Hello - I'm new to this fantastic forum. I have a '96 F-250 7.3L Diesel and the A/C has stopped working for me. First, it worked, then #6 fuse blew, then it worked for an hour and #6 blew, then 10 minutes, now the #6 fuse blows the second I turn the A/C on every time.

-I unplugged the compressor coil and #6 still blows immediately when I turn the A/C on
-I put the ohmmeter on the connectors to the compressor coil and it read out '0.0'...which means it's a dead short, I gather?

So, I'm stumped, if #6 fuse is blowing when the coil is unplugged, what could be the problem? Could it still be the coil or am I reading the ohmmeter incorrectly? Also FWIW, every time I turn the truck off I hear the expansion tank pressure equalizing for quite a few minutes even if the A/C has not been running. I don't have pressure gauges, but not long ago I had it tested and the Hi/Lo pressures checked out.

Thanks!

 

Unplug the low pressure switch mounted on the expansion tank. Try again. If the fuse blows, you have problems up in the dash area. If it does not blow, I suspect your diode is shorted out, it's in the harness out near the compressor coil plug and is sort of made into the harness.

 

Thanks Dave! So, I unplugged the low pressure switch on the expansion tank (photo #1 if i'm not mistaken). and just like you said, fuse #6 did NOT blow when I turned the A/C on. Progress!

So, now on to the diode. This is photo #2, correct? It's the plug going into the compressor which is behind the smaller coil plug (photo #3). Sounds like this diode is the part that I need to replace?

Photo #1: low pressure switch





photo #2: diode? is this also known as the high-pressure switch?



photo #3, coil plug

 

I am not sure exactly where the diode is. It's usually very near the plug for the coil. It is not the high pressure switch. Unwrap the tape off the harness starting at the coil plug and see if you can find it. If you find something with this symbol on, then that is the diode. It can also be a round barrel thing with a band on one end.

 

Here's a picture of the barrel type.

 

Here's a link to Napa, they sell the plug with the diode made into it.

https://www.napaonline.com/en/p/TEM207496

What the diode does is clamp down on the kick back from the clutch coil. When you put power on a large coil, it "fills up" just like a capacitor would, storing energy. When you take the power off the coil, all that energy runs out of the coil in reverse polarity as a very large power spike. Over time this can damage the low pressure switch and any other control relays and switches that power the A/C coil.

The diode is just like a electrical check valve. It will pass current one direction only. So when normal 12v is put to the coil, the diode is installed in a reverse direction, with the band on the + side wire, and the other side of the diode is on the minus side of the power coming in. No power can flow through the diode in reverse, so the circuit does not even know the diode is there. But when power is removed, the spike from the coil is reverse polarity. So when it comes out of the coil it sees the diode as a short circuit and the diode snuffs out the spike and protects the circuitry and components.

It's very important to install the diode in the right direction. If you install it backwards, the 12v coming in to power the coil will see the diode as a short and blow the fuse, and usually blow the diode also.

 

I peeled back the tape coming from the coil plug and didn't see anything that looked like a diode. I'll have to start fishing my way back through the harness. Before I go too crazy with that, just checking to make sure that this diode in the junction box is not somehow associated with the A/C circuit. It's in the back left of the box, position 12, and the manual says it's the power steering diode...so this is not anything useful, i gather.

 

I see a PCM power diode in the diagrams. That is the line that feeds the PCM power relay. That powers all the electronic controls for the engine. It looks like the diode keeps the relay from activating if someone put the battery cables on backwards. The diode will let the power flow through in one direction only.

 

So, i started stripping back the conduit from the compressor plug wires looking for the inline diode. I got back about 2 feet until it got much thicker and didnt see anything resembling a diode. So, maybe it's back farther... but according to a wiring diagram i found, it seems like it's got to be right up there between the coil plug and the hi-pressure (refrigerant containment) switch. So, the search continues.

 

Be looking for any wiring problems while you are looking for the diode. The diode being bad was a guess, it could also be something else like a worn through wire in that circuit. But it's not intermittent, so it's probably not a bad worn through wire.

Something else you can do. Take a meter and measure in ohms the resistance at the clutch plug. I would assume it's very low or near zero ohms if it's blowing the fuse. Then if you didn't mind cutting the wires at a good spot, you could ohm them again, see if the short is toward the plug or still back in the harness.

P.S. That wire going to the PCM tells the engine computer the A/C has power. The PCM takes this info and modifies the idle to keep the engine from stalling and give the A/C a little bit of rpm to help it work better.

 

So, after further inspection, I found the problem. Turns out my busted-up bottle jack retention bracket was all bungled up and not securing the jack properly. It had come loose, worked its way forward and through a wire that didn't have any conduit on it. It wasn't obviously associated with the A/C circuit--but with all the junctions on these harnesses, it's hard to figure everything out. Lesson learned--Look all over for obvious issues before diving in and focusing on the current symptom exclusively. A/C is working!



bottle jack shorted wires out

 

I see some green colored wires in that harness. Maybe one of them does go to the A/C circuit. One of them would double back into the computer harness for it's signal. Glad it's fixed.