the hose leaving the front differential is a vacuum tube? and i cant find the part in a diagram.
thanks

 

What year? Some early years have a vacuum disconnect front axle. That vacuum hose would be off to the side of the differential. There is also a breather tube located nearly center-top.

 

Yeah we can't really help without the year of your Jeep.

 

Where is it leaving the axle housing from? A picture would help...

If it's leaving from near the top of the centre section, and looks to be about 5/16" ID, then it's a vent line that goes up into the engine bay. It's there to allow air to move in and out of housing as it heats up and cools - or you'd blow seals.

If it's (more probably "they're") leaving from the passenger-side tube, is a small diameter Nylon line, and is probably three lines in three different colours, it would likely be the axle shift motor on the earlier (thru 1992?) XJ/MJ, used as the CAD (Central Axle Disconnect) in an effort to improve fuel economy in 2WD by reducing parasitic driveline drag. (This was eliminated later, simply because it was shown to not make that much of a difference.)

But, vehicle information and a picture would be a huge help here!

 

2001 the hose does go up in to the engine bay,it is connected to the upper part of differential on the driver side, the lower half was hanging on something cause it has just about melted in half

 

Breather tube. Replace with standard rubber tubing, re-using the vent at the top. Zip tie it away from heat and moving parts.

 

Thanks bud

 

I just noticed the same thing on my '01 Limited XJ when I was under it day before t-giving. I had no idea what this tube was for but my front drive shaft joint wore a hole in it. May sound like a dumb question but why does the front differential need a breather tube? I cannot find one on the rear, nor on my other two 4x4s.

 

To account for pressure differences between the outside and inside of the differential. The moving gears, and altitude changes create a pressure differential, and if there was not a vent, the axle seals would leak.
The tube itself moves the venting location higher off the ground, so crap doesn't get in the hole and get stopped up. It's very important if you drive in water to make sure the tube is there, and in good shape. Otherwise, you get milky diff fluid, which is bad.

Edit: The rear diff has one, but it is on the axle tube, IIRC. Since the seals are at the wheel end of the tubes in the back, they can move the vent away from the diff, to simplify the casting

 

The "moving gears" indirectly cause increased pressure in the case - by generating heat. But, you're otherwise correct.

The only thing I really don't like about the PRV fitting on the case is that it's Nylon and breaks easily - sooner or later, I'll D&T for a small pipe thread (probably 1/8" NPT) and do a brass fitting there so it doesn't pop loose.