LMTV Front Diff Pinion Seal

tennmogger

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My front diff puked out most of it's lube on a short drive (1994 S&S LMTV) . The pinion seal blew and the protective shield on the yoke acted as an oil slinger and threw oil all underneath even on the tires. Had to clean it up before source was found.

I'd like to have a replacement seal available before disassembly. Does anyone have the size or/and industry standard part number?

This truck has served well with almost 3000 miles since I got it. Pinion seals might be a good part to keep in the spares box.

thanks

Bob
 

NDT

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Let me know how it goes with removing the pinion nut and flange. I pounded on that nut with a 1" impact for 5 minutes and it would not budge. On another diff, I got the nut off, but the flange would not pull off with my biggest puller even adding heat.
 

tennmogger

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Let me know how it goes with removing the pinion nut and flange. I pounded on that nut with a 1" impact for 5 minutes and it would not budge. On another diff, I got the nut off, but the flange would not pull off with my biggest puller even adding heat.
Ughhhhh. I am not looking forward to that! Thanks for the warning. Last night a friend with an LMTV pre-warned me that even the caps on the U-joints can be difficult to remove.
 

tennmogger

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Followup: the seal has not leaked again. I got to wondering if the leak (which happened in a very short time on the coolest morning this fall) might have been caused by pressurization of the axle. If the axle got pressurized, fluid could have been blown out.

Went looking for a vent cap anywhere on the diff or axle (to check for plugging) and found a couple of hoses to the left side axle tube. I'm thinking these are part of a vent system. Too wet under the truck to go crawling in the dark. A look through manuals didn't reveal anything about any vent system possibly being pressurized for fording purposes.

Could the CTIS system be involved? I don't think the central axle gets pressurized but the ends do. Seal leak into the main axle body maybe?

Any specific knowledge of this guys?

thanks,

Bob
 

Retiredwarhorses

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It's no longer leaking cuz all the gear oil is gone...LOL
reminds me of when a news reporter came on Ch47 I was on and stated "wow, look at all those leaks on the floor"
my reply was "you better hope those leaks don't stop"
 

tennmogger

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Got the pinion seal changed today. I've made some notes and taken some photos in case someone else has to do it.

Yesterday, in preparation to pull the flange/yoke, I made up a puller to allow use of a hydraulic jack. Thanks NDT for the warning so I could be prepared. The job went well with a little preparation (no, not Preparation H).

The parts diagram is in manual TM9-2320-365-24P (available here on SS in the manual section). See page 366, and the following 2 pages for part numbers.

The part numbers are:
- #21: Seal, plain, encased, A-1205-F-2502
- #22 Seal, plain, encased, A-1205-U-2257

I pre-ordered the two seals from Find-it Parts. They have been a good source for me, and prices are better than many other sources.

There are three hurdles:
- Pulling the caps off the U-joints
- Loosening the big pinion nut
- Pulling the flange/yoke.

Pulling the caps is no big deal if you think to use a big mallet to drive the whole driveshaft and fixed yoke toward the cap you want to pop out. Bump out one then the other using the U-joint as the push tool.

The big nut was on there seriously tight, but, a heavy wrench can be put on the nut (one I had was a drive-on-fit, but shims could be used) and held in place. The wrench could even be lashed on around the flange. Let the handle come up under the left spring pack to hold backup while you turn the tires on the front axle. Turn the tires by putting the tranny in 'Mode' and backing up. 'Mode' is required or the front shaft will just spin. The front driveshaft will turn as the rear one turns so has to be suspended to allow rotation. The big nut popped loose with a truck-shaking thunk. There must have been some Locktite or worse on it.

The puller I made worked great on the flange.

The bottle jack is a 6-ton and I was at MY limit of pull before the flange started moving. It had to be pulled almost all the way off with the jack.

puller in place.jpg

The seal was in such bad shape it pulled out with the flange. It is supposed to be a drive-fit into the nose of the pinion bearing housing. pulling yoke and seal came too.jpg

This differential only had the #22 seal. There was no #21. [edit: yes there was!! it was destroyed! read on] I put in #21 when it went back together. [edit: there WAS a #21 seal in there. It was destroyed beyond recognition.]

This photo shows a new seal #21 pushed onto the sealing surface of the flange. #21 runs on #22, face to face. #22 holds fluids in the diff, #21 keeps fluids, like water, out of the diff.

this seal was missing.jpg

The photo of the flange puller ended up at the bottom of the page.

