have you checked to be sure the vacumm pump is working?

Fuse or ???

Does the damper in back work?

You might be able to hear it if it is a leak with the key on engine off??

I think is on the right fender linner very close to the little valve we unplug when we want 2WD-low


Keep asking

 

Mine is at the dealer right now for a vac pump i would check it out first if you pull the vac line off and pump should run. Use test light and you should have 12v with key on. /ubbthreads/images/graemlins/biggrin.gif

 

I've got the same issue on the 99 F250. My four low has also been out for a few years now.

 

Bruce,

exactly right on the location of the vacuum pump and of course the solenoid that controls the automatic hub lock/unlock vacuum.

I would check first to see if the pump is running on startup. If it is, then start checking for a good pump, by pulling the lines off the pump and using the fingertip gauge to feel for a vacuum from the pump.

If the pump is making vacuum, try disconnecting the electrical plug that goes to the hublock vacuum control solenoid. Then shutoff and restart. If this eliminates the vacuum leak, you know it is somewhere in the vacuum hoses that go down to the front spindles or knuckles.


The low range on an ESOF 4x4 (all Excursions) is controlled by an electronic stepper motor on the transfer case. In the ESOF 4x4, only the automatic locking hubs are controlled by vacuum, but you can always turn the dial on the hub and change them from automatic locking/unlocking, to manually locked in.

 

OK, I'm back home where I can crawl around the truck and have dependable internet access.

The vacuum pump does't make any noise with the key on. When I pull a line off of the pump and turn on the key and flip the ESOF to 4x4 I get nothing. No noise or air movement. I assume this to mean the pump is not working. Where would the fuse be for this pump? Want to check that before I drop $105 for a pump.

Also, you mentioned the rear air. It's working normally, thank God.

Thanks for the help.

 

The rear air/heat does not have any vacuum controls.

The front unit uses vacuum to control outside air versus recirculate (when there is no vacuum, it defaults to outside air).
It is also used to control the air direction to the floor/panel/defroster outlets (when there is no vacuum, they default to full defrost). The hot/cold blend doors are electronically controlled by a stepper motor.

 

Thanks gentlemen. One new vacuum pump later and everything is back to normal.

 

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The rear air/heat does not have any vacuum controls.


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Should have known that but we all come here to learn and contribute

Thanks

Bruce

 

[ QUOTE ]
Should have known that but we all come here to learn and contribute

Thanks

Bruce

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Ditto, Back at you spring king!

They use vacuum with spring return to get a 'safety fail mode' sort of effect. Default to outside air, and defrost.

 

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They use vacuum with spring return to get a 'safety fail mode' sort of effect. Default to outside air, and defrost.



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Now that I did know to tell the truth I worked as an engineer at Ford Heavy truck in Body-Electrical secton HVAC group for a while after school, they used servo vacuum back then it worked but only because of the temp feed back loop this is why they were reluctant to put actual temps on the dial since each sytem would be a bit different. Now talk about finding vacuum leaks those tripple (left right and rear) ATC systems were fun, three of everything except the compressor and condensor.