My sons 93 defroster was not blowing much.I found a vacuum motor with 2 hoses,red & yellow, located behind the ashtray.I disconnected the yellow hose and the defrost opened up.The hose has vacuum with the switch on defrost.Is my problem with the motor or the switch? Thanks

 

The main vacuum control on mine is under the hood at the firewall beside the fender on the passenger side. That vac hose on the control is connected to another line that runs inside a wire harness. I believe it is a white line. They are notorious for breaking at the harness. Just make sure it is connected on both ends. Sandy

 

The default (no vacuum) position for the system is defrost only. I spent a bunch of time on mine, not knowing much until somebody here posted the vacuum diagram for the HVAC system. If the 93 is like my '90 and '91, the vacuum comes in from the engine compartment on the passenger side, via small diameter tubing that is inside the wiring harness that goes through the firewall near the heater. After assuming that my control head was defective, removing it, dismantling cleaning and lubricating it, tracking down the vacuum motor you mentioned, and doing vacuum tests at all settings, I found I had a vacuum leak in the short section of tubing inside the harness. Ran a new line, and all problems solved. All the other work was just educational.
While you symptoms aren't the same as mine were, I would check for proper vacuum where that line enters the passenger compartment before tearing into other stuff, that is hard to access. The line (along with the one that operates the blend door in the engine compartment) comes in to the right of the glove box, black and white if I remember right, and there is a coupling that is easily disconnected to check for vacuum. I also pulled vacuum on the inside line with a pump to check my controls before running a new line.
Hopefully the person who posted the vacuum diagram will see you post.

 

His vac diagram should be on a sticker under the hood. If not it is available online.

 

The diagram under the hood is only for under the hood. You need the one for the HVAC. I printed out what I saw here, but it will take me a bit to find it.

 

https://www.ford-trucks.com/forums/1...-firewall.html

 

Here's the one for your truck:
https://www.ford-trucks.com/forums/1...m-diagram.html

 

The black and white coming through the firewall near the glove box (I took the box out for better visibility and access):
Black provides vacuum to the HVAC, white sends vacuum out to the blend motor. They are connected to a plastic junction block which is easily disconnected.
Check for good vacuum coming from the engine compartment on the black line. Mine was very low, so I checked everything under the hood, and eventually ran manifold vacuum to the black line where it entered the harness under the hood, confirming that the leak was in the foot or so of line in the harness. I had already spent countless hours under the dash.
To confirm everything, I ran a long length of tubing from under hood from a good vacuum source, to the junction block. Everything worked (I hadn't screwed anything up under the dash). I couldn't find a safe place to drill a hole on the passenger side, so I routed a new vacuum line through the drivers side to the junction block.
If I had checked for vacuum at the black line near the glove box first, the whole repair would have been about an hour, instead of days.

 

Thanks fellas,thats a lot of info.I think i am getting vacuum, because when i pulled the yellow hose OFF, the defrost opened and i could feel vacuum on the end of the hose. I think our problem is in the switch. I have a parts truck, how hard are these to swap?

 

Not all that difficult, although a pain in the butt due to having to disconnect things without much slack. I think the cable attachment was the most awkward. Couple of electrical plugs, and the vacuum attachment.

 

Thanks for the help