Fj40 Clunk when release clutch.. (1 Viewer)

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Jan 26, 2009
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kentucky
Hello,, quick back story, 1978 Fj40 JimC 2f, 4 speed, stock t case, 4 inch springs, super skinny swampers and lock-rite in the rear.. It's torque-y, twitchy, and sticky.

2f was rebuilt with a new clutch and the T case was rebuilt.. These only have a thousand miles on them.. Then I tore it down due to rotted frame. Few wheeling trips, but mostly parked.


I'm just now getting it back on the road after a doing the frame swap and full cage install.. So it was completely disassembled and reassembled.. I did leave 2f/trans/tcase together during frame swap.. But it's mostly been sitting during this last 3 year period..

As I said earlier, It has a lock-rite locker in the rear, and super skinny swampers.. So it's kinda sticky on pavement.

Soooo,, When I release clutch to take off (whether 1st or reverse), there is quite an audible single pop/clunk. As if something is binding and then releasing. It shifts fine once rolling. And rolls down the road great. But it has a single thud when I release the clutch from a stop. If I release the clutch extremely slow with no torque there is no thud..

The clutch does seem to release higher up the pedal than I remember.. But I removed clutch inspection cover and it seems to be doing what it should be doing upon initial inspection (not running with wife pushing in clutch).

I've went around and re-torqued and greased everything I could think of (just to double check my initial work). I tightened all the suspension bolts, u bolts, engine/trans mount bolts, drive shafts bolts, shackle bolts, etc.. Still pops/clunk when I release clutch to go.

Before this rebuild. I didn't have the clunk.

My driveshafts appear to be in phase, they are old, but no real slop in them that I can tell.

Will and old drive shaft cause a that loud of a thud/bang that you can feel/hear if you can't detect any slop in them?

It sounds as though something is perhaps binding then clunking and then releasing.. You can definitely feel it too..Especially if I kinda pop the clutch a little more than normal (not revving engine, just a faster clutch release than normal).

When I'm driving, if I hit 2nd or 3rd under fast acceleration, i can sometimes get a little clunk (but only if I'm really ON the throttle, power shifting and NOT all the time), but not loud like initial take off. And no sounds under normal acceleration.

I know it's hard to diagnose via this post... But I've searched and searched and searched and can't seem to find anything regarding the thud I'm referring to..

I'm really itching to start driving this thing again because my sons are old enough to enjoy it with me now.., but I don't want to destroy something else during shakedown. Especially if it's the beginning of a detrimental failure...

Any quick suggestions of ways to find the culprit? im worried it's could be a bad output shaft bearing or shaft? How do I check this?

I removed rear driveshaft, put in 4hi, locked front hubs, still clunks!


Does this sound like possible loose output shaft issues ? Ujoint issues? Locker issues? Clutch adjustment? Or 4inch springs and un-shimmed rear end pinion angle issues? Drum brakes sticking?

I've greased the driveshafts, checked the e brake bolt 90lbs. I did noticed there is about 1/32 of play if I REALLY REALLY YANK/PUSH up on the e brake bell via driveshaft... But it doesn't feel excessive by any means.. But maybe this is enough?.. Is this a sign of a loose output shaft bearing? Bad bearing? Bad shaft (splines looked good inside e brake service e Brahe during rebuild). Haven't inspected the rear Dif yet,, but Zero slop at pinion..

I can't pinpoint where it's coming from.. And it's driving me nuts.. If I just release clutch slow, it would never happen.. But I know there is an issue lurking..

Thanks in advance.
Kevin
 
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micruz60

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just out of the J-Hole
A mystery.
Can you locate WHERE the thud comes from, feels like it is right under your right foot?
Adjust slave cylinder pushrod?
I dunno, first and reverse only? Linkage off slightly?
 
Joined
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kentucky
A mystery.
Can you locate WHERE the thud comes from, feels like it is right under your right foot?
Adjust slave cylinder pushrod?
I dunno, first and reverse only? Linkage off slightly?

It is definitely centrally located, I'm pretty sure it's between transmission to rear dif? Hard to describe, but you can definitely feel it and hear it.. Bummer..
 

micruz60

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just out of the J-Hole
Uh, hmm.
Raise the rear axle, two wheel drive. Try to find the clunk with a helper?
Safety first, block the front wheels. I wonder if the lockrite is bound....
Seems more clutch related tho.
. . . .
 
