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Discussion Starter #1
It started with my AC not working while I was at a stand still, say in traffic. It would then work when I got back on the gas. But about a week ago air just stopped coming out of all the vents and would only come out of the defroster vents. No matter what setting it's on, or speed, it all comes out the defrost vents. So if I turn the AC on it will eventually blow cool air through the defroster vents while I'm driving, if I come to a stand still the AC air coming through the defroster vents will not be as cool, more musty. This is what it would do through the 4 dash vents before it stopped all together. What could be the deal?thx.
 

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It defaults to defrost when the motor/vacuum that turns the vents fails. Its vacuum controlled. The vacuum lines are under the hood, but I have no idea where.
 

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WFO 4life
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Passanger side fire wall over the valve cover. Black cylinder with a plastic nipple. The gray hose with black rubber tip goes on the nipple. The hose/tube is about the dice of a straw and is flexible. Push That's on the nipple and all functions should come back. If not your vacuum pump is toast.
 

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Discussion Starter #5
Passanger side fire wall over the valve cover. Black cylinder with a plastic nipple. The gray hose with black rubber tip goes on the nipple. The hose/tube is about the dice of a straw and is flexible. Push That's on the nipple and all functions should come back. If not your vacuum pump is toast.
That is exactly what popped off easy when I started moving hoses around to get a look at the valve cover. Took a pic of it but obviously not needed you guys are on it.

Great write up. Thank you!
 

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40 lug nuts
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It may not be the vacuum pump or that disconnected hose. A vacuum leak anywhere in the system will cause the exact same symptoms. The bad news is that if you have ESOF 4x4, the hubs are also controlled by vacuum and the troubleshooting is more challenging.
 

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the valve u plugged in is the heater bypass valve it is sent a vacuum signal when cool is on max and stops the flow of coolant to the heater

u can usually tell when the ESOF is the culprit because it will only make the vents fail when the dash control is actuated and it will only last less than a minute before it recovers and goes back to normal
 

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Discussion Starter #8
what about this black plastic piece, i noticed it moving around freely. concern?

Thanks Snake, if it's anything like that I got a guy to call.
 

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Discussion Starter #9
the valve u plugged in is the heater bypass valve it is sent a vacuum signal when cool is on max and stops the flow of coolant to the heater

u can usually tell when the ESOF is the culprit because it will only make the vents fail when the dash control is actuated and it will only last less than a minute before it recovers and goes back to normal
This happened through the dash vents before it quit coming through them all together.
 

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Discussion Starter #10
So plugging that hose in doesn't seem to have worked. About 4 weeks or so before I started noticing an issue with the air through the vents I noticed that the vacuum pump had been running after I turned the truck off, it doesn't do it anymore, normal?
 

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Discussion Starter #11
Passanger side fire wall over the valve cover. Black cylinder with a plastic nipple. The gray hose with black rubber tip goes on the nipple. The hose/tube is about the dice of a straw and is flexible. Push That's on the nipple and all functions should come back. If not your vacuum pump is toast.
So that grey hose did nothing, if I replace the vacuum pump isn't what caused it to go bad gonna rear it head again on the new vacuum pump? Thanks
 

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Discussion Starter #12
The fuse #27 had dirty 'pegs', not sure what they are called, but the metal A shape piece inside the plastic was in tact.
 

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Our trucks are also notorious for the Auto hubs not engaging 4x4 because of vacuum. The vac lines run all the way to the front hubs. Those lines will degrade and cause these issues also.
 

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Discussion Starter #14
The hubs say auto/lock on them. Left arrow auto, right arrow lock. Since I can remember if I had to use 4lo or 4hi the hubs never locked on their own I have always had to get out and turn the hub to the lock position, usually easily done with my hand. Am I confused?
 

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bump. am i confused?
Sounds like it!

The hubs were probably leaking somewhere from the sounds of it and it caused the pump to run excessively and burned it up. Get a hand vacuum pump Mityvac and hook it up to the side that goes to your hvac see if putting some vacuum to it fixes the issue. If it does then you know the vacuum system is good on the hvac side. Dont bother trying to 'fix' the esof side. Just get new line and replace the old sh!t. If that doesnt fix it cap off the system and keep engaging the hubs manually.

Good luck! A bs job that nobody seems to understand but is relatively simple to check.

P.S. the vacuum troubles you are having are completely independent of your a/c going from cold to just blowing air through the evaporator. Sounds like a low pressure switch or a compressor clutch problem.
 

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the hubs aren't under constant vacuum it's just a short signal sent to them when u select 4x4 so it would only cause a brief issue like air going to the defrost instead of vents ( under 1 minute ) in the hvac if those lines were compromised unless the ESOF valve faild to seal and thats easy to bypass since yours isn't working anyway
 

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Discussion Starter #19
So while I was at the dealership to pick up my degas bottle I talked to the service manager and he came out to look at the vacuum pump. He noticed this contraption was disconnected, the red and black hoses had broken off inside the black rubber grommet. He removed the hoses and gave me back the piece and I left, I reinstalled at home to no avail. Air still coming through defrost, visually inspected lines going to hubs, look fine. The air coming through the defrost is AC if I put it on AC setting and not AC if I turn it to regular air. I haven't had weather to see if the defroster setting would work, do you guys know if it would and could this vacuum issue compromise my 4hi or 4lo? Thanks !


 
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