Vibration On Decel

I have an 87 Bronco that has a really bad vibration when decelleration on the highway. It doesn't do it at low speeds, and only when you let off the gas, and only if you decel a certain amount. I was thinking a u joint, well, hoping it isn't the real axle.

You could also have a tire out-of-balance, or a bent rim. The factory rims are really bad about getting bent and causing strange vibes. Check for any play/wear in the u-joints, too.

Other things to check: Rear drive shaft slip joint, pinion nut, u-joints, make sure the drive shaft bolts are tight

You could also have a tire out-of-balance, or a bent rim. The factory rims are really bad about getting bent and causing strange vibes. Check for any play/wear  in the u-joints, too.

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Wouldn't that vibrate all the time if it were a rim or tire?

Not necessarily. A bad tire/rim can seem fine until you get around 50-60mph, speed up, slow, down, whatever. Even a chunk of mud stuck inside a rim will give you fits at certain speeds.

He is right. i can cruise at 70 vibration free but when i hit 82 exactly, harmonics start working and they wont stop until i am below 60. but i dont go that fast so i dont worry about it.

Something I found with mine was I forgot and left the manual locking hubs in the lock position. That caused a strange vibration at higher speeds. Of course I had a few other problems with the front axel as well like the right side seal blowing out so the fluid was a bit low. Check to see if the seals are all dry, if not then you could have a problem there.

Just stuff I learned with my "Beast"

Found the problem and I have to clarify, I said on decel, it actually was when under no load, neither speeding up or slowing down. The spindle nut was loose on the rear axle. I replaced the rear u-joint also since the grease zert was gone. Problem is solved.

Do you mean the pinion shaft nut??? I gots to know these things when I'm 2,000 miles away and not ridin' with you! J/K, glad you found the problem. You wouldn't believe the strange sounds I hear when driving with the top removed, only to find it's somone mowing their lawn or a rattle in the car sitting next to me at a stop light!!!

Pinion shaft nut, hmm, the nut that holds the piece on that the driveshaft connects to on the rear axle. Pretty techy talk there huh. I took that name from the haynes manual, of course, a whole night had passed before I posted and I have a 4 month old that with ear infection. So the sleep deprivation could have changed the name in my head.

This is great - I am experiencing the exact same problem. 1988 Bronco. Had the U-joints replaced and the double cardian replaced/rebuilt. The truck went from very noisy at any speed to now it just vibrates under no load above 55 mph (even that vibration is much less). Well last night I went under and checked the bolts on the 2 U joints. Three were wrench loose and one was only finger tight. I tightened them all and it made an improvement too. Still getting the vibe under no load at 55 mph (at least it doesn't feel like it will shake the truck apart any more). Can you tell me where to look for the pinion shaft nut? Do I have to disconnect the ujoints? If I am lucky it is loose on mine too.

BTW - should there be any play in the drive shaft where the smaller diameter rear connects to the larger diameter shaft under the rubber boot? I can move mine if I get underneath it and press up and down at this point. Memory says it moves a 1/8 inch or so. I appreciate any input. This vibration is getting better but it is starting to drive me nuts.

Thanks,

John

The pinion nut is on the end of the pinion shaft that comes out the axle center-housing where the driveshaft bolts to the "ears" on it. Someone will know the proper torque on it. I don't think the slipjoint in the driveshaft should have alot of play like you stated. That may very well be the cause of your vibes. You can try greasing the snot out of it and see if that helps. If it's dry you can get some binding and excess wear.

Yep, just undo the back drive shaft from the rear axle, you will see it then, pretty large nut. I think it was 190 ft-pounds.

Thank you for your help. I'll keep my fingers crossed. . .

John

Its definately the rear driveline U-joints or splines (slip yoke) I had the same problem when reducing the throttle on the freeway. It was suprised to see all the needle bearings were gone on one side of the u-joint. Pressed a new one in and its a perfect ride now on and off the gas.

I will say mine is way better but, you can tell there is still something. I will have to look at the rest of the u-joints. I think the nut being loose just amplified the problem.

check rear pinion