A/C troubleshooting
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Logistics Pro
Joined: Feb 2000
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From: Mesa
A/C troubleshooting
I think I have multiple electrical issues with my a/c system. I have no fan and no power at the compressor clutch. I was able to find a wiring diagram for a non a/c truck but not one for an factory a/c truck. Does anyone have this or a link to one? Need to test to see if I have power going to the system, then do I have power after the switches(that should see if the switches are good) Once I get the electric figured out then I am on a search for ducting and then I need to check the heater side of things. (core, vacuum valve etc) Thanks
Hotshot
Joined: Apr 2009
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From: Stanley, VA
The diagram you have will work fine. Ford made a change in the wiring in late '75. From late '75 through '77 is the same.
The most common electrical problem on these trucks is the blower relay, if you're looking at the factory diagrams it called a safety relay. It's located behind the glove box although if it's been replaced before it might be bolted underneath the glove box. The second part to look at is the blower resistor mounted in the plenum duct. There is also an "anti-ice" relay that is mounted behind a cover, under the dash, next to evaporator.
The most common electrical problem on these trucks is the blower relay, if you're looking at the factory diagrams it called a safety relay. It's located behind the glove box although if it's been replaced before it might be bolted underneath the glove box. The second part to look at is the blower resistor mounted in the plenum duct. There is also an "anti-ice" relay that is mounted behind a cover, under the dash, next to evaporator.
Ford Parts Specialist
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From: Simi Valley, CA
1973/79 F100/350 & 1978/79 Bronco: Factory installed integral A/C is combined with the heater. The A/C controls are in the same panel as the heater controls.
Deluxe dealer A/C combined with the heater, has two A/C switches mounted in bezel. HUGE bezel sweeps across the entire right side of dash, covers up glove box. Hang-on glove box bolted to bottom of dash.
Economy dealer A/C is not combined with the heater, has two switches in panel that also contains two registers. Panel fits into dash to the left of glove box.
Deluxe dealer A/C combined with the heater, has two A/C switches mounted in bezel. HUGE bezel sweeps across the entire right side of dash, covers up glove box. Hang-on glove box bolted to bottom of dash.
Economy dealer A/C is not combined with the heater, has two switches in panel that also contains two registers. Panel fits into dash to the left of glove box.
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Logistics Pro
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From: Mesa
Today was a great day! 
I traced the issue to the relay under the glove box. $10 later and she is working. All a/c, heat, defrost selector slide positions work. Temp selector slide works. All 4 speeds on the fan work. Water heater vac valve works and no leaks from the core. I think I got very lucky that there were no other issues. Still no power to the a/c clutch. I did verify power for the a/c clutch switch (anti ice switch) so now I have to check that. 
I traced the issue to the relay under the glove box. $10 later and she is working. All a/c, heat, defrost selector slide positions work. Temp selector slide works. All 4 speeds on the fan work. Water heater vac valve works and no leaks from the core. I think I got very lucky that there were no other issues. Still no power to the a/c clutch. I did verify power for the a/c clutch switch (anti ice switch) so now I have to check that.
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
Today was a great day! I traced the issue to the relay under the glove box. $10 later and she is working. All a/c, heat, defrost selector slide positions work. Temp selector slide works. All 4 speeds on the fan work. Water heater vac valve works and no leaks from the core. I think I got very lucky that there were no other issues. Still no power to the a/c clutch. I did verify power for the a/c clutch switch (anti ice switch) so now I have to check that.
I traced the issue to the relay under the glove box. $10 later and she is working. All a/c, heat, defrost selector slide positions work. Temp selector slide works. All 4 speeds on the fan work. Water heater vac valve works and no leaks from the core. I think I got very lucky that there were no other issues. Still no power to the a/c clutch. I did verify power for the a/c clutch switch (anti ice switch) so now I have to check that. The switch you need to check out (anti-ice switch) should have voltage on both terminals. If it has power on one side and not the other, it needs to be replaced. BTW, it's listed in the Ford parts catalog as "Thermostat assy., air conditioner evaporator temperature control".
EDIT: My explanation above didn't come out quite right
.Look at the picture of the switch. With the green wire hooked up and the A/C on, the terminal that the brown wire connects to should have power. If the green wire has power and the terminal for the brown wire doesn't, then replace the switch.
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Logistics Pro
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From: Mesa
Well more good news. I found the switch and I do have power at the green and brown wires. I kept finding no power at the compressor. Turns out I was testing the wire at the clutch not the wire comming from the switch. (was unplugged) DOH!! I checked that wire and it was hot so I plugged it in to the compressor and it works. I put on a belt and it spins. No cold air tho. On to the next challange.
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Logistics Pro
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From: Mesa
Not much of an a/c guy. lol I am guessing draw down a vaccuum and see if it holds. Still don't know if I want R12 or 134A. Thank god I have till March or so to get it ready for summer. 
Elder User
Joined: Oct 2009
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From: New Albany, MS
You want R12, however r12 is very hard to get at this day and age at a decent price, so you may be stuck with 134a. Freeze 12 is a great alternative, but unfortunately it recently has been discontinued, so it will be hard to find as well.
Another option is a mixture of Isopropane and Isobutane. It sounds scary, but I have actually tried it before and it does indeed work. Some people argue that it is dangerous, and I guess it could be, but so is 12 gallons of gas in a metal container behind the seat.
I'm not recommending that you use flammable hydrocarbons as refrigerant, but simply stating that 80% R290 (Propane) mixed with 20% Isobutane performs similarly to r12.
Another option is a mixture of Isopropane and Isobutane. It sounds scary, but I have actually tried it before and it does indeed work. Some people argue that it is dangerous, and I guess it could be, but so is 12 gallons of gas in a metal container behind the seat.
I'm not recommending that you use flammable hydrocarbons as refrigerant, but simply stating that 80% R290 (Propane) mixed with 20% Isobutane performs similarly to r12.
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Logistics Pro
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From: Mesa
R12 is fetching about $25-$40 a can on craigslist. Thing is if I am going to replace a lot of the parts anyway I could go either way R12/134A. 134A being about $10 a can at the wallyworld. But a functioning R12 system would rock. 
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