Loud clonk while shifting gears

[tomm998]

Hi guys!

I´m having a small but annoying problem, and I would appreciate this groups expertise in this matter. My truck is a 1985 M998 from the USMC, and I use it and maintain it on a regular basis.

I have changed all the fluids and done all other maintenance that should be done since i got it home, and the truck runs perfect, shifting gears up and down , and everything else works as it should.

When truck has been parked overnight, and I put it in drive or reverse , the gears engage like a dream. But when it´s hot and I´m changing gears, I hear a loud clonk. This appears when i´ve been driving and changing from D to R. Other times i get a clonk when I change from R to D. Always when the truck is warm and has been driven

I have also checked play in the drive shafts, but it´s rock solid.

The feeling i get (I could be far out)… Its´s like there is a build up of pressure in the gearbox/transfercase during driving, that creates these clonks when shifting gears. Next day when the truck is cold, the shifting is smooth and silent again.

The idle rpm:s are exactly according to the TM so that wouldn´t be the problem with too high rpm:s while shifting. I always do the shifting between gears with truck stopped. Always!

Could it be some vent lines somewhere that are clogged?

Could it be the transfer case chain slack that´s the problem?

I´m a former military and a good maintenance man, but i´m a lousy mechanic…

All ideas and advice appreciated.

Cheers from Sweden,

Tom

 

[DREDnot]

An easy thing to do is torque all your half shaft attaching bolts.
A common source of a clonking type sound like you describe.
Every truck I've inspected has had at least one axle that needed its bolts tightened to spec.

 

[tomm998]

An easy thing to do is torque all your half shaft attaching bolts.
A common source of a clonking type sound like you describe.
Every truck I've inspected has had at least one axle that needed its bolts tightened to spec.

Thanks!
I´ll check the bolts, but I´m still trying to understand why the problem only occur when truck been driven.....
Cheers, Tom

 

[papakb]

I'd be willing to bet that you have a u-joint failing and that's something that needs to be repaired before something catastrophic happens. The u-joints are often overlooked during PMs because they're a PITA to lube. To properly lube them you need to rotate the drive shafts to open up the clearance enough to get the grease fitting between the yokes and it just doesn't get done. I just replaced mine and when I got them out the grease in the center cross was solid all the way thru. I had to run a drill thru them to open them up!

 

[rcamacho]

Ditto ref universal joints. I just replaced all the front and rear shaft joints in my M998. All were loose/worn to some degree. One was bone dry and approaching failure immanently.

Also a good opportunity to check and replace the front shaft carrier bearing if you have the early model front shaft.




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[FKAM]

I have the same problem. I couldn't find anything obviously wrong. I actually replaced all the universal joints in the rear driveline (the originals appeared fine but still replaced them), checked the torque on the half shafts and replaced all bolts and washers. I had rebuilt the geared hubs prior for other reasons so I know they were good. I'll be interested if your bang goes away after you check yours. I'm down to possibly the problem is somewhere in the differential/tranny/transfer. I isolated the front drive train and still had the same bang when going from R to D or D to R. Did all the same checks on idle speed ect.

One thing that was suggested to me by a mechanic (that I trust) is if the tranny is worn or dirty it may be engaging the drive train too soon because the valves are left in a slightly open position. He said if that is the case a good tranny flush may solve it. But he also said if the tranny is worn baldy enough a good flush may actually cause it to stop working, bcause you clean out the debris that keeps the valves just working enough. I replaced the filter and the pan had a lot of debris in it, so the bang may very well be associated with the tranny. So I've been holding off on flushing until I figure other things out first. Hate to go from bangs to nothing at all.

One thing I have found is, if I place a good deal of pressure on the brakes, it seems to mitigate the bang and at times eliminate it completely. Which makes me think the problem is in the diff/trans/transfer, because the brakes are inboard of the halfshafts and geared hubs in the drive train.

I have also have an acceleration issue, when the tranny is heated up, that may be associated with this one and I'm trying to figure it all out.

So let us know what you find. It may help me out too.

