The truck I'm working on is a 2003 F150 King Ranch 5.4 4x4. I'll try to describe the issue as best as I can. I've added pictures with descriptions on each.
The original issue was the usual vacuum problem causing the heater/AC to only blow from defrost. While diagnosing that issue I found, what looks to be, a vacuum line that is broke/disconnected. I have Identified the line but can not find a missing line or connection.
In the pictures you can see the black plastic tube I'm referring to. It is connected to what looks like the vent tube of the front differential. I have checked every connection, connector etc and there is no place for this tube to connect. I have vacuum checked every tube individually, each is good and drop tests well. I pressure tested the tube in question and it only achieved -10hg (all others reached -21hg) and held drop test fine (1hr no loss of vacuum). This is the stock setup.
I can find nothing on this tube, all help would be appreciated.
If this is the venting for the front diff why does it route to the vacuum cluster? Why would it change over from rubber host to rigid plastic vacuum tube?


Notice black tube in front of red tube.

Another pic of black tube. Notice it goes into bundle with tubes that operate front 4 wheel drive.

Shows connections at vacuum reservoir, all good and hold vacuum when each drop tested separately.

This pic shows bundle of tubes (including the black tube from previous pics in question) going into 3-way connector on frame below transmission fill/check. At connector black tube changes to 1/4" rubber vacuum tube. Rubber tube connects above vacuum actuator on front differential.

Another picture of the connector. This one shows the black tube change over to rubber hose.

On top (with the new clamp) is the rubber hose that the black plastic tube connects to. Below the axle shaft is the front diff vacuum actuator.

 

That top tube is a Axle Shaft Vent. It regulates the pressure in the Axle Differential. I don't think it goes all the way to the Top. At lunch I will check to see where mine is running to.

 

yes its a vent tube according to the Haynes manual.

 

It is indeed a vent. But "regulates" would imply that it is maintaining a certain pressure. It's just simply a vent.

 

If it was closed the pressure within the Differential would get higher once it got warmed up causing leaks, or it would create a vacuum once it got cold. Therefore the Vent tube regulates atmospheric pressure within the Differential.

 

Thank you for the information and identification. Looks like I'm heading out to buy a Haynes manual. I searched here and Alldata, found nothing relevant.

 

No, it's a vent. Period.

 

As per Google...
A differential / axle breather serves the important purpose on a 4WD vehicle of venting the air pressure in drivetrain assemblies such as the differentials, transmission and transfer case. ... When the drivetrain assembly cools down the air inside contracts and creates a vacuum.



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The Axle vent tube does not plug into to anything. Its just a open end tube by design. It allows the air pressure to escape when the air expands and contracts during heat and cold cycles. Its connected correctly on your truck.

 

Thank you for proving my point.