot, and depending on which position the switch is in would determine where the power goes out. I have power coming IN on two poles. I don't have a Haynes for this van, so any electrical diagrams would help. I can't find a short anywhere. I read something about a diode that could cause this issue, it's in the power distribution box under the hood. I pulled the diode (Which looks like a blade fuse) and tested it for continuity (which I believe to be the proper test procedure) and it doesn't have continuity regardless which leads are put to which blade. But, I put the diode in my pocket thinking it was bad, and when I went to start the van, it wouldn't. I put the diode back in, and it runs. So I'm assuming the diode IS good, or else it wouldn't have started. I have no idea why it's blowing fuses..and it does it every time you try to turn on the A/C. When the fuse blows, it shuts down the entire climate control system..no fans/blowers or anything....any thoughts or opinions? Thanks in advance!



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The pins with the arrows under them have power (First, and 3rd from the left)

 

Without some special equipment to pinpoint a short circuit you might try disconnecting the different components like the compressor clutch,blower motor,cycling switch and thermostatic controls.
If the short is still present then it may be safe to assume a pinched or rubbed through wire somewhere in the wiring that is allowing the circuit to go directly to ground instead of to a component.

 

Would you please explain to me what kind of special equipment do you own that would pinpoint a short? I've never heard of such a thing. And We have gone through those components, disconnected everything and tested.

 

There are two that I have used. They get connected to the side of the fuse that goes to the shorted wire/component. They generate a signal into that line and you move a receiver over the wires . Where the signal stops is where the short is located.
There is an inexpensive tool you can make and use. It involves a sealed beam lamp with a fused supply. What you do is connect the one side of the fused lead to the supply for the A/C system fuse that is blowing, and the other wire of the sealed beam to the other half of the A/C system wire coming from the fuse block. As long as there is a short to ground the sealed beam will stay lit and you can't miss seeing it.
Once you move the wire/part that is shorted away from it's ground the sealed beam will go out. Simple, but effective.

 

Something to check,
if you have a copy of the owner's manual does it show an A/C relay in the system?
If you think it does have one you could pull the relays out noting their location of course and see if any of those relays allow the fuse to not blow when the relay is removed.
There is another method which I hesitate to mention and that is to put a circuit breaker in place of the fuse and give it the "smoke test". The circuit breaker should keep it from igniting the wiring, but may reveal the location of the short to ground by visible smoke.