Thunking in 2WD

[Greg C]

I've searched prior posts but have not discovered any similar problems, so I'm asking for help..

My X is a '95 4WD XLT with limited slip in the rear. In 2wd, under slow acceleration, it makes a thunking sound up to about 20-30 mph. It also feels like the truck slows down slightly. When I engage 4 auto, the sound goes away. It appears to do this more when the truck is cold (not operating for ~4 hrs but in 70 deg air temp).

It seems like the front driveline is being engaged in 2wd.
I am guessing that either two things are happening: 1) the transfer case is energizing the front driveline while the driveline is not spun up, or 2) the front axle lock (not sure of the correct name but I know its vacume actuated) is not fully disengaged in 2wd.

Anyone know how I could disable the transfer case to see if the problem goes away? Its a Borg Warner 44-05, according to the manual.

Regarding the front axle lock, I'm getting vacume to it for both 2wd and 4wd. I also bypassed the vacume switch by running a vac line straight from the vac manifold to simulate 2wd setting. The thunking was still there.

I'm sure you guys will come up with something!
Greg

 

[Robb]

Greg,
Try a search for autohub,auto hub,or POS autohub. Most people on this forum (I believe) would tell you that the X autohubs are JUNK. I remember a thread just last week pertaining to this issue (I remember because I was in it, discussing how my F-150 did the exact same thing). I don't remember the name of it though. But, believe me, there are plenty of threads on autohub problems.

BTW, Welcome To The Forum! Beware of BUG!

Robb

 

[Greg C]

Thanks for the clues and warning Robb, I'll check it out.

 

[Greg C]

Need more help!

Ok, I've checked prior posts and most refer to vehicles with auto hubs on each front wheel. I think my 95 only has one "auto hub" between the front diff and the right front wheel.

Is it common for this hub to not fully disengage? Can I just replace it or remove it?

I've pulled the electrical connector off of the transfer case and the thunking problem is still there, so I think the problem is the auto hub.

thanks....

 

[Greg C]

Just talking to myself now...

I pulled the auto hub (Ford calls it the Motor and Cover assembly). A vacuum driven piston moves a collar to connect the output from the front diff to the rt side half axle. It works fine. When I pumped the fluid from the front diff I did notice flakes and chunks of metal .

Feeling brave, I jacked the truck up with all 4 wheels off the ground (using jackstands of course). In 2wd (the auto hub was open and manually moved the collar to ensure 2wd) the front driveshaft would spin slowly then the thunk would happen and the front driveshaft would spin up faster. I believe this tells me the transfer case is engaging in 2wd. I've pulled the electrical connector off the transfer case, but the thunking still happens. There are some electrical conductors that bypass the connector and go straight into the transfer case. Any ideas? I'm running out of ideas....

 

[Tom Wilk]

Greg,

It does sound like the transfer case is engaging intermittently in 2wd. I had forgotten that the earlier control-trac had a 2wd setting and a vacuum disconnector in the front axle.

If I"m not mistaken, there's a thread here somewhere where some people were trying to prevent the 4wd from working in their later control-trac explorers. I don't know which wire it was, but there is a wire that if you disconnect it, will prevent the transfer case from engaging the front axle.

That is, unless there's something mechanically wrong with the transfer case. What could be wrong? Well, this is jogging my memory here so I could be completely off base (and my notes are still neatly packed away somewhere with a bunch of my office stuff). Anyway, I think that the clutch in the transfer case that engages the front axle is electrically actuated, but has a spring that returns it to the disengaged position. Now, if that spring were broken, it would perhaps sporadically engage and disengage the front axle as the mechanism floated freely with no spring to contain it. Just an idea.

 

[Greg C]

Tom,
Thanks for the comments. I'll do some more searching. Although I did pull the connector off the transfer case, it may still be engaging mechanically like you said, or maybe (a longshot) electrically since there are hardwired electrical connections that bypass the connector.

I'm starting to think its the front diff. I did see chunks of metal when I pumped it dry. I also have removed the front driveshaft and the clunking goes away in 2wd.

Repair of the front diff is going to be costly since the front axles need to be removed then the diff lowered down (the front frame covers the diff cover bolts).

 

[Greg C]

Bump!
I've pulled the front driveshaft out and have ignored the problem for too long.
Anyone know a good source for transfer cases?
Any opinion on rebuilding vs replacing?
Anyone have a manual for rebuilding?