Page updated on 06-18-2018

how to check bad u-joints?

02-23-2005, 08:44 PM
I'm getting tired of my rear end moaning, it seems to be getting worse. I don't think it's the bearings but I'm questioning the diveshaft and the u joints.

Are there any tell-tale signs of bad u-joints or driveshaft disalignment? I think I have to go to a shop for the driveshaft, but what do you guys think?

02-23-2005, 09:03 PM
Grab the propellor shaft and wiggle it around, up-n-down-left-n-right, then grasp it tight and twist. Any play or clunking about is a sign that the u-joints are on the way out. One of the many things I checked while under my truck today with my mechanic and getting the tranny done right... However, this will not tell you if it is out of balance. That is another possibility. ;)

EDIT: Afterthought - if you know for sure that it is coming from your rear end, there is the possibility that you have a hung shoe. Once in a blue moon, this could happen, leaving either the primary or secondary shoe (in a rear drum - this does not apply to rear disc brakes) partially engaged. The light riding of your drum will make the whole rear end moan and sometimes shudder. Does the moan/shudder get better or worse with ground speed and with right/left turns? Also, when was the last time you had the rear diffy drained and refilled? I used to have a low moan coming from the rear axle in a Jeep 4x4 under acelleration in right hand turns. It turned out that I forgot to add 2 oz. of friction modifier to the hypoid GL oil and it was causing limited-slip chatter...something else to ponder...

02-23-2005, 09:10 PM
I had one rear wheel jacked up (in park) today. The lifted wheel itself had rough turning play in it.

With both wheels on the ground something in the drivers side axle would click if you slightly rocked the whole car just right.

Are the propeller shafts the two axles coming out of the rear differential? What do you think?

02-23-2005, 09:15 PM
The propellor shaft (A.K.A. "Drive shaft") is what connects the rear axle to the rear output shaft of your transmission/transfer case.

EDIT: Here is a photograph of a typical propellor shaft with U-Joints installed on each end - hopefully this will give you a better idea:

lou S.
02-24-2005, 07:59 AM
If you hear/feel moaning while driving put it in nuetral---this takes the load off the rear end and u-joints, if the moaning/vibration goes away-----could mean u-joints.

02-24-2005, 08:57 AM
O yeah, when I try coasting at 10-20 mph and then let off the gas I hear a clunk underneath like something is disengaging. I'm almost positive that has to be u-joints. I wonder if this is happening at higher speeds.

02-24-2005, 01:29 PM
yip, u-joints

02-25-2005, 07:27 PM
Would a bad u-joint also make a "clunk" or jerk when you start from a dead stop?

02-26-2005, 12:59 AM
For sure it would.

02-26-2005, 02:21 AM
I have found by looking around in the fourums' past history of posts on the subject a common means of detection amongst all of the threads:

Find a nice open parking lot at least half a block long. (This is for your safety and others. Listening for sounds will honestly distract you to a degree)
Roll the truck in a line, perfectly straight for at least a few dozen feet. Come to an easy stop. Shift her into reverse, and roll backwards in a straight line. At the moment the tranny goes from drive - through neutral - right into reverse and you hear a nice, solid "CLUNK" coming from under your seat... you have a strongly suspitious U-joint somewhere in your rear shaft. ;)

Now, a bit of anecdotal experience! Over this past winter I experienced a clunk whenever going into gear, in either direction. Especially the first time the truck moved after a night of sitting in freezing rain/snow. However, whenever the temperature was over 32 degrees for at least an hour, the problem went away. I waited for the next rain/snow and freeze cycle - sure enough, that clunk was there.
I got under the truck to look at the propellor shaft and the U-joints. They were solid! Odd. However, I glanced to one side looking at the passenger side shock absorber. The aftermarket Midas exhaust that some monkey put into the truck before I got it was bent in a manner that was too close to the upper shroud, melting the plastic boot around the cylinder. I guess as I drove, snow, water/slop got in through that hole, but sure enough, all around the shock tube was a solid jacket of mud/ice. The clunk was the rear end sinking, or rising slightly under torque when going into gear and shattering the coat of ice off of the shock absorber tube! After the initial "clunk" of going into gear, I put it in park, got out and looked under the truck. There was suddenly a small pile of shock absorber tube shaped shards of frozen mud and glop on the ground right next to the inside, rear passenger tire wall. Gonna look around for OEM or upgrade shocks here after I take care of tomorrow's project - rebuilding the front brakes. ;) My wear limit tabs started squealing on me about 4 days ago. Picked up the parts today after work that I ordered that day. Anyway - good luck with your rig. I know I love mine - especially after beating it back into shape with a torque wrench on weekends. :p

04-04-2005, 10:11 PM
Man, I hate to keep posting stupid questions..but I gotta...

