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Discussion Starter #1 (Edited)
I have a 99 F350 crewcab PSD, 4X4 manual locking hubs and a 6 speed manual trans. I just got the truck a little over a month ago. The truck has 79,000 original miles and I got it from the 1st owner.
No matter where the switch is pointing it blows up through the defroster. no matter if its the air conditioning, heater or just the vents it just blows up the defroster vents on the winshield.
I recently read a thread on here about either a blown fuse or a vacuum hose or pump causing the problem. If memory serves me correct, the writer was saying something about the vacuum hose leaking, but referred to the auto locking hubs as a problem. Any:dunno: help would be great. Thanks
Sorry guys forgot to add that it has a build date of Aug of 98. I don't know how to locate the exact date. It does have the fender air draw and there isn't any foil on the intercooler. Thanks again:icon:confused:

I found the problem. The vacuum pump thats on the passenger side engine compartment behind the battery. I was just checking on vacuum lines and switching the vent control switch back and forth. I just tapped the pump with a wrench and now it works. Tried it several times today and still working OK. I also noticed that my over head display that reads the MPG, is reading more miles per gallon. I take the same route into town and drive the same speed but for some reason instead of the display reading around 16 MPG it was reading almost 18 MPG. Just curious if that vacuum pump controls something on the motor. Thanks to all who gave advise.
 

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I had the same problem with mine and it cleared up on its own...it was wierd one day it was fine the next nothing but defrost...then it started working

Luke
 
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John Wood Nut Swinger
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when you have the key on engine off(KOEO) and hood open. Move the switch from defrost to face vent, you should hear the vacuum pump come on, it is under the battery on the driver side. If you move the switch around and no sound of the vacuum pump, check the fuses.
 

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Discussion Starter #4
I checked every fuse under the dash, I'll try to hear if the vacumn pump does anything .Thank guys
 

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My vacuum pump is pretty loud, I can hear it in the cab with the doors shut. 4x4 only uses vacuum for a minute or so after you flip the dash switch so if it is blowing defrost all of the time and not just when you switch 4x4 then 4x4 aint the problem, unless the 4x4 regulator/solenoid is messed up, but then the vacuum pump would pump all the time. I’d try finding the pump and making sure it is working first. Seems like mine is on the passenger side fender near the map sensor.
 

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You've got a fuse box up under the hood too by the way (although I'm not sure if any of those are related to the pump).

Also, if your pump was out I would think that your brakes would not be working properly since isn't that where our trucks get their assist from?:dunno:
 

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You've got a fuse box up under the hood too by the way (although I'm not sure if any of those are related to the pump).

Also, if your pump was out I would think that your brakes would not be working properly since isn't that where our trucks get their assist from?:dunno:
Power brakes comes from the power steering pump, vacuum only operates AC controls and SOTF 4x4.
 

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another area you can get vacuum leaks is up under the glove box where the lines for your heater controls come in, especially if you have a passenger with big restless feet. the most common would be the 4x4 lines or hub seals if you get the problem when in 4x4.
 

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yea make sure the pump is coming on my 01 did that and the pump wasnt kicking on i gave it a good shake and it started working but if its shorting out i would replace it because that also controls your 4wd and when it gets cold it tends not to work and that sux when it is snowing outside and you have no front feet
 

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Check vacuum switch! I had this twice....

Theres a vacuum operated switch on the pass side firewall. Mine has gone out 2x in 5 yrs. mines a 6-98 production 7.3 f-250. Some people call it a 4wd actuator too. Its a simple swap. part runs about $30 from dealership.
let me know if this works for ya..

[email protected]
 

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Discussion Starter #11
Thanks for the info. I'll check it out this morning. The pickup has manual locking hubs so there shouldn't be any problems there.
 

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My vacuum pump is on the passenger side fender and the ABS pump is under the drivers side battery? Is there an early early 99 (2/98)?

STROKER
 

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lewiston montana?

I flew in there a few years back for an elk hunt. Must be nice living in gods country....
 

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Discussion Starter #14
Yep it sure is. A local saying goes"THE LAST BEST PLACE" that is as long as your not living in Bozeman, Missoula, Big Sky, Whitefish. Where all the greenies and hollywood types live. Retired out here almost 9 years ago. 4 wheelin, ride snowmobiles,hunting and other outdoor stuff you couldn't live in a better place. The local wages suck big time though.
 

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vent control switch

I have a 2003 F350 4x4 6.0 with the same problem. The Ford dealer told me it was a vacuum cylinoid, they ordered the part but it hasn't been put in yet. I am going to try these other things first. I also have trouble with my four wheel drive going on (it does it when it wants to just like my vent switch turning from defrost to vent, mostly in defrost mode)
 

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I have a 2003 F350 4x4 6.0 with the exact same problem, stuck on defrost mode. The Ford dealer told me it was a vacuum cylinoid, part is order but haven't had it installed yet. Does anyone know if it is easy to install? I am also going to try these other things before I fork the money over to them. Also, my 4x4 is acting up, it works when it wants to just like the control switch.
 
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