When you put the big nut back on, it needs a little RTV sealant on it's inner surface to keep water from creeping under the flange into the splines. The shim pack behind the bearing unit housing also needs a film of sealant. Be sure to not damage those shims as the housing is removed.
 

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tennmogger

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The front pinion oil seal has blown out again. The seal is completely intact but was blown out of the pinion bearing housing. It was stopped by the yoke/flange. The sealing surface was still seated on the surface of the flange nose. The seal was a tight, drive-in fit when installed so I am convinced, as mentioned before, that there has been high pressure air built up behind the seal.

This seal blew on a cold morning, after just a couple of miles, just like the previous/original one did. Heat build up, air expansion, and lack of vent is not doing this, I think.

Pre-load of the pinion was less than specified so one of the pinion bearing spacers was lapped to increase preload. I wanted to be absolutely sure the pinion was not wobbling to contribute to seal loosening.

It's going back together this afternoon but this time I am not putting the diff fill plug back in. I'll put in a reducer with a hose barb and run that to a gauge so I can watch it. Hopefully I'll see any pressure build up before this second new seal is blown out. Maybe I can associate the blow-out with some event, like CTIS pressurization, etc.

I made an observation after putting in the first replacement pinion seal. The 'chatter' in the drivetrain, that common chatter that occurs after acceleration ends and when coasting begins, almost totally subsided after putting in the new seal. Could the chatter be accentuated by low pre-load on the pinion? A brand new seal should tend to subdue the chatter if loose pinion is indeed the cause of the chatter. I had also put in heavier front diff lube, too, using a GL-5 74W140 that I had available. I know that loose spider gears in the hub reductions has been credited with causing the chatter and I'm not saying that is not the cause.
 
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NDT

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If it were the rear axle I would guess the CTIS seals were leaking.
 
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Suprman

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The axle has to be vented somehow. Maybe the vents are clogged. Happens all the time on M939 trucks. Fluid will usually seep out of the hub ends I think its the path of least resistance.
 

tennmogger

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The axle has to be vented somehow. Maybe the vents are clogged. Happens all the time on M939 trucks. Fluid will usually seep out of the hub ends I think its the path of least resistance.
There is now a tube coming from the front diff into the cab, and a pressure gauge on the dash. I will see if there is pressurization when I take the truck out for a parade tomorrow. If I come home with a lap full of differential lube you can all have a laugh. So far the CTIS has cycled a couple of times with zero pressure showing on the diff.

Any idea where the vent tubes terminate on the LMTV? I tried to follow a probable vent tube from the front axle but can't twist and turn well enough to follow it along with others in the frame channel. Oh, to be agile and to hear air leaks again.:)
 

10Lugger

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Awaiting discovery, but willing to bet it is CTIS. My 1078 blew a rear pinon seal just after we bought it, same scenario as yours. Temperature in the 20's 1.5 miles from house and POW! High pressure blow from pinion diff fluid everywhere. Unhooked the CTIS and used that for a reason to do so called High speed gear swap. We did change out CTIS axle seals during diff swap, but have yet to hook up CTIS.
 

tennmogger

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After changing out the second blown front pinion seal, the cause of seal being destroyed is obvious.

When I pulled the original bad pinion seal (A1205U2257) I thought the secondary seal (A1205F2502) was missing. It was not missing, it was completely destroyed. When I pulled the replacement seals this past week, the secondary seal was again completely destroyed. That prompted me to make some measurements: there is no room for two seals!

By installing the seals as shown in the manual diagrams and parts list, failure is guaranteed. What happened (twice) is the secondary seal is destroyed as it is pressed into the primary seal. Eventually the remaining metal band in the secondary seal will be forced into the inner hole of the metal part of the big seal, and bind it up. Remember that the main seal is stationary and the secondary seal is moving with the yoke. Eventually the interference between the metal parts of the seals will work against the main seal seated in the nose of the pinion bearing housing, wearing out both.

The unknown is why the seals do not fit as shown in the manual. Maybe the design on the pinion bearing housing was changed? Maybe the seals have changed? Maybe an earlier or later model axle only used one seal?

Or, maybe the 'replacement' seal (120501092E) which is commonly available instead of A1205U2257, is designed to operate without the secondary seal. My axle had that replacement number originally and has been twice repaired with the 120501092E.

