Jeep: Jeep Cherokee Drive Shaft Problem, jeep cherokee sport, cherokee drive


Question
Hi Chris,
 I have a 98 Jeep Cherokee Sport, 4.0L 6Cyl. I believe I have a problem with my driveshaft. I had a shimmy at certain speeds on the highway and a friend of mine helped me diagnose it. The shimmy turned out to be a bad rim, but in the diagnosis process we replaced the U-Joints. We put the drive shaft back in, and I did not notice it at first, but between 55-60mph I have a definite shaking. It does not seem to be from the front or rear wheels. I do not feel the shake in my steering wheel. It feels like it is coming from the bottom of my Jeep. I had spoken to my friend, who originally helped me and he said that he forgot to mark the driveshaft when he took it out. He suggested I take it out and turn it 1/4 turn and see if that helps. He said to keep turning it until the shake goes away. But I have also read that a driveshaft needs to be balanced by a technician. Can you give me any advice regarding this issue? Thanks Chris.

Jon  

Answer
Hi Jon,

This is a toughy to answer on the 'net. First, you really need to determine what frequency the vibration is at. If it's a driveshaft, the frequency will be more than 3x the rotation speed of the wheels -- remember, the differential provides that gear reduction (3+:1). So, any vibration before the pinion shaft will be a very high rotational vibration -- very much like running over closely-set rumble strips at high speed. Generally, you will feel this vibration in the body of the vehicle.

Now, realistically, driveshafts *should* be balanced. In practice, they are almost never marked, and rarely cause issues if done so. I would suggest checking the spring clips on the yokes of the shaft -- if they are not seated firmly in their grooves, the U-joint will be off-center, and definitely vibrate. Inspect the installation, and verify that no play exists -- installing an incorrect joint has caused more than one mechanic to scratch his head.

Still, if everything looks good, you can measure the runout of the driveshaft on a rack with a dial indicator (fitted with a roller plunger) -- look for around .003". Careful manipulation with a large deadblow hammer can often correct small runout problems. Depending on the length, check it in at least 3 positions, 2 of them at each end. If there is a problem, then you can R&R the shaft and rotate -- but unless it's been bent by something, most shafts are true enough to install in any position. Make sure you inspect for the cause, and not the symptom. Also, I've assumed that this isn't the propeller shaft (front), but keep in mind that these are often bent by service personnel (like oil change techs) inexperienced with setting the rack to lift a vehicle. However, the same principals apply for the front as well.

While I always recommend a reputable shop for any work, ask before having the driveshaft balanced. Often times, and even documented in Jeep manuals, balancing can be achieved, albeit loosely, by applying hose clamps (yes, radiator!) at certain spots on the shaft. Crude, I know, but it does work -- the screw portion of the clamp does offer a small counter weight. However, most shops would just as well sell you a new shaft, so keep that in mind.

Still, if the vibration remains elusive, some shops (mostly dealers) should have a tool called an EVA (Electronic Vibration Analyzer), that measures vibration frequency at certain RPM's, which helps the technician narrow down the problem area. Don't be surprised if they give you a blank look when you mention that -- most people, unless they are learned techs, have no clue about this. It's a great tool, but requires some setup, time and training to operate it accurately.

If it's wheel speed vibration, then it's a whole 'nuther ballgame. From your question, it sounds like the vibration surfaced after the shimmy was resolved -- it could have been masking it, but not likely. Good luck!