strange "bump" when decelerating quickly

[Matt1031]

I'm just getting educated in the CUCV world, but this complaint is a common one for owners of full size Broncos. The fix for Broncos is to lube the slip joint in the rear driveshaft. Although I've never seen a M1009 rear driveshaft, I imagine it's also a DC with a splined slip joint since both vehicles deal with a short wheelbase and a relatively steep rear driveline.

Anyway, there's one more thing to consider while your underneath checking bushings and mounts and what-not.

Good luck.

If it's still stock driveline, the OP's m1009 has a different setup than what you're describing. CUCV's all use a slip yoke style tailhousing, so there isn't a splined coupling in the driveshaft itself. If you want to get rid of it, you need to change out the T-case output shaft/tailhousing to a fixed yoke style or get a SYE kit and have a custom shaft built.

Note that on a M1008 or any 1 ton CUCV variant some "clunk" sounds at low speeds are totally normal because of the Detroit Locker in the rear. If you ever take one apart you'll see why it makes those noises when you suddenly let off the throttle at low speeds (engagement cogs need a few degrees of "slop" or lash to work as designed)

The M1009 uses a G80/Gov-lok rear "locker" which is a totally different setup than a Detroit Locker. It shouldn't really be making any noises - unless something's damaged. However, Gov-loks are well known for breaking parts. Maybe worth pulling the rear diff cover to check and see if everything looks intact? M1009's also use a NP208 transfer case which is well known for making clunk sounds if worn or sloppy. If you can rule out U-joints, my guess would be probably some kind of excessive lash problem at the rear diff, or at the T-case. The diff is relatively easy to check, but the t-case involves a lot of disassembly to check internals.

 

[chief1983]

My truck was making a noise kind of like that when I let off the throttle, and I just lost another driveshaft. Not sure if that was related or not, but I feel I've read somewhere before that it could be ujoint related. I had a salvage yard shaft and brand new NAPA HD u-joints. They did seem a little tight when I installed them, so I'm wondering if I had one of the caps in a slight bind. Be sure to inspect the joints and check the shaft for play. Next time I'll be jacking up the rear end and putting it in idle so I can watch the driveline roll and see how it looks.

 

[Whitey]

I've encountered this a few times and it usually ended up being a worn bushing or motor mount. Rubber does deteriorate. Also, my 1009 had the entire underside caked with mud when I got it from GL and it took the good part of a day powerwashing followed by a nice regreasing, a few times actually, then the clunks and sqeaks went away.

The front torsion bar suspension of a '70 Dodge Dart I was working on also made this sound, and loud as **** to.

-Whitey

 

[Wyofj40]

Pull the rear drive shaft and check your input pinion. you will probably be able to turn it by hand and make the clunk you r hearing. It is the backlash in the gearing which is normal for how the differential is made.

 

[stampy]

Wow thanks for all the great replies! I haven't had a chance to get under the truck yet. I have replaced the driveshaft allready. I don't think it is a bad rear axle as it has been there for over 10k miles and I figure I would have had to replace a rear axle by now if it was that. it's almost as if the rear end of the truck is flexing forward when I am coming to a stop and then it "feels" like the clunk is it going back in place....does that make any sense at all? If we figure it out TRUST ME I WILL POST IT, as I have searched all over for something that explains the habitual bump

 

[littlebob]

I agree with the motor mount/transmission mount theory. I'm not that familiar with that exact vehicle, but I had a 74 LWB chevy truck that had a 2pc drive shaft and support bearing in the middle and it would make that kind of noise when the rubber in the support bearing came apart.

 

[biggestc69]

I'm with Matt1031. Back when I was a heavy right footed teenager I have exploded many 10 and 12 bolt axles. Pulling the diff cover and looking for carnage would be my first move.

 

[Davepo99]

could be rear end gear slop?
would be easy to check just see if you have slop in rear dif pinion with truck in park and tcase in neutral

 

[stampy]

Well, I haven't gotten to it yet as life got in the way. But it happens when I am using medium to heavy braking to stop, then when I let off the brakes (to move through a stop sign for example) is when the clunk happens.

 

[GreenBull]

To confirm driveline windup as the cause get the condition to occur and instead of releasing the brake to create the clunk, instead shift to neutral or reverse. If that causes the clunk do the inspection and repairs below. the noise is a product of stick/slip at the Tcase output and slip yoke.

Remove the driveshaft. Inspect the splines in the slip yoke for wear. You will see a pattern of wear. Reach in to see whether a lip has formed. If the wear isn't bad lubricate the splines with ahigh pressure grease. If the rear spring eye bushings are worn change them as this allows excessive axle fore/aft movement.

 

[O.D. Fever]

If you have time to play McGyver, you can work on the truck. I mean Jack.

 

[mudman]

If yo have a broken Motor Mount, it will be the one one the passenger side for deacceleration, and the drivers side for acceleration, you might take a look at your trans mount as well. It's a cheap fix!!