Bumped from behind....

[JohnC]

I know this has been discussed before, but....

Occasionally when stopping, usually just after coming to a full stop, I get the sensation that I have been bumped from behind. Of course, that's never true. Anyhow, I recall that it was attributed to the slip joint in the rear drive shaft binding up. Well, I dropped the drive shaft a few weeks back and the slip joint was clean and well lubricated by ATF from the transfer case. I greased it up anyhow and put it back, but, no surprise, that did not fix anything. The other day I was pulling a trailer, first time in a while, and the problem was much more noticeable than without the trailer. At one point I though it was related to warmer temps, and maybe still is (if you call 40* warmer). Also, I have ABS issues, but I think they are due to a flaky front hub.

Anyhow, all suggestions are welcome!

TIA,

[DmaxMaverick]

That should be caused by spring wrap, which is normally caused by a "less than well lubricated" yoke spline. Lithium grease is recommended, but I use extreme pressure moly with success. Others ("regular" wheel bearing/chassis grease) doesn't work.

Another possibility is the "two foot driving habit". A low-speed downshift can feel similar. The early models wouldn't shift to 1st unless near zero ground speed with zero APP input (not sure which year models changed this, if any). Some folks with the habit of covering both pedals with both feet may experience this at times.

Lastly, a bad U-joint or worn out spline can cause a similar effect.

[JohnC]

That should be caused by spring wrap, which is normally caused by a "less than well lubricated" yoke spline. Lithium grease is recommended, but I use extreme pressure moly with success.

EP Moly grease is what I used, but I doubt if it is still there. The spline appears to run in a bath of ATF. It was clean and in like-new condition.


Another possibility is the "two foot driving habit".

Not me!


Lastly, a bad U-joint or worn out spline can cause a similar effect.

Nothing seemed out of order when I had it out, but thanks for trying...

It's almost like the rear brakes are causing the springs to wrap and then they release suddenly as I come to a full stop.

Maybe I should change the front hub. Wish I could read ABS codes...

[DmaxMaverick]

Yes, the spline is bathed in ATF, but it should take a while to wash out the grease.

It would be a good idea to get the ABS issue behind you. I have a couple known good sensors off my 2001, if they are compatible, for diagnostics, if you need. It would be helpful to know the codes, though. Have you tried cleaning and greasing the sensors/reluctors? If you pull a sensor and it's covered with grimy crud, you may have salt contamination, which breaks down the grease and reluctor ring, and fouls the sensor.

[Mark Rinker]

I have experienced the same thing, and believe it is rear leaf spring 'wrap' as you've suggested.

Maybe lubricate the bushings?

[DmaxMaverick]

"Spring wrap" is one condition, happens all the time, can't be avoided, and most of the time, is normal and unnoticed. Spring wrap is stored energy in the rear leaf springs, and increases with greater loads placed on the drivetrain (acceleration, heavy loading, etc.). Problems happen, like the "bump", when that stored energy isn't managed properly. One common cause of this is the sticky spline condition. What happens is, the rear spring "wraps" (coils/bends), changing the driveshaft angle/position. This causes the tranny/TC output yoke spline to take up the difference. When the spline doesn't telescope smoothly, it sticks/releases, causing the "bump" feeling after the load is released, usually when the brakes are released or reduced. Other conditions can cause a similar feeling, but the yoke spline is by far the most common, and not unique to only the late model trucks. Spring wrap can be reduced, or almost completely eliminated, using traction bars, links/ladders, etc.

Lube the bushings? Not the cause. The spring deflection during spring wrap is extremely slight, compared to the normal deflection during normal suspension extension/retraction. If this were a condition, there would certainly be heard/felt/seen indicators during normal driving.

John's problem is probably mismanaged spring wrap. The difficulty is identifying the root cause. It shouldn't be too difficult, if the condition can be repeated. I suggest cleaning and lubing the yoke spline, again. Perhaps too little grease in the right place is the cause.

[JohnC]

It would be a good idea to get the ABS issue behind you. I have a couple known good sensors off my 2001, if they are compatible, for diagnostics, if you need. It would be helpful to know the codes, though. Have you tried cleaning and greasing the sensors/reluctors? If you pull a sensor and it's covered with grimy crud, you may have salt contamination, which breaks down the grease and reluctor ring, and fouls the sensor.

Thanks for the offer, but I think the problem is a worn hub bearing. While I get the false ABS activation almost every stop, I also frequently get an ABS warning light when making a slow turn. Not mentally awake enough yet to tell if it's always a turn in the same direction, but I think I also have a little bearing noise.


I suggest cleaning and lubing the yoke spline, again. Perhaps too little grease in the right place is the cause.
Any specific suggestions are welcome. Last time I put a liberal amount on the inside of the yoke and a little on the outside sealing surface and pushed it all back together. (Too dang cold do do it again any time soon...)

[DmaxMaverick]

A bad hub bearing is easy enough to diagnose. Jack the wheel off the ground, and push/pull the top of the tire. Any noticeable movement indicates a bad bearing. If it's tight, you could try the cleaning method. The TSB recommends this step, before hub replacement (a warranty thing, not unlike the steering shaft issue). Hubs aren't cheap, but not too difficult to replace.

[DickWells]

Don't know if this means anything, but, what condition is your intermediate driveshaft bearing/carrier in? Or does the ex. cab even have one? Reason I ask is, that for many months, I could feel just the slightest little bump on take-off, loaded, empty, or towing, and I had known for a long time that the rubber around the bearing was shot. Now, I don't get any axle wrap, as I have air suspension back there, and the control arms that replaced the steel springs pivot at the old front spring mounts, preventing any wrap. So, last Summer, when I finally got around to ordering up a new bearing from Rock Auto, and installing same, the little bump disappeared. The old rubber isolater was still hanging on by a couple of very small rubber threads, but it couldn't have been very effective, by the time I pulled it out. BTW, the Rock Auto replacement was much more well-made than the factory job, (Fed. Mogul, I think), at a somewhat better price than anything available at one of our local parts stores. Not sure, but I think the DM//Ally all take the larger of the two bearings available. I know I ordered one the year before at the local store, and got it home, only to find out that, even though I had told them (NAPA) all the info, HD, 4x4, 2500, 6.6, etc, etc, they gave me the wrong one, and then told me, upon return that the big one was special order only! Got my refund, and waited almost a year to take care of the problem, myself.

[JohnC]

A bad hub bearing is easy enough to diagnose. Jack the wheel off the ground, and push/pull the top of the tire.

Did that on the side I thought was acting up and it seemed tight.


Don't know if this means anything, but, what condition is your intermediate driveshaft bearing/carrier in? Or does the ex. cab even have one?

Nope, single piece, but thanks for asking! ;)

[cabletech]

Does it seem to pull to the right or left when you are almost stopped? Pull the power to the ABS and take it for a drive (will cause warning lights and alarms). I bet the problem is gone. If so one or both of the front ABS sensors is bad or moved away from the hub by rust. If it pulls to the right it's the left sensor. If it pulls to the left it's the right sensor.

I went through this this past summer with my 2006. It almost felt like the brakes were letting up when the truck was almost stopped.

Jay

[JohnC]

Pulls slightly left. That's why I jacked the right wheel and checked the bearing.

Good idea about pulling the fuse. Unfortunately, though, the bumping is very unpredictable so I won't know for sure if it is "fixed" for a long while...