I left out the secondary seal when I reassembled the pinion housing. So far so good. The only down side that's obvious is that the main seal now has to also protect the pinion from outside contamination.

Following up on the theory of pressure building up behind the seal, well, no pressure was seen. I installed a pressure gauge on the differential housing and ran it into the cab. No pressure was ever seen during several long trips, including during many CTIS cycles and mode changes.

I am making a picture file to show the damaged seals and other stuff. Will post it eventually.
 
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tennmogger

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I hope this is the final chapter in the blown seal saga. Today was warm and sunny (a rarity this winter) and I was determined to find the vent on the front axle. Following the hoses again it became obvious I had jumped the track on earlier tracing of the hose, and this time I found the vent! It is hidden up inside the big upside down "U" that supports the cab rear. The vent is a simple hose end with a vent cap on it. Mine was completely crudded up. I had to work a wire at least 4" back inside the hose to clear it out. Air pressure was applied at the axle end of the hose during the cleanout to keep dirt out of the system. (Suprman suggested this was cause of failure weeks ago!)

In summary, two problems caused the failure(s): First, having two seals as shown in the manual drawings and parts lists allowed them to destroy each other. I am convinced the 'replaced by' part number primary seal was designed to eliminate the secondary seal. Second, the axle vent was plugged, allowing pressure buildup in the axle housing.

This probably points to another problem on my truck, a leaking CTIS seal that allows air to ingress into the axle housing. That's the next project when my high speed diffs go in.

Bob
 

Suprman

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I have the ctis seals that go on the axle. Good to know you found your problem. I just started getting into another truck working thru a whole set of new issues.
 

scottmandu

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I hope this is the final chapter in the blown seal saga. Today was warm and sunny (a rarity this winter) and I was determined to find the vent on the front axle. Following the hoses again it became obvious I had jumped the track on earlier tracing of the hose, and this time I found the vent! It is hidden up inside the big upside down "U" that supports the cab rear. The vent is a simple hose end with a vent cap on it. Mine was completely crudded up. I had to work a wire at least 4" back inside the hose to clear it out. Air pressure was applied at the axle end of the hose during the cleanout to keep dirt out of the system. (Suprman suggested this was cause of failure weeks ago!)

In summary, two problems caused the failure(s): First, having two seals as shown in the manual drawings and parts lists allowed them to destroy each other. I am convinced the 'replaced by' part number primary seal was designed to eliminate the secondary seal. Second, the axle vent was plugged, allowing pressure buildup in the axle housing.

This probably points to another problem on my truck, a leaking CTIS seal that allows air to ingress into the axle housing. That's the next project when my high speed diffs go in.

Bob
I've run all the vents to the engine air filter.
 

MaiNiaK

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Thanks for documenting your repair so thoroughly.

I have a rear pinion seal that's gone on my 1995 M1078. After reading the parts tm I've discovered the rearend uses the same 2 seals as the front. (I also noticed the replaced by part #)
That puller you built with the bottle jack, looks slick. I might just borrow your idea there.

I also believe I have a rear right hub seal that's allowing CTIS air into my axel. Which helped to push all my gear oil out while parked for a few weeks. (Right rear tire is slack)
Mine also started leaking on a very cold day.

Ive just been waiting on a little warmer weather to tackle my repair.

Thanks tennmogger.!!
 

tennmogger

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Hi Mainiak,

Of course that pulling jig is generic, copy as you please. I did the job the harder way the first time, when the pictures were made. Don't work so much under the truck. Remove the nose of the differential (the pinion bearing housing) and bring it to the bench to use the jig to pop off the yoke. Preserve the shim pack the housing sets on. Once the yoke is off, the seals are easy to remove with the pinion slipped out.

What seals do you find?
 
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MaiNiaK

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These are the pages I found in TM 9-2320-365-24P (#'s 21 and 22)

image.jpg image.jpg


This is what I found looking up A1205F2502 and A1205U2257 on ISO Group.

image.jpg image.jpg

This is is all I could find on FinditParts.

image.jpg
 

walshjuice

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I've run all the vents to the engine air filter.
Can you provide a picture(s) of how and where you relocated into the air filter? Any problems or issue since relocating? It seems like it'd be worthing doing. I'm in the same boat as the original poster.... a nice even coat of oil on everything around the rear axle. It also happened on a very cold day so I'm guessing my lines are obstructed/clogged. Thanks!
 
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