Source of Driveline Vibrations?
 

I just purchased my '92 D250 and I'm trying to track down a very severe vibration that occurs from about 30 to 40 MPH. It gets worse the harder I'm accelerating, and I get a less violent vibration anytime I'm under firm acceleration.

The vibration isn't coming through the steering wheel, so I'm ruling out front end for now. What do you guys think? U-joints?

Not much info here. Is it auto or manual? What does it do at steady speed or under deceleration? Does it do this in all the gears you use at this speed? Could be u-joints/loose u-joint straps, an unbalanced tire, a tire bubble, a thrown drive shaft weight, loose lug nuts, and the list goes on. You need to get under the truck and start looking around. Grab stuff and shake it to see what moves in a way it shouldn't.

Sorry, it's a manual. It does vibrate under deceleration, but it doesn't seem as bad as under acceleration. It occurs in both the gears I use at this speed (3rd and 4th), and even in neutral.

I got under the truck tonight and started yanking on the driveshaft. The U joint directly aft of the carrier bearing is completely shot. Lots of play in all directions. I'm hoping this is the culprit.

Any tips on replacement? I've never done one. Thanks.

have you had your tires balanced? that can cause some bad vibrations depending on how far off they are.

If you have a press or a large vice you can press the old u-joint off and the new one on using different size sockets. I have done it with sockets and a hammer in a pinch but the chances of a bearing falling down inside the cup are increased that way. A bearing falling over in the cup is what you must avoid.

I'm not sure what you mean by a bearing falling inside the cup. Like I said I'm not real familiar with the process and how u joints are built.

There is a cross that has 4 bearing races, 4 cups, and a lot of little loose roller bearings that make up a u-joint. The inside of the cup is totally lined with these little bearings standing on end and held there by the grease. It is like the centrifugal force ride where everyone stands along the wall in a round room that spins really fast and the bottom falls out but you stay plastered to the wall. If you knock one of these bearings over while inserting its race on the cross it will go to the bottom of the cup instead of being where it is needed and the u-joint will destroy itself. When you buy the new u-joint and pull off the cups you will see what I mean. Type u-joint in the search box on, You tube, there are installation videos there.

Well it sure was that U joint. I replaced it and the truck no longer feels like it's going to self destruct at 35mph. A contributing factor may have been that on one side of the U joint at the diff, the previous owner used bolts that were about a half inch too long, and used small pieces of copper tubing to make up the difference :icon_doh: That got fixed as well. Thank for the help.