Can the front axle engage by itself?

Coming home tonight about 100 yards from the house a large clunking noise started, I stopped, it sounded like the four-wheel drive was trying to engage, I rolled along slowly as it clunked, so I actually shifted into 4H, (NP 231, automatic transmission) and the noise went away as I drove along. Then I shift into 2H and there was a grinding noise, upfront. Got to the house left it in 2H and backed into the drive-way.

Can the vacuum unit on the front axle engage by itself due to a vacuum issue of some type? Also, on occassion the part-time light will come on at randon. I assumed the switch that turns the light on was going because I did not touch the transfer case shift lever when it came on.

I did a search but probably did not know what I was really looking for, lots of posts for clunk etc. Most happen when turning, mine only occurs when driving with the wheel straight. Thanks for any help on this issue.

I should mention it is a 1988 XJ, all stock, 160 k. No use off-road as far as I know.

sounds more like a hub unit bearing

Well, I went outside and shook the front wheels and no play at all. No sweaking when I drive, very minor shimmy at 50 mph but very minor and I believe my tires need to be balanced.

I'll jack it up this weekend and check it closer, just not really sure what to look for.

Check the linkage?

If 4wd is engaging, that has more to do with your t-case than the vaccum disco. Consider that later XJ's got rid of the disco all together.

Check and tighten your linkage for the t-case shifter.

Don't know if you found your problem, but for my two-cents worth, I don't think it's a vacuum leak. I'd look to improperly adjusted linkage with transfer case lever. Clunk..or grinding might indicate partial engagement (i.e. linkage adjustment). A simple Haynes manual has pretty good instructions on proper adjustment.

Let us know if you figure out the problem.

Re: Clunking gone and so is the part-time light

Well I finally managed to look at my transfer case linkage. I've only had this jeep for a few months and I'm still learning about the particulars. The linkage looked fine, it was tight with no slop at all. The whole area was soaked in gear oil. I checked the fluid in the NP231 and it was basically empty, I drained out 1/2 pint of nasty gear oil and refilled with Mercron to the correct level. Cleaned everything up and drove it down the road to a deep area of sand. 4X4 worked great, easy to shift in all gears 4h and 4L. No clunking or any bad noises all was normal,

Except this: my part-time light use to come on whenever it wanted, on the freeway or in stop in go traffic, the jeep was always in 2H. Now the part-time light does not come on at allM even in 4H or 4L. The four-wheel drive works great, and yes I'm sure, I put it in 2H and almost got stuck in the sand leaving to head back to the house. I climp a step bank and the 4L was perfect, just no light.

So with all this said...... I think the clunking was due to a hot transfer case? No idea about the part-time light. Haynes said to use Mercon, kinda odd using ATF instead of 90W gear oil. Previous owner used 90W so I hope there is no damage.

my part-time light has not worked since I had the transfer case changed (used one installed). My 4x4 works great just no dummy light to tell me that it is in 4 (guess I will just have to notice the handle pulled up :eeks1: )