I've got a suspicious "clunk" that happens sometimes when I'm pulling out from a dead stop.
I got under the truck & tried the "push/pull/rotate" test described in this thread & couldn't get any movement.
I then tried Wolfox's other suggestion & tried driving really slow in a parking lot, coming to a gentle stop & shifting. Felt a noticable clunk there when the tranny goes into reverse, though it was very slight.

The question: could the u-joints be bad even though I couldn't move them with the hands-on test?
I figure that there's a difference between me pulling on the driveshaft by hand vs 6000 lbs of Blazer moving it-right?

(on the other hand, u-joints aren't all that expensive...with 112,000 miles on the original ones, maybe I should replace 'em anyway....)


04-04-2005, 10:54 PM
Yes, most bad u-joints can be found with arm strength.

The back u-joint is the one that fails most often.

04-09-2005, 01:32 PM
When I did the hands-on test that I mentioned in my last post, I had done it with the tranny in Park.
I went back & did the same test a couple times, with the tranny in neutral (with parking brake on & someone ready to hit the brakes, just to be safe) & this time, I could make the shaft rotate approximately 3-5mm.
Strangely, I put it back into park & repeated the test & got the same movement, even though when I tried this last time (in park), I couldn't get it to move at all by hand.

The movement looks to be where the joint enters the rear differential (the joint itself doesn't move about) ; hoping the problem is the u-joint itself and not something worn in the differential.

04-09-2005, 04:16 PM
Yip, the rear u-joint is the weak on in the rear driveshaft.

Replaced mine twice in the trucks life.

Grease it at every oil change.

04-10-2005, 08:43 PM
BlazerLT, (and everyone else for that matter)
I know I'm probably driving you insane with all the questions, but I'm tryin' to learn here, Please bear with me on a few more...

The play I seem to have on my Blazers' shaft looks like it's just forward of the differential casing, where the yoke joins up with the pinion (hope I'm using the right terms there).
When I try to move the drive shaft near the joint, there's no movement in the joint where the cross, caps etc are, just right at the differential.

With that in mind, is it still most likely the u-joint that needs replacing, or the yoke and/or pinion? Or are the yokes considered part of the u-joint?

I'm taking my Blazer in to a mechanic tomorrow & I'm gonna let him check it out & fix it, but I'd just like to know what's going on. I appreciate all the help I've gotten here so far!


04-10-2005, 09:29 PM

I would have the mechanic check it out.

Might be something other than the u-joint but I still wouldn't rule it out.

07-19-2005, 06:11 PM
My 95' BlazerLt has 83,000 miles on it with the original u-joints. I'm getting the vibration in the wheel and the floor, seems to be coming from around the driveshaft. The vibration is very noticable while I'm accelerating, but as I ease off the accelerator and coast, the vibration diminishes, no matter the speed. I do get the clanking noise when shifting from reverse to drive at a complete stand still. I'm assuming I need to replace the U-joints, is this a complicated task, or can I, being and average mechanic, be able to do this.

Next question, just for kicks I decided to engage the previously working electronic four-wheel drive. My 4-HI lite doesn't lite up, but, when shifting into 4-LO, it flashes and then the 4-LO lite stays on. The flashing tells me the light works, but could something be causing the 4-HI not to engage, but allow the 4-LO to engage. The clanking noise I heard made me think, maybe, my four wheel drive was stuck engaged, but after reading these posts, I believe my front U-Joints, and possible the rear u-joints are shot. However, can the U-Joint problem be causing a problem with my four wheel drive also. I would appreciate any help you guys can give, thanks.


07-20-2005, 02:01 PM
Search this forum using the search feature. Electronic 4x4 issues have been successfully troubleshot in the past with information and articles here. :)

JA no Y
07-20-2005, 10:36 PM
"I'm assuming I need to replace the U-joints, is this a complicated task, or can I, being and average mechanic, be able to do this."

Replacing the u-joints isn't very hard. The hard part is getting them out of the drive shaft which doesn't take a rocket scientist to perform. Just use a hammer and a rod aprox. a tad smaller in diameter than the u-joint cups to knock them out. I like to use a solid brass rod to minamize any denting or damage. Just make sure you don't knock any of the "bearings" out of the new u-joint cups when installing them. Its best to have the cups on the u-joint to minamize any bearings from falling out and the other cups not being inserted into the drive shaft taken off to keep them from being knocked off while inserting the u-joint. And make sure they are centered in the drive shaft. (both caps are inserted into the drive shaft the same)

Add your comment to this topic!