4x4 drive line thumping in 2wd only?

[bajawind]

1995 Mazda B4000, ext cab, 4.0L v6, 5 speed, 4x4 with auto hubs and transfer case. 150,000 miles.

In 2wd at about 40 mph (or sometimes when I hit a bump) I get an persistent thumping from the drive train, it sounds worse at slower speeds, like something is almost binding--not quite the sound of mashing gears.

There is a definite clunk when the noise starts.

As i am driving the noise stops when i switch to 4wd, but as soon as i switch back to 2wd the thumping starts again--minus the initial clunk.

The 4wd hi and low works, though sometimes it takes a couple of trees. the universals and steday bearing look fine, though there is an occasion drive shaft ping as i take off.

any suggestions?

 

[bajawind]

should the front drive shaft spin?

I raise the front today and the wheels bearing are loose. i missed them the first time through. the hubs seem to engage/ disengage properly.

Parked in 2wd, using my hand i can spin the front drive shaft maybe a quarter (or a little more) before it engages the front differential, then locks and is fully engaged with the differential. i am puzzled that the drive shaft can seem free, then engage--and still seem to work okay.

is it possible the loose wheel bearing are somehow engaging/disengaging the hubs and spinning the drive shaft? with all that play(?) i can see where that might be where the thump comes from.

the rear wheel bearings seem fine, though the brakes shoes look funny (somewhat checked in the center) because the brake adjusters were sized solid.

 

[martin]

That's normal for the front shaft with auto hubs, they try to engage when you rotate the shaft. Check the u/j's in the front axle, I suspect the centre one is bad.

 

[Jspafford]

As noted, your style auto hubs are locked in by the spinning of the front driveshaft. They also unlock by spinning it in the opposite direction.

 

[bajawind]

Thanks for the advice. i had a ball joint (there was play in the wheel [i thought it might be the wheel bearing] when i moved it up and down) and the driver's side universal (it was dry and rusty) replaced for $600--it appears the universal on the passenger side is the same. In fact, it i think that is where thumping was coming from. It seems the hub(s) are not disengaging which is why when i put it back in 2wd the wheel (is likely) spinning the universal rather the driveshaft--cause the thumping stops the moment i put it back in 4hi. Next step is to fix the passenger side universal. cheers.