A/C issue, only blows as defrost, nothing through other vents

Hey all, need to see if anyone has had a similar or the same issue. For reference I have a ‘93 ford f250 XLT with a 5.8L.

My A/C and heat will only blow through the defrost vent, and none of the others. It’ll blow as hard as it always has on any of its 4 settings.

I replaced the belt and air filter with the last oil change about 300 miles ago. I also fixed a charging system issue so I kind of doubt its an issue with those, but could it be a switch or relay issue at the board?

If anyone has ideas or diagrams and any other references that woumd be greatly appreciated. image

Sounds a lot like a vacuum leak affecting the blend door.


Or the blend door actuator, they do fail.

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Sounds a problem with the vacuum operated mode doors.

When vacuum is lost to this vent system, the system defaults to the defrost mode.

Item 16 is vent system controller with the vacuum manifold.

These plastic manifolds crack over time where it no longer directs the vacuum to the proper vacuum motors.


How very interesting… I will check that out shortly. If I do not find deterioration the best way to test it would be to?

It has to be in the control manifold.

If the AC/heat blows out the defrost, the blend door vacuum motor is working, but vacuum isn’t reaching the mode doors.





Now, are you referring to the white fitting or the one with the stainless-looking hose?

Also, any idea if that wire that looks like its been broken that leads to the top of the engine might be a factor?

By the white fitting its plug has melted that hose just a touch, could that be another possible culprit?

There is one more photo I took but ill have to redo it

No! No! No!

The HVAC controller is what you operate on the dash.

See the vacuum hoses coming out the back?


Ok, im looking at it and I have 3 possible areas to check in on. Let me look behind the panel and see where they lead so I know exactly which ones they are.

Update: those look like the ones that run into the cab based on what I see from inside.

Your first two pictures are of a broken vacuum line, repair that so there is a vacuum supply to the controls on the dash.

oh… well then. Suppose that would do it. So, is it supposed to run over the engine or around it?a

Also, anyone know what the part is called or its number on rock auto or the like?

You might also consider the possibility of a failed one-way valve. That is item 14 in the picture and is usually located under the hood. Engine heat can often do them in.

IF that is the problem you can buy universal replacements at AutoZone,etc for a couple of dollars.

Hey, that diagram, what did you search so I can find the layout that went with it? Trying to figure out what to look up that line under and match parts up.

Is that the one that is broken or can you ID it from the photos?

Plastic line was used from the factory, you can replace it with common vacuum hose. Run a new hose from that check valve in the first two pictures to the intake manifold where the old line is connected.

Sure enough, 14 inches of tubing reconnecting the two, now everything works like a charm. Cost me $1.72 to fix. Cheapest fix I have ever had.