 

[Milcommoguy]

IMO..... Accumulated gear backlash at every point in the power train. Some points more that others. Your on the right track... transfer case, drive line joints, diff's, hubs. I had the same issue and chased, turned, rocked, locked, blocked, removed and replaced components to isolate the bigger clunks. Knowing what I have done, IMO there a lot of slop / wiggle room in all the fit up. One point of wear was discovered in the differentials. A good design, but if abused (and who knows how they have been driven ?????) easy to go clunky mode. See pictures.


I wonder how many pull the covers and take a look for the ckunk,

Here's a look at a new one. I think clunks were designed into the power train. Could be wrong.

It's a HumV, Not a pocket watch, CAMO

 

[tomm998]

I'd be willing to bet that you have a u-joint failing and that's something that needs to be repaired before something catastrophic happens. The u-joints are often overlooked during PMs because they're a PITA to lube. To properly lube them you need to rotate the drive shafts to open up the clearance enough to get the grease fitting between the yokes and it just doesn't get done. I just replaced mine and when I got them out the grease in the center cross was solid all the way thru. I had to run a drill thru them to open them up!


View attachment 822804View attachment 822805View attachment 822806

Thanks Frank!
I think I´ve been able to grease them, but I´ll have to check them all over again.
Cheers,
Tom

 

[Action]

Jack one corner up and have someone quickly turn the tire back and forth. You get under and find the spot with the most noise.

 

[tomm998]

I have the same problem. I couldn't find anything obviously wrong. I actually replaced all the universal joints in the rear driveline (the originals appeared fine but still replaced them), checked the torque on the half shafts and replaced all bolts and washers. I had rebuilt the geared hubs prior for other reasons so I know they were good. I'll be interested if your bang goes away after you check yours. I'm down to possibly the problem is somewhere in the differential/tranny/transfer. I isolated the front drive train and still had the same bang when going from R to D or D to R. Did all the same checks on idle speed ect.

One thing that was suggested to me by a mechanic (that I trust) is if the tranny is worn or dirty it may be engaging the drive train too soon because the valves are left in a slightly open position. He said if that is the case a good tranny flush may solve it. But he also said if the tranny is worn baldy enough a good flush may actually cause it to stop working, bcause you clean out the debris that keeps the valves just working enough. I replaced the filter and the pan had a lot of debris in it, so the bang may very well be associated with the tranny. So I've been holding off on flushing until I figure other things out first. Hate to go from bangs to nothing at all.

One thing I have found is, if I place a good deal of pressure on the brakes, it seems to mitigate the bang and at times eliminate it completely. Which makes me think the problem is in the diff/trans/transfer, because the brakes are inboard of the halfshafts and geared hubs in the drive train.

I have also have an acceleration issue, when the tranny is heated up, that may be associated with this one and I'm trying to figure it all out.

So let us know what you find. It may help me out too.

Thanks!
It`s kind of a mystery, but I have to check the tranny. I have changed all the fluids in the entire truck, but I´ll probably have to do a methodic search in order to rule different problems out. As I wrote, i was wondering of a slack in the trannys chain could cause the problem.
We´re getting snow here now, so I´ll wait a while before I´ll get to it.
Cheers,
Tom

 

[tomm998]

Jack one corner up and have someone quickly turn the tire back and forth. You get under and find the spot with the most noise.

Okay, thanks!
I´ll try that.
Cheers,
Tom

 

[tomm998]

IMO..... Accumulated gear backlash at every point in the power train. Some points more that others. Your on the right track... transfer case, drive line joints, diff's, hubs. I had the same issue and chased, turned, rocked, locked, blocked, removed and replaced components to isolate the bigger clunks. Knowing what I have done, IMO there a lot of slop / wiggle room in all the fit up. One point of wear was discovered in the differentials. A good design, but if abused (and who knows how they have been driven ?????) easy to go clunky mode. See pictures.
View attachment 822838 View attachment 822839 View attachment 822841 View attachment 822840

I wonder how many pull the covers and take a look for the ckunk,

View attachment 822843 Here's a look at a new one. I think clunks were designed into the power train. Could be wrong.

