Jeep: Jeep Wrangler vibration, michelin ltx m s, propeller shaft

Question
I have a 1998 Jeep Wrangler, 135000engine miles and another 45000 behind a motor home. I have developed a signifant vibration at about 55-58 MPH. I have been to three mechanics and none seen to have a fix. In desperation I went to Jeep dealer. They cannot find anything wrong. All have test driven the vehicle and tried many things: tire rotation, alignment, road balance, balance, and nothing shows up. I t has Michelin LTX M/S tires, P235/75/15 tires with about 30000 miles. The treads are down about half.

Answer
Hi Robert,

First, is the vibration continuous at that speed? Or does it come and go, initiated by a bump, etc.? I'm assuming its continuous, at that speed.

Can you tell what frequency the vibration is at? This will lead you to the problem, most times. I doubt its at wheel speed, since you've covered most of the obvious, and since its at a particular speed, I would tend to rule out water or other foreign objects (fix-a-flat compound) in the tire(s). It doesn't rule out a bent axle shaft, but that would normally show up when checking wheel runout on the car -- *if* that was checked. Basically, you rotate the wheel while the vehicle is on a rack, and eyeball the rim. You'll notice if the axle shaft, or hub, is bent. Technically, you should measure, with a dial indicator, at the rim bead, and then across the tread.

If the vibration is of a higher order (roughly 3x wheel speed), then chances are either the front propeller shaft or rear driveshaft is bent. We had a real problem with the front prop shafts getting bent by novice technicians not setting the rack properly prior to lifting the vehicle. Ironically, that's also a popular spot that off-roaders would smack when crawling over rocks. That same dial indicator (even eyeballing it might clue you in) can measure shaft trueness on a lift.

This is a bit disheartening, particularly with the dealer. They have a rarely-used tool (or they *should* have) called an EVA -- Electronic Vibration Analyzer. It's used for this EXACT problem, but techs need to be trained (NVH class) on it to use it properly. The unit comes with sensors that you mount on various components, and they interface with the Analyzer itself. Then, the tech drives the car at the speed of the vibration. The Analyzer measures vibration frequency, and with the use of a calculation worksheet, will direct you to probable areas of concern.

Good luck!