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AC Wiring - Blowing fuses (78 F150 w/400)

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Old 06-23-2016, 08:18 PM
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AC Wiring - Blowing fuses (78 F150 w/400)

I'm readying my truck to get the A/C "turned back on" and was looking to verify that the wiring is good.

(Note: Engine is running but I don't have the belt on the compressor)

Test-Results:
1. What I think is the "AC wire" for the compressor (#1 in the picture), this wire also runs to (#2 in the picture) on the carb. The other end of this wire runs back (#3 in both pics) and then back to the firewall by the AC.


Click the image to open in full size.


Click the image to open in full size.


2. When I hooked, what I think is the AC wire up and switched from Vent to AC the fuse blew. I switched back to Vent and replaced the fuse (35amp), worked fine - switched to AC - fuse blew again.

3. I UNHOOKED the wire at the compressor, put a new fuse in and have it on Vent. I could move to AC and back to Vent multiple times with no issue.

4. I then put a multimeter on the "AC Wire" while it was switched to AC to see if I could read anything. Fuse blew...

Q1: Is the "AC Wire" #1 the correct wire for the compressor?
Q2: Any idea what would be causing the fuse to blow?

Appreciate any help on this.
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Old 06-23-2016, 08:40 PM
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sounds like the clutch in the ac compressor is shorted. how many ohms resistance between the connector on the compressor and ground?
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Old 06-23-2016, 09:16 PM
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In Test 4 I didn't have the compressor wire hooked up and it still blew the fuse (so I figured this wasn't a compressor issue).
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Old 06-23-2016, 09:37 PM
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What setting was your meter on?
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Old 06-23-2016, 09:52 PM
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I had it set on 12v to see if I could detect any voltage coming in on the wire.
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Old 06-24-2016, 04:52 AM
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Old 06-24-2016, 08:02 AM
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Test for power with both the #1 and #2 wires disconnected. It sounds like #2 was left connected when you switched the AC on. If so that #2 wire may be the cause for blowing a fuse. It is going to the carburetor throttle emissions control solenoid. Maybe it is faulty and grounded out somehow.
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Old 06-24-2016, 08:02 AM
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Yes, you have the correct wire for the A/C clutch. If the fuse blew with the compressor unplugged then I would look at two other possibilities.
Its possible the A/C mode switch is shorted (doubtful). The other area that I would look at is the A/C wiring harness itself. Ford uses two accessory wiring harnesses that plug into the main harness to power the A/C system. I've personally worked on several that had the insulation melted. It looks like Ford used some undersized wire when they designed these harnesses. The last one I rebuilt, I used a gauge larger wire to repair the harness.
Here are some pictures of one of the harnesses that I found melted:
EDIT: One thing I would do, unplug the solenoid on the carb. I really doubt this is the problem, I've never seen one short out. I would unplug it just to take it out of the loop, eliminate a possibility.
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Old 06-24-2016, 04:23 PM
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Thanks for the feedback guys. In test #3 I did have the solenoid (#2 wire) on the carb hooked up (it's actually always been hooked up) and it didn't blow a fuse when I switched to AC. I'll try another test with it unhooked to see what happens when only #1 is hooked up.

I will also trace-check the wires to see if I see anything suspect - the junction where the AC Clutch and Carb solenoid split had some bare wire on the Carb solenoid side but didn't look damaged/melted otherwise, I'll clean that up.

Mike, in the bottom right picture, where is that on the truck? (glove box opening, main gauge panel opening, ?).
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Old 06-24-2016, 05:07 PM
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Mike, in the bottom right picture, where is that on the truck? (glove box opening, main gauge panel opening, ?).
That picture is right below the HVAC control looking up into the dash. You can see the wiper motor at the 12 O'clock position.
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Old 06-25-2016, 10:06 PM
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I cleaned up the wiring and that looks to have done the trick. Could turn the AC on and off with both the AC Clutch and Carb Solenoid hooked up (or not) and no blown fuses. Was getting ~12v out the end of each so I think I'm ready for some R134! Thanks everyone!
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Old 06-26-2016, 12:41 PM
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Congratulations on getting it sorted out.
Thanks for the update.
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Old 07-12-2016, 09:24 PM
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I've got COLD air in the dent, and not just cool air !!

Hooked up the compressor, new lines (a 2nd new dryer) and a charge.

AC project is completed, bring on the heat !!


Click the image to open in full size.

I made the lines a little long so that when I replace the motor (not anytime soon) I can tuck the AC compressor out of the way.


Click the image to open in full size.


You can see there was a little fab work done, I had the old York mounting bracket in place because it has the tension pulley attached but since Im using a Sanden style compressor it didn't bolt on this side so I had to make a piece to join the two for stability on the tension pulley.
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Old 07-13-2016, 02:57 AM
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Awesome! My truck is in the shop getting charged. Just waiting for him to call me to pick it up. Thanks for the help with my problems i had. Now you can enjoy your dent a lot more. Lol
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Old 07-13-2016, 08:39 AM
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Quote:
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You can see there was a little fab work done, I had the old York mounting bracket in place because it has the tension pulley attached
Congratulations, looks good. I did the same thing on my conversion.
I used the "fixed" conversion bracket instead of the swivel bracket but still fabbed a small bracket to tie everything together for rigidity.
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