A/C blows fuse #9 immediately

[MtnHaul]

So as soon as I activate A/C or Defrost fuse #9 blows. I can run heat and ambient air through the vents no problem. I removed the diode which seems to test OK but I still ordered some new ones. I tested the resistance at the A/C clutch connector on the compressor unit and got 2.2 ohms so I think the coil is good, maybe I am wrong? When I disconnect the low pressure switch at the expansion tank the fuse does not blow. If I disconnect the A/C clutch connector the fuse does not blow. I have checked somewhat for chafed or melted wires but so far nothing obvious. Aside from the diode what part might have failed and caused this problem? Again, fuse #9 blows the instant A/C or defrost is engaged.

 

[Nero]

Check for short to ground. That is what blows fuses.

 

[IDIBRONCO]

Check for short to ground. That is what blows fuses.

It has to do with the compressor kicking on. If the low pressure switch is unplugged, the compressor won't kick on.
On a side note, thanks for recognizing what the thing that blows heat onto the windshield is called. YEARS ago, I had a couple of guys from Houston try to tell me that it "def" on their controls means "de-fog". They never would believe me that it means defrost.

 

[ihc1470]

If you are disconnecting the compressor clutch and fuse is not blowing then I would suspect the clutch is bad. 2.2 ohms sounds a little on the low side.

Why not test the clutch for amp draw? That will tell you exactly what is going on.

 

[franklin2]

Sounds like you are easily installing and removing the diode. Are you installing it the correct direction? If you put the diode in the wrong way, it will be a dead short.

Leave the diode out and see if the problem goes away. If it does, I think it's installed backwards. Using the compressor once or twice without it in the circuit should do no harm, It will do harm long term though.

The diode should be installed across the clutch coil. The line on the diode should be on the + voltage wire feeding the coil.

 

[MtnHaul]

Sounds like you are easily installing and removing the diode. Are you installing it the correct direction? If you put the diode in the wrong way, it will be a dead short.

Leave the diode out and see if the problem goes away. If it does, I think it's installed backwards. Using the compressor once or twice without it in the circuit should do no harm, It will do harm long term though.

The diode should be installed across the clutch coil. The line on the diode should be on the + voltage wire feeding the coil.

I found some wiring diagrams for Ford A/C systems and across a range of years, gas and diesel, they show the anode attached to a black wire(Should be ground) and the cathode(The side with the line) attached to a black/yellow wire. Now oddly when I traced the wires down where they go into the loom there was another black yellow wire that had been cut--no idea what that wire is.

I am starting to think the clutch might be bad as the resistance is quite on the low side and the diode I removed passes the diode test using my multimeter. When it stops raining I'll see if can at least get the system to activate without the diode in place.

 

[gandalf]

For what it may be worth, I scanned that portion of a wiring diagram for a 1992 Ford diesel. The A/C is at the right in the picture, or the lower part depending on your orientation. Let me know if it doesn't come through properly.

 

[franklin2]

I found some wiring diagrams for Ford A/C systems and across a range of years, gas and diesel, they show the anode attached to a black wire(Should be ground) and the cathode(The side with the line) attached to a black/yellow wire. Now oddly when I traced the wires down where they go into the loom there was another black yellow wire that had been cut--no idea what that wire is.

I am starting to think the clutch might be bad as the resistance is quite on the low side and the diode I removed passes the diode test using my multimeter. When it stops raining I'll see if can at least get the system to activate without the diode in place.

That sounds right. When installed properly the diode is actually installed backwards, so it does not conduct when the clutch is supplied voltage, but does conduct when the voltage is removed and the kickback from the coil(which is reverse polarity) is trying to go backwards into the circuit.

 

[MtnHaul]

Installed a new diode with the cathode or strip side to the black/yellow wire and anode side to black/ground wire and everything works perfect now. New blower motor, heater resistor, heater core, and a freshly charged and converted A/C--bring on the heat & bring on the cold!

 

[Codmcg]

Having the same problem with my 1991 F350 7.3idi. Where is the diode located?

 

[franklin2]

Having the same problem with my 1991 F350 7.3idi. Where is the diode located?

Usually out near the compressor. Usually wrapped up in the harness so you will not see it unless you unwrap the harness.

 

[DirtyWood]

I've had to splice in a new diode as well. Super easy and cheap to do. I ordered a pack of diodes off ebay and no problems for a while now.

 

[Codmcg]

Usually out near the compressor. Usually wrapped up in the harness so you will not see it unless you unwrap the harness.

Thank you for the info. What does it look like? Kind of like a regular fuse? Forgive my ignorance.

 

[franklin2]

Like a mysterious piece of plastic in the wiring. It varies depending on where Ford got sourced the diode at the time. It does usually have a line on one end though, to denote the proper direction of orientation. Think of it as a electrical check valve.

 

[Codmcg]

Like a mysterious piece of plastic in the wiring. It varies depending on where Ford got sourced the diode at the time. It does usually have a line on one end though, to denote the proper direction of orientation. Think of it as a electrical check valve.