Diagnosing Drive Shaft U-joints

driveshaft u-joint

It is most common to find drive-shaft u-joints on rear wheel drive automobiles. You may also find them driving the front wheels on 4 Wheel drive models with I beam suspension like on older Ford trucks and sport utility vehicles.

In some cases this flexible joint will have a grease fitting located near the center of the cross. If you have one, it will require regular service. In other cases they are sealed from the factory and will require no lubrication at all.

Drive-shaft u-joints have been around for a long time. In fact, I became familiar with these parts at a young age when I got my learners permit and began driving. My first car was a 1974 Dodge Charger.

Dodge Charger 1974Dodge Charger 1974

I made the classic mistake of putting a shackle bar lift kit above the rear leaf springs, so I could install a pair of giant L60 tires and deep dish Craiger wheels on the back. This was cool at the time.

It was not long after these alterations that the mighty Dodge began to have problems with the rear universal joint. This was because my modification drastically changed the angle that the drive-shaft operated at. The new angle on the old part coupled with aggressive driving habits destroyed it quickly.

Common Universal Joint Problems

One of the most common signs of a damaged or bad universal joint is when a clunk is heard when the transmission is shifted into gear or moved from reverse to drive. This clunk can be caused by other problems but may be a sign of excessive clearance in the u-joints.

When a drive shaft flexible joint is on its way out it can also cause vibrations at higher speeds. This may feel like a wheel balance problem. A way to tell the two problems apart is in most cases a worn u-joint will cause vibration during acceleration or deceleration. A tire balance problem will create a steady vibration regardless of throttle position.

The road test would be just the first step in confirming a problem with the drive-shaft u-joints. It is easiest to inspect the parts with the vehicle up on a lift. When you inspect the universal if you see grease leaking out of the end caps it's a sure sign of a problem.

With the vehicle raised in the air and the transmission placed in neutral you can usually grab the driveshaft and rotate it to spin the rear wheels. Moving it a half turns up and back in either direction is a good way to inspect for looseness. It should have no noticeable play when moving up and back as described.

Removing and Inspecting the Drive-shaft

Before you remove the driveshaft it is a good idea to mark the it in relationship to the differential yoke. In some cases a shaft can be specifically balanced. If you remove and reinstall it and a vibration that was not there before surfaces it might be out of phase.

When the propeller shaft as the British call it is removed it's easier to check the condition of the universal joints on both ends of the propeller shaft. You should be able to rock the flexible joint in all directions and it should provide smooth steady movement. Any stiffness or bumping that is felt is a sure sign of a defective u-joint. Although it's not a common problem, repeat universal joint failures can happen.

I've seen some weird things cause stubborn vibration complaints from drivers. More then once the root cause was undercoating installed on the vehicle. A sloppy undercoat job can get on the drive-shaft itself and cause out of balance problems that can have negative effects on longevity.

If you find u-joints with any problems just go ahead and replace it. In most cases they are not expensive. Different manufacturers have different types of joints and therefore different service procedures are needed.

Example: General Motors original universal joints with plastic inserts can be difficult to replace. The Ford universal joints with removable spring clips are much easier. I recommend that you check your online auto service manual for your specific procedure.

There is not a lot of information on the internet in regards to drive trains. This is why I decided to dedicate a repair module to the manual drive trains subject.

I also put together a couple of pages for front wheel driveshaft u-joints better known as (CV) constant velocity joints.

Get a rundown on the kind of information that's available here at the you fix cars website. Visit the homepage and learn more about Diy auto repair.