We have a 1999 F250 SD 4x4 crew cab with a 7.3L. It is new to us and has a few issues i need to fix.

One of them is air from the a/c doesnt blow out the dash vents, just out of the defrosters, no matter what position the knob is in.

The knob is turning the control. But the air doesnt move from defrosters to dash or floor.

I am guessing this is a vacuum problem, but not sure exactley what controls this or if there is a common problem on these.

The hot/cold control seems to be working correctly.

Thought and ideas would be appreciated.

Also is there a place on the sight to buy factory Ford service manuals for this? I saw some on e-bay but i dont know what a set costs new or used.

Thanks, Ken

If you have auto locking hubs, the seals may be leaking. This is a common problem. To isolate the hubs as your problem, remove the vacuum lines at each hub and plug them - then see if the a/c works properly.

Definitely a vacuum leak-- those controls operate on vacuum. The temperature knob is connected to a blend door via a physical cable, no vacuum, so that's why it still works.

There is an electric vacuum pump somewhere...it defaults to defrost from a safety aspect...(winter), I have the same problem in the 750 but it is intermittant, only seems to do it when it is very hot outside.

DNFXDLI said:

There is an electric vacuum pump somewhere

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Atop the inner fender on the passenger's side.

Bricks said:

Atop the inner fender on the passenger's side.

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I can't find the damn thing on the 750...no fenders, hood lifts forward!

Thanks for all the fast replies. I have to find a manual for this thing.

I checked the vacuum pump on the fender. It is running and pulling vac.

The line comes out of it and goes into an oval shaped canister. I dont have vac on the other fitting, what i am guessing is the output side of the canister. There maybe anther output i dont see.
This line comes out of the canister and t's off, one side goes to what looks like a selonoid on the inside of the fender. The other runs someplace i cant get to right now.

How do the control switches come out of the dash? Or gain access to the backside of the control switch. I dont want to force something and break it.

Thanks, Ken

kdv said:

Thanks for all the fast replies. I have to find a manual for this thing.

I checked the vacuum pump on the fender. It is running and pulling vac.

The line comes out of it and goes into an oval shaped canister. I dont have vac on the other fitting, what i am guessing is the output side of the canister. There maybe anther output i dont see.
This line comes out of the canister and t's off, one side goes to what looks like a selonoid on the inside of the fender. The other runs someplace i cant get to right now.

How do the control switches come out of the dash? Or gain access to the backside of the control switch. I dont want to force something and break it.

Thanks, Ken

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I'm not sure, but I would guess that the dash trim comes off like the old trucks. I have no idea what the controls would be like, the old ones routed the vacuum through a series of passages...hopefully someone will chime in.

the vacuum line should go to what looks like a plastic flute, under the dash and then to the dash pots. that flute-looking thing is a pneumatic slide that uses a low-volume vacuum to interface the full vacuum with the control. Very often that slide will break.

the best thing to do is eliminate the 4x4 solenoid first. these pieces are bad about leaking. the solenoid is right behind the battery on the right side, its black with a white cap and has 2 vacuum lines going to it ( red and green if i remember correctly). plug the one that has vacuum on it or use a vacuum pump to measure the amount the pump is pulling down 18 - 25 inches. see if this helps.

DNFXDLI said:

I can't find the damn thing on the 750...no fenders, hood lifts forward!

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look on top of the evaporator core.

WOO HOO!!! I am glad i forced myself to stop messing around with this untill i had a manual or new what i was looking for.

It looks like the problem is between the selonoid and the hubs. I pulled the lines from the selonoid and plugged the red vac side.

The A/C, heat, defrost, etc. all works like it is supposed to now.

I noticed last night that the auto hubs didnt seem to be locking in when i was trying to check things. I will have to look at the lines and hubs and see what needs to be repaired.

Thanks to everyone for your help!

I am sure you will be hereing more from me. One problem at a time.