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Uh, hmm.
Raise the rear axle, two wheel drive. Try to find the clunk with a helper?
Safety first, block the front wheels. I wonder if the lockrite is bound....
Seems more clutch related tho.
. . . .
Finding a helper is essential.. Just doing some preliminary investigation.. But I will definitely jack up the back and do some testing. The only reason I don't think it's clutch is when I remove rear drive shaft, and put in fwd it doesn't clunk.
 

1911

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... But it's mostly been sitting during this last 3 year period..

Drive it some more before you get too worried about the noise. Could be something as simple as some surface rust on the flywheel making it a bit sticky for the clutch. Maybe bleed your clutch slave cylinder to make sure it hasn't leaked a bit in three years?
 
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Check the Nut that holds the transfer case and transmission , I would have a clunk between first and second for almost a year couldn't find it. When I took the transfer case off the nut for the transmission spline was loose.
 
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Finding a helper is essential.. Just doing some preliminary investigation.. But I will definitely jack up the back and do some testing.
Drive it some more before you get too worried about the noise. Could be something as simple as some surface rust on the flywheel making it a bit sticky for the clutch. Maybe bleed your clutch slave cylinder to make sure it hasn't leaked a bit in three years?
Good call. I will bleed clutch system again tonight. Only reason I don't think it's clutch, is no clunk when clutch released in fwd with rear shaft removed.. But it doesn't hurt to check.. Thanks
 
Joined
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Check the Nut that holds the transfer case and transmission , I would have a clunk between first and second for almost a year couldn't find it. When I took the transfer case off the nut for the transmission spline was loose.
Can I get to that nut with trans and t case in truck?
 
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i just had a similar clunk and it was the nut on the pinion came un staked. Put in neutral with wheels chocked front and back and e brake off. See if pinion flange has any play. If so remove driveshaft and check to see if nut came loose. My 2c
 
Joined
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kentucky
i just had a similar clunk and it was the nut on the pinion came un staked. Put in neutral with wheels chocked front and back and e brake off. See if pinion flange has any play. If so remove driveshaft and check to see if nut came loose. My 2c


Thank you.. It does have a newer pinion seal in it.. Maybe I didn't stake it enough.. That would be great if this was the case. Will check when I get a few. Thanks
 

78fj40mg

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It sounds like a slop issue in your diff.

I had the exact same issue with my '78 so I rebuilt my third member...

The 3rd member bearings were in sad shape, both carrier bearings and races were pitted, both pinion bearings were bad and had to cut one out since it was stuck on the pinion shaft, so there was a lot of bad juju in there. I'm glad I decided to take it apart completely and rebuild it hence no more clunking.

Clunking of diff before rebuild vid
 
Joined
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kentucky
It sounds like a slop issue in your diff.

I had the exact same issue with my '78 so I rebuilt my third member...

The 3rd member bearings were in sad shape, both carrier bearings and races were pitted, both pinion bearings were bad and had to cut one out since it was stuck on the pinion shaft, so there was a lot of bad juju in there. I'm glad I decided to take it apart completely and rebuild it hence no more clunking.

Clunking of diff before rebuild vid
Thank you.. I guess it's time to dig into the rear diff too..
 
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Good call. I will bleed clutch system again tonight. Only reason I don't think it's clutch, is no clunk when clutch released in fwd with rear shaft removed.. But it doesn't hurt to check.. Thanks


Actually removed the rear shaft today AGAIN. And put in fwd. AND LOW AND BEHOLD, It does still make the clunk..

So maybe it is in my clutch, trans, or t case.... Gonna re bleed and adjust clutch and see if that helps.. Thanks again.
 
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Actually removed the rear shaft today AGAIN. And put in fwd. AND LOW AND BEHOLD, It does still make the clunk..

So maybe it is in my clutch, trans, or t case.... Gonna re bleed and adjust clutch and see if that helps.. Thanks again.
did you check that the nut for transfer case ?
 

allan man

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There's a cone shaped cover on the back of the t/case, its usually black. You remove the cover and tighten & stake the nut. It's not uncommon for that nut to loosen up, and when it does, it creates slop in the driveline.

Interesting to know...what is the recommended torque setting on that nut?
Anyone know off hand.
 

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