[Willys2TJ]

I had both front tires off the floor the other night and noticed that when I turned either wheel, the driveshaft going back to the transfer case was turning. Is this normal? I thought that the vacuum motor disconnected the wheels from the differential and that they free-wheeled when not in 4WD. The Vac.motor must be working because I've had it in 4WD w/no problems.

Also, I left the right side off the ground overnight so the axle was leaning towards the drivers side. There was a puddle/spot of diff fluid about the size of a silver dollar under the wheel area and I could see where it had dripped from the steering knuckle. I'm guessing that the axle shaft seal is going bad. Question is: Is this sort of normal or does it need immediate attention. It appears that both sides may be leaking some from the look of oil on both knuckles. The diff fluid is filled up to where it starts to run out of the fill hole on level ground.

Jeep has 90K miles, I-6, auto trans.

Thanks for any input.

[duffy]

There are a lot of people in this forum that know way more about Jeeps than I do. I know that on my '89, when I am moving forward (in 2H), the front driveshaft turns as well. I asked the same question and was told that this can only happen if you have lockers installed, have a one-piece axle, or the angle of the drive shaft is a little off and is in a bind between the front axle and transfer case. All that said to say this, mine does it too and I would like an answer that I can comprehend as a Jeep dummy.
Thanks

[sentinal02]

Originally Posted by duffy

There are a lot of people in this forum that know way more about Jeeps than I do. I know that on my '89, when I am moving forward (in 2H), the front driveshaft turns as well. I asked the same question and was told that this can only happen if you have lockers installed, have a one-piece axle, or the angle of the drive shaft is a little off and is in a bind between the front axle and transfer case. All that said to say this, mine does it too and I would like an answer that I can comprehend as a Jeep dummy.
Thanks

you need to understand how the open differential works to understand why your front DS spins under forward motion. But I'll get to that in a minute,

Mark, if you can turn your front DS (drive shaft) by turning the pass side wheel, your shift fork is not disengaging the two piece shaft in the axle. in 2wd, there should be no connection between the inner and outer axle shafts and thus no link from the tire to the DS. if you can tunr the tire and thus turn the DS, then you have a connection. Now, on to the science lesson

see here for lots of cool graphics and I'll try to tie in the jeep's setup to the pics

http://auto.howstuffworks.com/differential.htm

First off, Duffy is partly right when he mentioned binding at the DS. under no load conditions (2wd) there is only one power input into the system and that is the driver side wheel. remember, the vac disco means that the passenger side shaft can spin however it wants, forward, backward and at whatever speed it wants (because it's an open diff). likewise, the front DS is also free to spin forward or backward becuase the t-case effects neutral for the front in 2wd. Now, take a look at the pics under "Spinning at different speeds". The jeep's setup matches the pic, so that the left shaft is the driver side and the right is the pass. see the two red differential pinions in the second and third pic? what would happen if the ring gear were held tight in those pics, while the left shaft was turned? assuming that the right shaft was totally free (which in the jeep it is) it would rotate in the opposite direction that the left shaft would spin. just look at one gear if you have trouble picturing it, i know it's tricky when you see the pics moving. but trust me, if you hold the ring gear steady, the two shafts will spin in opposite directions. this is what happens if you have a bit of binding from the u-joints in the front DS. that bind transmits through the input pinion to the ring gear, making it easier to transmit the force to the inner half of the passenger shaft which just has to deal with bearing friction holding it back. also, remember that the gear reduction in the differentials (those 3.07, 3.73 and 4.10 numbers you're always hearing about) gives any binding in the shaft the mechanical advantage. in other words, if you put the same amount of holding force on both the front DS and the passenger shaft, the passenger shaft will still turn first because it takes the tire 3 to 4 times that holding force to move the DS but only the same amount to move the pass shaft because of the reduction.

now, in the event that you have a really free DS with good bearings, no binding at the u-joints and smooth operation inside the t-case, it's possible for the bearing friction on the passenger shaft to be the greater holding force, even accounting for the advantage. In mark's case, this is happening because the pass side tire is still connected to the inner shaft for some reason, and it wants to turn the pass shaft in the SAME direction as the driver side. in order for that to happen, the ring gear must spin. and because the ring gear is spinning, the input pinion spins which in turn moves the DS to spin. Does that help clear things up a bit?

[Willys2TJ]

I was thinking that maybe the vacuum motor wasn't releasing and your reply gives me some info to help troubleshoot, Thanks.

I'll hold the DS in place and try to spin the right front wheel and see if its just the various frictions turning the DS or if there is a mechanical connection. If its mechanical, I'm guessing that the vacuum motor and/or connections is the best place to start.

This vacuum motor seems like one of those things that must have looked good on paper or in the lab but not worth a crap in the real world.

I don't think I'll worry about the seals. After searching the archives, they seem like a lot of work to change and I only drive this thing about 3-400 miles a month. Maybe a winter project.