It's a HumV, Not a pocket watch, CAMO

Haahaa, thanks!
I´m used to WWII Dodges doubleclutching and all, so no watches for me....
The thing that gets me confused is why it only clunks when it´s been driven/varm. The damage shown at your pics would probably clunk when shifting, cold or warm?
Cheers,
Tom

 

[tomm998]

Ditto ref universal joints. I just replaced all the front snd rear shaft joints in my M998. All were loose/worn to some degree. One was bone dry and approaching failure immanently.

Also a good opportunity to check and replace the front shaft carrier bearing if you have the early model front shaft.




Sent from my iPhone using Tapatalk

Thanks, I will!
Cheers,
Tom

 

[Milcommoguy]

With the trans clutches coming on/off, all the gears meshing, planetary in the trans and transfer case, chain flopping and spinning parts at right angles to get the spindles to turn, it a wonder they even move.

Might work to have a helper working the selector and roll around underneath looking for what parts are winding up and listening for DaClunk. Be Safe.

I would like to ask the gang here... is this somewhat normal or are they tight as a rubber band ready to launch.

Asking and lots of parts to isolate, CAMO

 

[tomm998]

Thanks,
I will try everything that you and the others in this forum so kindly supplied me with.
Have to wait a while because the winter make i tough to work on my truck due to freezing temperatures. (No heating in my rented space in garage at a farm....
Cheers,
Tom

 

[jkcondrey]

Haahaa, thanks!
I´m used to WWII Dodges doubleclutching and all, so no watches for me....
The thing that gets me confused is why it only clunks when it´s been driven/varm. The damage shown at your pics would probably clunk when shifting, cold or warm?
Cheers,
Tom

The "warm" part would to me say its use related for sure. As the vehicle is used, heat builds in alot of moving parts and thus expansion, exacerbating the clunk. Id start simple. Check your fluid level in the transmission and transfer case when warm and see if its incorrect. Then those u joints after its been driven awhile, get under before they can "cool" off and see if any play can be found. It does not take much movement in a joint to make that clunk.

 

[papakb]

Mine clunked like an SOB with only that one U-joint bad! I discovered it by accident on a monthly inspection when I grabbed the drive shaft to scoot around on my creeper. I guess I'd never done that before or it was always in the right spot when I grabbed it. All the others seemed to be solid but with the grease in that one solid as a rock I guessed that none of them had been serviced properly.

I'm extremely lucky that it didn't come apart somewhere on the road. It was certainly welll on the way.

 

[tomm998]

The "warm" part would to me say its use related for sure. As the vehicle is used, heat builds in alot of moving parts and thus expansion, exacerbating the clunk. Id start simple. Check your fluid level in the transmission and transfer case when warm and see if its incorrect. Then those u joints after its been driven awhile, get under before they can "cool" off and see if any play can be found. It does not take much movement in a joint to make that clunk.

Yeah,
That sounds like a good idea to start with. I´ll have to wait for better weather before getting it out in order to obtain working temperature.
I know my oil level int the transmission is okay, but I´ve never checked the level in the tranny "hot". Only cold when I changed the tranny oil.
Cheers,
Tom

 

[tomm998]

Mine clunked like an SOB with only that one U-joint bad! I discovered it by accident on a monthly inspection when I grabbed the drive shaft to scoot around on my creeper. I guess I'd never done that before or it was always in the right spot when I grabbed it. All the others seemed to be solid but with the grease in that one solid as a rock I guessed that none of them had been serviced properly.

I'm extremely lucky that it didn't come apart somewhere on the road. It was certainly welll on the way.

Thank you!
I´ll do a thorough check of all my u-joints when weather improves here.....
Cheers,
Tom

 

[tomm998]

Mine clunked like an SOB with only that one U-joint bad! I discovered it by accident on a monthly inspection when I grabbed the drive shaft to scoot around on my creeper. I guess I'd never done that before or it was always in the right spot when I grabbed it. All the others seemed to be solid but with the grease in that one solid as a rock I guessed that none of them had been serviced properly.

I'm extremely lucky that it didn't come apart somewhere on the road. It was certainly welll on the way.

Do you remeber wich u-joint that were bad?
Tom