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Grinding Vibration on Deceleration

MaineJeepGuy

NAXJA Forum User
Location
ME
Hello,

I have read several threads on what sounds like similar issues but I'm going nuts trying to figure out what the issue is. I recently acquired a 1999 cherokee this being my 6th cherokee so i know my way around them pretty well.

I'm just looking to drive this beast daily not trying to wheel it or anything like that just keep it running and enjoy it.

I installed a 2 inch spacer lift since the shackles were toast with new leaf springs shocks etc. Prior to the lift it ran totally smooth, i installed the lift which i have done on several other cherokees and now i get a grinding sound when i let off the gas between 32-40 mph and sometimes higher speeds coming off the highway that are intermittent from the center area of the jeep.

I lowered the TC 1/2 an inch, maybe not enough?

I reworked and welded up the exhaust for clearance and strength so its not rubbing anywhere but i cant decipher whats causing the sound.

with the leaf springs and the shackles id say im more like 3-4 inches in the rear with the shackles, is 1/2 and inch drop not enough? The pinion angles seem pretty mild.

are there other things in the TC that can cause grinding after a lift? it runs and shifts smooth it just makes this awful on/off grinding sound

any thoughts would be helpful.
 
Lube up the slip joint. How about year and other info?
 
A slip yoke requiring some grease does not typically cause a grinding sound. Since MaineJeepGuy mentions a new lift and recent exhaust work I would suspect the driveshaft u-joints first, worn motor transmission mounts, and then exhaust interference issues next. Often a lift install will bring to light a worn out u-joint that is not happy operating at an increased angle due to the lift. With 3-4 inches of lift, MaineJeepGuy should start saving for an SYE.
 
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welcome aboard, i have been tracing grinding noises on my 99 for years, i always seem to think i find them and am never quite there,, there are just so many spots to look, motor mounts, transmission mount, pinion angle, front and rear, track bar, brake slides, rear shoes loose, balanced tires, .. anything that may have been a problem before the lift that you could not feel will be amplified that much more, most opt out of the idea of lowering the transfer case in favor of sye, the homemade version works pretty well, i installed mine several years ago and has been trouble free
 
Might be the front. You dont say what year but 2000 and 2001 have low-pinion D30 that is somewhat more susceptible to pinion angle vibes than earlier years. And the TC drop will worsen the angle on the front driveshaft.
 
The Jeep is a 1999 Cherokee spot automatic AW4 with a NP 231 TC 160K miles

I replaced all 4 brakes,all 4 wheel bearings, axles shafts, u joints etc. Just general maintenance stuff. The transmission mount seems soft but it should be to a certain degree. I may pop a new one in just to eliminate that but i don't want to go down a rabbit hole of spending. I didnt plan to do an SYE just from the simple fact that i plan to just drive it not wheel it too heavily.

It almost sounds like something knocking around in the rear that why i thought it was exhaust at first but thats definitely not the case.

I opened up the rear diff (8.25) and it looked pretty good normal wear for a 20 yar old jeep.

when i did the rear leaf springs i left the rear drive shaft attached would that hold the correct angle of the axle? Im concerned that i may have the axle angle in the rear slightly off but my understanding is that it can only go back together at one angle because of the spring mounting plates?

Could it be chain slap in the TC?

Its just an alarming type sound and anyone who is mechanical would be driven up a wall by it....

I appreciate all the response thus far.
 
Now you need to start taking numbers, that's the only true measure.
 
At about 3 inches of lift is when driveline angles become too severe to use a t-case drop or axle shims as a band aid. Excessive driveline angles will likely be the root cause.

Also check that the shackles are not contacting the rear bumper bracket bolts on the rear crossmember.
 
Check out this site for good info.

https://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml

Here's an excerpt that may help, from the "Fixing Driveline Problems" section:

Vibrations at relatively high speeds are often due to balance problems.

These vibrations may come and go as speed increases as you move in and out of resonant frequencies of the drive shaft.

Vibrations that get worse when transitioning from acceleration to coasting to deceleration, like when you back off the gas over the top of a hill and before you go into full on engine braking, may be due to loose or worn parts, like loose flange bolts, worn or over-extended slip yoke, etc.

Vibrations that get worse say going uphill or accelerating at speed than when going the same speed on the flat or downhill, or vice versa, might be due to a slight alignment issue.

If worse uphill/accelerating, the lower u-joint angle may be moving too high as the axle and pinion tip up under load, if so, tip the static pinion angle down a little lower than it is now.

If worse downhill/decelerating, the lower u-joint angle may be moving too low as the axle and pinion tip down from the lessened load, if so, tip the static pinion angle up a little higher than it is now.

Vibrations at very high speeds may be due to approaching the drive shaft critical speed, which is essentially the resonant point where the shaft begins to whip and vibrate violently.
 
I'd suspect driveline angles and the rear u-joint as well. Longer rear shackles without doing anything to correct the angle tips the rear diff up. Lowering the t/c makes this worse.
 
lawsoncl what would you suggest axle shims if the angles are off?

Its a 2 inch lift i dont truly want to spend the money on SYE if i dont have too?
 
See post #7
 
just a question to throw out there, when you installed the lift did you tighten everything up with the jeep in the air, or with it on the ground with the full weight of the vehicle on the springs?
 
also, did you put your transfer case back to stock location yet, you will either need an sye or not, i think dropping your transfer case even 1/2 inch will only compound any problems you have and add new ones.
 
The transfer case drop definitely alleviated drive line vibrations throughout the vehicle on both deceleration ad acceleration as well as just cruising.

What i realized over the weekend was that one leaf spring is more arched then the other, so the Jeep was leaning about an inch and half toward the passenger side. The leaf springs were from Crown automotive just stock leaf springs.

I put some weight in the back on the driver side and boom the vibrations are totally gone. Thing runs smooth no grinding no vibes as if its perfectly running.

Has anyone experienced this, if so should i just leave the weight in the back for a few weeks in hopes that it bring down my leaf spring?

I tightened everything on the ground BTW.
 
The leaf springs should result in a level vehicle. I would wonder if they sent you one stock spring and one HD (+1 inch lift) spring.
 
That wouldnt be ideal, I will say they came in separate boxes in separate shipments from Qudratec.

You do make a good point though, when looking at them prior to installation i didnt notice anything severely different.

I will have to see if i can find some part number information on the actual springs themselves to identify.
 
Hello,

so putting weight in the back has helped some reducing the grinding sound but it's looking like my pinion angle is off because of the longer rear shackles. The spring packs appear to be the same which are heavy duty spring packs. I have read that they also provide some lift as well upwards of 2 inches.

I took some measurements and I'm trying to determine if axle shims are the best method. I need to bring my rear pinion down it angled pretty much straight at the TC and the angles are definitely different.

Bottom of the yoke on the TC is 5* straight to slightly downward

Drive shaft is 11* upward

Top of the Pinion Yoke on the Diff is 10* Upward

My understanding is that i need to subtract the angles from the driveshaft angle and account for some axle wrap maybe a 1 - 1.5*? (11-5=6 & 11-10=1) The total would be 7* minus 1* for axle wrap.

Would a 6 * shim in the rear work to bring the axle pinion angle down or am I doing this wrong?

Any and all help is greatly appreciated.
 
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