JTX150

Hey guys,
So I recently bought a 93 F150 4.9 I6 with 96XXX miles. It's a base model XL. Anyways, when I bought it, the AC would blow through the defrost whenever I would accelerate. So I knew there was a vacuum leak and quickly found that it was the white vacuum line that runs on top of the blower motor housing. I replaced that and the ac works as it should now.

The problem I'm having now is that a little bit of air leaks through the defrost vents no matter if I'm sitting on idle or driving. The AC still blows through the front vents normally as it should, but I guess some leaks through the defrost. It's just enough to barely feel it when you put your hand on the defrost vents. Normally It wouldn't bother me, but with our weather here in South Texas, it fogs up my windshield a bit in the mornings and such. Any advice on what I should look for?

Thanks!

wde3477

Welcome to the site!
Suggest you either still have a vacuum leak not allowing the particular vacuum actuator develop full force, or have a binding in either the vacuum actuator internals, linkage, or damper itself not permitting a full seal. Suppose there's a possibility that the main HVAC control head has a problem, but I haven't heard of anyone having this issue.
If you can lay your hands on a manual vacuum pump such as and connect it directly to the defrost actuator to see whether it holds vacuum and whether the observed defrost air leakage changes - could speed which direction needs to be chased.
Good luck, and keep us posted.

JTX150

I was also leaning towards a small vacuum leak somewhere. Do you know where the defrost actuator is located? I've tried reading the vacuum line diagram on the inside part of my hood but I dont understand those things one bit.

wde3477

Eh, no, never had to really fool with that part of the HVAC, and I no longer have the shop manual which might have an expanded view.
Best I can recall, there are two actuators - one for defrost / floor, and the other for vent / floor, er, or something like that. If your luck runs like mine, if you find one, the one you want is the other one. :-)
No idea where under the dash they're located - removing the glove box is easy enough to look in from that side, not sure whether anything of interest can be seen looking up past the accelerator pedal. Pulling the radio may be the ticket for a peek?

JTX150

Ok great. I might look around and see what I can find. I already took off my glove box the other day and didn't see anything. It might be a while until I find it sense I'm not really in a hurry to fix it. It is annoying, however, in the morning having to use my wipers

stxlt

The actuator where you fixed the white vacuum line is for the Blend door; whether defrost, floor, or dash vents operate. As you found, the door defaults to Defrost without a vacuum signal present. With some vacuum present, you can gently, manually stroke the throw of the actuator and listen to the blend door move and feel for binding as it operates. As wde3477 stated, linkages or binding of the blend door may not allow it to fully close and seal. Any partial loss (reduction) of vacuum signal will reduce the stroke of the blend door and allow it to lay open with air escaping through the defrost vents.
The fresh air/recirc/Max AC actuator is below the ashtray. This is the actuator that controls whether the air goes past the heater or AC coils during reciculation or is drawn from outside as fresh air.

Not knowing what you know...first things first: this HVAC actuator system operates through vacuum, a mechanically-caused suction of air within this closed system. There is no open air source in this system unless a crack or other opening has developed. The engine's intake manifold is the vehicle's source of draw for vacuum. The actuators are near the other end of the HVAC control vacuum system branch of the vehicle's vacuum system.

To find leaks into cracked tubing, broken connectors or dry rubber boots, follow the vacuum lines back toward the intake, spritzing starting fluid on the lines and fittings, with the engine running. I start at the blend door actuator and work toward the manifold, spritzing in the actuator's direction as I work toward the manifold. Use small bursts of starting fluid sprayed through a straw. BE DELIBERATE where you spray. You are looking for small leaks; not fogging for mosquitos. The starting fluid will be drawn through a crack or void in the system and cause the engine to rev slightly. When a broken area is found, replace the entire section to the fittings on each end or the individual fittings.

**FIRE is a possibility! Small bursts of fluid! Be mindful to have your head and face clear. It is best to check your ignition system the night before for stray spark that could ignite the starting fluid. And a cold engine running after it comes down from high idle is best; no hot exhaust manifolds.**

Watch here for other, better and safer ways to find vacuum leaks. There will be comments!

If you search here on the site, there is a great write-up on the entire vacuum system that can answer questions. I don't have the link handy; it is on my other computer.
www.fordparts.com >VIN has Exploded views of every detail of these trucks!