Hey y'all. I've always wanted to own an 80's F100, and a month ago my dream came true. I came across a 1982 F100 with 46,000 original miles and jumped at the opportunity to own this beauty. Here she is:





She runs like a champ and is in fairly good condition for a 32-year old lady; I have very few things to work on. Nonetheless, something has been intriguing me:
When I bought the truck, the A/C was only blowing through the dash vents, and I attributed the hissing sound coming from the engine bay to some leak that was causing it to switch to the default state. I put it on the "to-do" list and worked on a few other things, when one sunny summer day, the dome light and the passenger side's fog lamp stopped working. I checked the fuses, everything was fine - the following day, the fog lamp was working again, but not the dome light was not.
But today, to my surprise, the A/C started blowing through the front vents again! Since I do not believe in miracles, I came to you guys for advice - can it be an electrical issue? Are these events (lights working one day and not on the other and A/C problems) connected? If it is indeed an electrical issue, what about the loud hissing sound coming from the engine while it's running?
Thanks a lot guys!

 

Welcome to FTE. I found this forum only recently and it is a great site. I can't help you with any of your problems as I'm not that sharp with electrical problems but I sure that there are guys on here that can give you some answers or at the very least, where to start to trouble shoot your problems.

That is really a great looking pickup!! Keep the photos coming.

Kenn in Texas

 

Thanks, buddy! It really is a great site, lots of knowledgeable and friendly folks around here. I'm glad I found it.

 

You will have these gremlins come up now and then, it's part of owning a old vehicle. Lift the hood and check your vacuum lines. The ones for the HVAC run over to the pass side firewall and go through there.

For your domelight, open the door and twist the pin sticking out of the switch back and forth and see if the domelight tries to come on.

 

Welcome to FTE! This is THE place to be.

And, Dave is right, as always. But, a little more info - the vacuum probably goes to a tee on the firewall. From there the HVAC vacuum line should run to a tee/check valve on or close to the passenger's fender, with one side of the tee going to a reservoir (a ball or an orange juice can) and the other side into the cab via a connection through the firewall by the passenger's fender. Any leak in that can cause the HVAC system to go into default mode, which is the Defrost position. (The HVAC system's various doors are run by vacuum, not electricity.)

 

welcome! she looks like she was well cared for!

I fully agree about the vacuum leak, and when you began to get attacked by those little electrical gremlins, remember to check the easiest stuff first, fuses, bulbs, and do a good visual inspection of the wiring and connections.

 

Wow! She's a beauty.

Congratulations, and welcome to the forum.

 

Thank you so much for the input. I will try to reply with pictures, to make things easier (and, why not say, lore entertaining).
Gary, I believe this is the tee you referred to:



Nonetheless, upon closer inspection, I found out that the noise is coming from this area behind the carburetor:



Any ideas? (By the way, even with the same noise as before, the A/C is still blowing through the front vents...)

Dave, as far as the dome light goes, I believe the PO installed a later model, as you can see here:



This is the same one I used to have in my 2002 Ranger. I checked the fuse and the bulb, they are good. One thing I can't figure out is, what is the cluster of wires hanging from the door hinge? There is a black, a red and a green wire.
Once again, thanks for the help.

 

Unfortunately if you have a loud hissing coming from the back of the engine I'm going to suggest it is the A.I.R. crossover pipe connected to the back of the heads.

You can see the hose to the checkvalve has been cut off next to the transmission dipstick.

If you don't have emissions inspections and the system has been disabled anyhow, you can remove the pipe and block the holes in the head with short 5/8-11 bolts.

 

If you think it is a 'hiss' I would check the vacuum 'tree' that you can see in the #8 intake runner.
Visible just under the kickdown linkage
.

 

That is the tee/check valve I was talking about. You can see where the hose is going up by the fender, and the plastic line gets brittle and frequently breaks there. But, if it is blowing through the vents I'd guess that's not broken. However, if there's a cracked line where Jim is talking about at the back of the engine it'll work sometimes and not others as things shift around. Check out all the lines to see if they are cracked.

 

Alright, I will make sure to check these lines for cracks, as well as the A.I.R. crossover pipe. Thanks for the help.

 

also for checking for vacuum leaks if the crossover pipes are good, some carburetor cleaner or a small propane torch with a hose attached can really help, just spray or stick the hose around while the engine is running, when the rpm goes up you have found your leak.

 

Lightgreen with a yellow stripe is your hot wire for the courtesy lights.

Black with a lightblue stripe is the wire that feeds your courtesy light(s).

Red with a pink stripe is the wire that goes to the warning buzzer to tell you the exterior lights are still on when you open the door.

 

Exactly what I was looking for. Thanks guys!