Knocking sound seems to come from front differential only in 2WD -
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post #1 of 7 Old 02-12-2014, 09:16 AM Thread Starter
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2003 WJ 
Join Date: Feb 2012
Location: Brentwood
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Knocking sound seems to come from front differential only in 2WD

Hey all,

I posted this over on Wranglerforum too knowing there are plenty of folks that only use one or the other and thought there might be someone on here that might be able to help that's not on the other forum.

My '94 YJ Wrangler has recently started making this strange knocking sound. It seems to be coming from the front differential and sounds like it's coming from the passenger side but it's impossible to tell when driving. I can hear it from the drivers side when the window's down too. It only happens in 2WD. The instant 4WD is engaged, HI or LOW, it stops. I have replaced the U-joint on the drivers side axle because it was completely shot and rattling constantly. It sounded like loose washers rattling. The seals had completely rusted out and most of the pin bearings were powerized. I could move it side to side in all directions. Anyway, the U-joint on the passenger side seems fine. It's not loose and looks good visually. I've looked at the both drive shafts and neither seems to have any play in the U-joints but that wouldn't make sense to me anyway. If it was the rear drive shaft it wouldn't matter if it was in 4WD or not, it would make noise any time the Jeep was in motion, and if it was the front, wouldn't it make more noise in 4WD i.e. under load? I was thinking it might have something to do with the silly 4WD vacuum motor and fork on the passenger side of the front differential. It kind of sounds like a rubbing sound like a bearing, like metal scraping an unevenly worn or rusted surface...not like a hard knocking sound like an engine knock or a pop from suspension or steering components. Anyone have any ideas or experience?

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post #2 of 7 Old 02-12-2014, 10:04 AM
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1989 YJ Wrangler 
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Location: quechee Vermont
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check the drive shaft to the front.. specifically the splines where it slides together and the ujoints.

1998 ZJ 5.2 Auto

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post #3 of 7 Old 02-12-2014, 10:25 AM
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1994 YJ Wrangler 
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Check for wear in the shift fork on the cad. Betcha it needs to be replaced

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post #4 of 7 Old 02-12-2014, 04:08 PM Thread Starter
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Earlier today I had a conversation with the manager at the local 4Wheel Parts. He said it would be difficult to say without actually having it in the shop which I completely understand but he still had a lot to say. He said it could still be the U-joint on the passenger side even though it looks perfectly fine. It could be that the needle bearings are just shot. He said he's seen it before where when it's in 4WD the force of the drive on the axle and the opposing force of road pushing on the tire can cause just the right situation where it makes a noisy joint shut up. He also said I should lift it and have someone roll the wheel while I listen with a stethoscope around the vacuum shift motor on the differential. He told me it could be the shift motor/actuator is going bad and the fork isn't sliding all the way back like it's just on the edge of shifting into 4WD.
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post #5 of 7 Old 02-14-2014, 01:51 PM
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1987 YJ Wrangler 
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Its four bolts and a splash of oil to pull the shift cover off. I pulled the e-clip off of the shaft slid the fork over and re-installed the e-clip thus keeping it engaged at all times then plugged the vacuum lines. It's temporary till I get one piece shafts with bigger u-joints but I hear of guys running like that all the time.
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post #6 of 7 Old 02-14-2014, 02:42 PM
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1992 YJ Wrangler 
Join Date: Oct 2009
Location: Orlando
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Are we sure it isn't the hub/unit bearing on that side? Mine made a metalic scraping sound when going bad.

Pretty easy to check. Jack up the suspected side front tire. Check for play in the tire by grabbing bottom of the tire and top of the tire and attempting to tilt the tire in towards the jeep (top toward the engine, pull bottom of tire away from engine).

If there is any movement detectable the hub is bad. You should really only be able to rotate the tire by hand. Any lateral (side to side) wobble of the tire means the hub is gone.

Putting it into 4 applied load to the hub and silenced the noise temporarily in mine.
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post #7 of 7 Old 02-14-2014, 02:51 PM
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1987 YJ Wrangler 
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Similar to what everyone else is saying, I'd jack it up and give the front end a thorough look.

Front driveshaft
Wheel bearings
Transfer case front output shaft
Vacuum shift

I also wouldn't completely rule out the front differential itself (based only on the info in the thread), whether it's bearings or gears or whatever, but that is a pain to take off and refill with oil so I'd personally save that as a last resort if I couldn't find the problem somewhere else.
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2wd only , 94 yj , front differential , knocking sound , not in 4wd

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