I have a '76 F100 w/ factory air that had not worked for a long time. I literally replaced everything except evap. core. New comp, comp relay, dash control, dryer expan valve. My problem is I have power from ignition but when I plug in the a/c switch the power goes out but not a fuse. If I unplug the switch the power is there. I jumped the comp to charge and is cooling well but the comp does not receive power from the relay. My Hayne's manual does not show a schematic for A/C. Any ideas?

 

Go here: http://fordification.net/tech/images...aster_4of9.png
This is the page from the '76 wiring diagrams that concern the A/C.
Just a note, what most people call a blower relay is the "accessory safety relay" on the schematic. There are two other switches that concern the compressor. One is mounted on the vacuum switch on the control panel and the other is mounted near the face of the evaporator, called the "evaporator thermostat switch".
One thing to look out for, I've had several of these systems torn down and it seems that Ford used some undersized wiring in the A/C accessory wiring harness. Look for burned or melted insulation in the wiring going to the back of the blower speed switch. Also check the plug for the switch, quite often its melted.
One other thing, if you need to replace the switch pigtail, all the ones listed in the various parts catalogs say they fit 73-79. Not true, the pigtail they show only fits 78-79. It won't fit the '76.
The last one I replaced I scrounged off of another wiring harness. The good thing is, it's the same as the one in trucks with just a heater, so just about any truck is going to have one.

 

Hello Mike,
I have a 1978 F250. The A/C was not working. I just had it compressor, etc. replaced and converted to new freon. I picked up the truck and drove it about 10 minutes down the road and the A/C just cut off. I took it back to shop. The shop called today and said it appears to be the A/C controller is not sending power to the A/C units. If they by pass the controller everything runs.

What you posted above makes me think it could be what you are describing with the wires melting, etc.

Does anything from my description sound close to being that issue?

I told the shop I would look over the weekend for a new A/C dash controller but I am having a hard time.

I am not a mechanic so my apologies if I am leaving details out that are needed or not conveying my issue in a way that makes sense.

Thanks for your time and any input.

Take Care,
Brett

 

Unlike modern cars, the HVAC control panel is actually pretty simple. There's the blower switch and two vacuum switches. One of the vacuum switches also has an electrical switch that kicks on the compressor.
If by "bypassing the controller" they mean they're jumping the switch I would say you just need to replace the vacuum switch, not the entire panel, which, by the way are getting very hard to find NOS.
The switch isn't available separatley, you have to replace the vacuum valve.
In the picture the switch is in the orange square.
Its Valve assy, vacuum selector
D5TZ-19B888-A Motorcraft p/n YH-285
Dennis Carpenter, Concord, NC has 1,741 (!) (704) 786-8139
Green Sales, Cincinnati, OH has 3 (800) 543-4959
There are more around at various dealers.

 

Whoa...deja vu

After R&R'ing my blower motor and trying to clean up the rats nest of wiring in the dash, this is what I found when investigating why my blower motor was running anytime the ignition was turned on.

Seems that the "Mode" switch in my controller was fried so the Previous Owner "Bubba'd" this fix.

NOT GOOD, see the crispy brown "jumper" wire? Yikes...

 

Hello Mike,
Thank you for the quick reply and the lead on a replacement part.

Yes, they said they were "jumping the switch".

Is there a way for the mechanic to test that port if I point him in that direction?
Also, has anyone had this happen before? Could be that the A/C has ran in 10 years that it burnt the part up?

Again, thank you. This is very helpful.

Thank you,
Brett

 

Scotch-locks, gotta love 'em.
As for testing the switch. Just use a multimeter to check for open circuits and continuity.

 

An update: The mechanic tested the vacuum switch that was mentioned above and found that it is bad. I have ordered the switch from Dennis Carpenter. It will be here Monday and the switch will be changed out. Hopefully that will do the trick.

i will post another update once the new switch is in.

Thank you,
Brett

 

They tested the switch and the switch was the problem. They have replaced it and now it is fixed. In pulling the on/off controller to replace the switch they have broken some of the vacuum lines. Seems they were pretty brittle after 36 years. They are now trying to find a vacuum diagram so they can fix that. If any one know where I might find one it would be appreciated.

Thanks for all of the inout an help. It seems as I fix one thing I find another to fix. lol.

 

I answered a similar question a couple of months ago.
Go here for a description of how the vacuum lines are routed on the late model a/c system, that's the one you have. The early system is before late '75 (s/n X00,001).
https://www.ford-trucks.com/forums/1...l#post14615712
This is a link to my post mid-thread.

 

Thank you Mike. I forwarded this onto the mechanic. I appreciate the time you take to answer these questions.

 

A/C is in. Vacuum lines connected. Everything is running. I now need to see about adjusting idle to smooth engine out when compressor is engaged and truck is in gear. When truck is in park with A/C on truck idles pretty well. When in gear with foot on the break and A/C on the truck idles a bit rough. Old truck, old everything. I am sure there are a number of things that need to be replaced/adjusted to get best result. Thanks for all of the insight and guidance. It was really helpful.