Jeep Wrangler, jerks when engaging gear

Synchros shot? Weird noises while shifting? Not sure what needs to be replaced?
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Jeep Wrangler, jerks when engaging gear

Post by Mercury »

I am a new owner (under three months) of a '99 Jeep Wrangler 5 speed. I'm new to standard shifts, but I've noticed a problem that I can't easily blame on being a newbie.

When I engage the gear, and it can happen engaging first, second, or third, sometimes (once a day, usually) I'll hear a loud pop and the Jeep will jerk to the side a foot or two. I haven't been able to recreate the problem, or notice any trend with it other than it *seems* to happen more when facing downhill rather than up or on flat ground.

Any ideas?
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Re: Jeep Wrangler, jerks when engaging gear

Post by IMBoring25 »

Are you in 4WD?
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Re: Jeep Wrangler, jerks when engaging gear

Post by Mercury »

No, not in 4WD.
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Re: Jeep Wrangler, jerks when engaging gear

Post by Rope-Pusher »

The entyre vehicle will jerk to the side a foot or two? Shirley, you jest!

My son tests axles - no, not by drag racing or 4-wheeling. When the axle shafts fail in torsion, they often crack at an angle. If one, or two, tries to pass torque thru the broken axle, one half may rotate relative to the other half until the two "long" edges of the crack align, pushing both axle halves away from each other. If the axle shafts can move far enough away from each other, the point of contact can go over-center and the shafts snap back into alignment of the long side of the crack to the short side on the other half.
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Re: Jeep Wrangler, jerks when engaging gear

Post by Mercury »

Rope-Pusher wrote:The entyre vehicle will jerk to the side a foot or two? Shirley, you jest!

My son tests axles - no, not by drag racing or 4-wheeling. When the axle shafts fail in torsion, they often crack at an angle. If one, or two, tries to pass torque thru the broken axle, one half may rotate relative to the other half until the two "long" edges of the crack align, pushing both axle halves away from each other. If the axle shafts can move far enough away from each other, the point of contact can go over-center and the shafts snap back into alignment of the long side of the crack to the short side on the other half.
Image
I'm not certain what you mean by all of this, but are you saying you think the axle may be cracked? It's possible, though the axles were replaced fairly recently. What would I be looking for if this were the case? Outside of the obvious "big freaking crack."

And don't call me Shirley.
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Re: Jeep Wrangler, jerks when engaging gear

Post by AHTOXA »

With the rear likely a D35, that axleshaft would have walked out of the tube a long time ago, had something serious like this occurred in the rear.

Now, the vehicle moves to the side? As is, right or left a foot or two?? Do you believe you can isolate the source of the noise somehwhat, as in front, back, middle?

Now, Shirley no offense was meant by resident rope there.
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Re: Jeep Wrangler, jerks when engaging gear

Post by Rope-Pusher »

Lemme Spline somethin' to you, Lucy : F N both halves of the axle look like the image I showed in my previous post, they will want to align with the long edges diametrically opposed from one another. When you try to feed torque thru this broken section, the ends will twist out of alignment. When they snap back into alignment is when you get the popping noise.

F N U drained the fluid and then took the cover off the differential housing, you should be able to see the side gears that the axle shafts are splined to. F N N axle shaft were cracked, there might be extraneous noises or rotational freeplay between the road wheel and the side gears. If nothing is obvious, reinstall the cover, refill the diff with lube, and jack both rear wheels off the ground. Then drive the wheels in first gear while you stand on the brake and throttle at the same time. If the axle is broke, it will probably start the popping.
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Re: Jeep Wrangler, jerks when engaging gear

Post by theholycow »

Thoughts from a non-expert...

Wheel hop?

Broken transmission mount or engine mount, so that when you suddenly apply torque it allows the transmission and engine to slap sideways against the transmission tunnel?

Malfunction in an automatically-locking differential (like the G80 on GM pickups)?

Broken body mount(s)?

...can you post a video of it?
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Re: Jeep Wrangler, jerks when engaging gear

Post by Mercury »

@Rope I'll see if I can get a chance to check out my differential tomorrow. Thank you for the info. And don't call me Lucy.

@Cow I wish I could get a video, but I can never create the issue upon demand. It happened twice on a seven-mile drive to work today, but I can drive for days without incident. I noticed, as an update, that it's happening either immediately upon engaging a new gear, or within three seconds of the shift.

The rear diff used to have a Detroit Locker installed, but it froze and blew out a rear axle a year ago and wasn't replaced.

My mechanic seems to think that it's the front suspension, but I'm not certain I can agree with that. This isn't an issue that happens while driving steadily in gear, even going uphill or downhill. It only happens during, or immediately after, gear shifts.

@AHTOXA It seems that it jerks to the right a foot or two, and it seems that the pop comes from the rear of the Jeep.
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Re: Jeep Wrangler, jerks when engaging gear

Post by Rope-Pusher »

Mercury wrote: and it seems that the Pop comes from the rear of the Jeep.
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Re: Jeep Wrangler, jerks when engaging gear

Post by AHTOXA »

What I would do is have someone get in the jeep, put the front bumper against a tree and give it some mild throttle in first. I'd be outside checking for things moving, that shouldn be moving or for any oddities. If nothing is found, I'd put the rear bumper against the tree and repeat the same in reverse. Look for control arm play, body mount play - all that good stuff.

They way you've descrived it, it seems like it becomes an issue during torque application or weight transfer when shifting.
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Re: Jeep Wrangler, jerks when engaging gear

Post by tankinbeans »

Rope-Pusher wrote:
Mercury wrote: and it seems that the Pop comes from the rear of the Jeep.
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Re: Jeep Wrangler, jerks when engaging gear

Post by theholycow »

That's Iggy Pop. He's always been severely ugly and a heavy drug user.
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Re: Jeep Wrangler, jerks when engaging gear

Post by tankinbeans »

theholycow wrote:That's Iggy Pop. He's always been severely ugly and a heavy drug user.
The name is familiar. I guess I'm lucky that I've never seen him; I'd have had to sear out my eyes a long time ago.
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Re: Jeep Wrangler, jerks when engaging gear

Post by Rope-Pusher »

theholycow wrote:
That's Iggy Pop. He's always been severely ugly and a heavy drug user.
But he sure outlasted Amy Winehouse!
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