Very low front differential oil level: hoping for the best, preparing for the worst

[kestrelnh]

I got my 1992 M998 a few days ago, and began to go through it. I changed the rear diff oil, and then went to go change the front, but close to nothing came out!! The diff had not made any noise to indicate things were in a bad way, and the magnetic plug had some shavings on it (for that matter, so did the rear), but nothing ridiculous. I've refilled it, and after I check all the other fluids in the truck and make sure nothing else is empty, I'll take it for a spin and change the fluid again to flush out what I can.

I have no idea how long the truck was like this, and since it doesn't seem to be leaking when static, I have no idea how many miles it was driven low/out of front diff oil.

Maybe I'll escape having to rebuild it, but I want to have a plan in my pocket in case I have to.

I've read the TM on differential repair (TM-9-2320-280-34, chapter 19), and there seem to be a largish number of specialized tools. I'm not sure I have the skill/equipment/time to be able to do it by myself. Who rebuilds these differentials? Is it something a local shop can do, or is it a specialized job to be sent out?

Currently, the big auction site seems to be full of 2.73 diffs for reasonable cost, but some research has indicated I probably have 2.56 gears, which appears to be a bummer from a parts availability standpoint.

And so it begins

 

[papakb]

The differentials use the oil for lubrication and to help transfer heat. If they are built correctly they can run with a bare minimum of oil without doing any damage to the gearing. If you change out the diffs make sure you get the same gear ratios front and rear otherwise you'll end up tearing up your transfer case. Higher number ratios are better for faster starts but they limit top end, lower numbers make for slower starts but higher top end speeds. There are also a lot of 3.08 diffs around too.

Kurt

 

[patracy]

I unfortunately ran a front (and rear) diff without fluid for over a thousand miles. It locked up on me after some metal got in the R&P. The rear looked fine though. If it were me, I'd run the front for a few hundred miles and drop the fluid again, checking for metal. Then judge accordingly.

I bought my replacement front end from CSI in Tallapoosa, GA. I know they had a large container filled with them. I can get you Tom's number if you like.

If your HMMWV is a earlier model, it's likely a 2.56 geared diff. 2.73's came in the later 90's trucks. 3.08's (not 3.06) came in the latest variants, with armor. I haven't had any issues with my 2.56 geared diff when it comes to their effectiveness.

 

[kestrelnh]

Thank you for the very useful information! I'm going to try to stick to 2.56 if I can, and you've given me some hope that the diff may have escaped certain doom.

For everyone new to this (like me) reading this thread, if my experience is any guide, I can't recommend enough to go through the truck before you take it out. Mine is a 1992 m998 with 25,000 theoretical miles on it. I think that overall it's in pretty good shape, and the engine appears to be a brand new 6.5, but I've found enough other things (largely in the steering system) to make me very glad I'm spending a few weeks bringing it up to snuff before going out on the trail. There's a lovely sticky thread in this forum about the items you should go through before taking out your HMMWV. Of all the items I've read in that thread, checking the spindle nuts appears to be one of the most important. I'd hate to have a wheel pass me!

 

[patracy]

It's hard/impossible to be 100%, but being a 92, I'd bet it's 2.56. I *think* it was around 94 or so when they started going to the 2.73's. I was worried about the ratio as well when I swapped the front diff.

I'd like to keep my 2.56 gears and add a 4L80e or 700R4 one day to mine. Then add some soundproofing to the floors/tunnel/doghouse.

 

[papakb]

Sorry for the clerical error Pa, I was running off of memory and it just ain't what it used to be. LOL
Kurt

 

[kestrelnh]

A quick update. After I put ~2,000 miles on the truck, it's time to put it away for the winter. In the spring I filled the diff, ran it for a few miles, then changed the fluid. Now 2000 miles later the front diff acts fine, but the fluid came out black, and there were a few shaving on the magnetic plug, and one of axle output seals leaks just a bit. I changed the rear diff oil as well, and that was as clear as the day I put it in with no shavings. We'll see how long the front one lasts, but I think it's time for me to keep my eye out for rebuild parts or a new diff. At the very least I need to pull it to replace the seal.

I also changed (again) the gear oil in the portal reduction gearboxes, and the oil looked fine, but there were shavings on each of the magnetic drain plugs, so I'll keep an eye on that too.

Thank you all for the guidance!