My heater/AC controls are limited to either off or defrost. The lever has full movement and does not feel different(dont think i broke the linkage). But when the lever is in any position besides off..the defrost is on and nothing else. What has happened?

 

Do you hear any kind of hissing in the dash when the switch is in different positions? It sounds like vacuum isn't getting somewhere. All of the doors that direct the air within the plenum are operated by vacuum motors. They are all controlled by a central switch which sources vacuum from the intake manifold on the other side of the firewall.

 

Yeah, what FMC said plus this...
"defrost is the default setting"
In other words...
No vacuum = the doors are "at rest" in "defrost position".

I recently had the same porblem and found out it was a leaky crack in the
end of my "can of tomatoes". Looked kinda like the rust had duplicated a
can-opener-cut for about 2". :)

That vacuum leak was driving me nuts, bought a hand operated vacuum
(with gauge) and discovered it no sweat. Had been thinking one or more
of my vacuum motors was leaking and it was cool that they weren't. :)

I just soldered-up my "can of tomatoes" and called it good for now.
It's easy when you know how and have the right tools. :)

Alvin in AZ

 

Hmmm...no one mentioned the following...and this part has been a problem child since these trucks were brand new.

D5TZ19B888A .. A/C Vacuum Control Selector Valve & Switch Assembly (Motorcraft YH285) / Use with FACTORY installed A/C ONLY!

Still available from Ford.

MSRP: $168.42 / FTEpartsguy.com price: $101.05.

Fits: 1973/79 F100/350 // 1978/79 Bronco.

This valve bolts to the backside of the A/C Heater control panel, and has 7 ports where all the vacuum lines connect.

It also has the kicker switch on it that turns the A/C compressor off/on.

When you move the lever on the A/C Heater control panel, this valve opens one damper door and closes another.

One of three things is causing the problem.

Either this valve is defective (and this is somewhat common), the vacuum motor that operates the defroster door is defective, so the door is stuck in the open position, or the vacuum hoses are cracked.

D3TZ18A318B .. Defroster Damper Door Vacuum Motor (Motorcraft YH218) / Obsolete

Dennison Ford-Mercury in Bloomington IL has one = 800-322-9431.

This is the only FoMoCo dealer that has it. No obsolete parts vendor has any.
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Make sure to check all the vacuum hoses under the dash for leaks.

The hoses are prolly the originals, making them 30 + years old.

The hoses are routed so some of them bend at 45 and 90 degree angles, and those angles are where the hoses usually crack.

 

thanks..ill have to check for vacuum leaks..
And no i cant hear crap with those new pipes. lol