"Thunk" in four-wheel drive

[Tim Smith]

So we are going through this northeaster right now and I just had to take my 88" to work today (arm twisted ).

With the roads so bad, I had to leave it in four-wheel drive while keeping it in a controlled slide most of the way to the office.

Then all of a sudden there was a thunk. A loud thunk. A thunk that sounded like a hammer against one of the gears in the transfer case. It occurs once every 200 feet or so and only when going above 25 mph while the engine was under load going up hill (but not down hill). Also, if going up a steep hill at speed the rear end felt like it was kicking out a bit right when the thunk happened. As if the rear end had just stopped momentarily.

It didn't happen under full lock in the covered parking garage at work but the truck did hop a bit as it should. So I know four wheel is working and that pressure build up in the transfer case doesn't make that sound at low speeds.

Any ideas? I'm thinking that it's a bad gear in the rear end transferring the pressure into the transfer case but only after a momentary lockup of the rear wheels.

Cheers,
Tim

[KingSlug]

I had it happen once, chipped 2 ring gear teeth. I would bet its the back.

[Tim Smith]

I had it happen once, chipped 2 ring gear teeth. I would bet its the back.

Thanks KingSlug. Do you mean that it was caused by two chipped ring gear teeth or that it, being the thunk, caused the chipped teeth? I'm guessing the former but wouldn't the truck do it in 2 wheel drive? Assuming it's the rear that is.

Dang I'm hoping to make it to Maine this weekend... a 7 hour drive. I assume this wouldn't be wise with a messed up rear end.

[KingSlug]

Ring gears can break just like axles, mine sounded worst at slow speeds because the pinoin wouldnt coast across the 2 bad teeth like it would at higher speeds. I would get a tow home or swapout the rear at work, it would suck if you lost more teeth, damaged your pinions and broke an axle. If the chips havent cause much damage just pull the third member and swap in another and you are off to Maine. You can do it in 30 minutes if you have a garage, lying on your back in the snow give it an hour.

[Tim Smith]

Ring gears can break just like axles, mine sounded worst at slow speeds because the pinoin wouldnt coast across the 2 bad teeth like it would at higher speeds. I would get a tow home or swapout the rear at work, it would suck if you lost more teeth, damaged your pinions and broke an axle. If the chips havent cause much damage just pull the third member and swap in another and you are off to Maine. You can do it in 30 minutes if you have a garage, lying on your back in the snow give it an hour.

Unwittingly I drove it home already so no tow needed. Lucky!

If I had another diff I'd surely swap it out. Since I don't have one I think I might be stuck missing the Romp this year.

I'll have to do some more testing because I really don't want to miss it. Thanks for your advice though. You may have just saved me a huge bit of aggravation.

[Tim Smith]

Okay, so I just did a test drive in 2wd with the front hubs locked and there is no thunk. By the way, I usually have the front hubs unlocked.

Could this be the front diff? Although the rear has been making some whirring noise since I installed it (used) more than a year ago, this particular problem has never cropped up till now. I've also never driven past 25 mph in 4x4 so I'm dealing with two firsts here.

Is there anything else that happens in a series truck in 4wd at road speeds that could be causing this?

[Tim Smith]

Okay, just tried it in 4wd locked hubs and then unlocked. When locked I get the thunk, unlocked no thunk.

So it's the front diff then, right?

[Mercedesrover]

Sounds like your drivetrain is getting loaded up and releasing. Are the tires all the same size? Are they worn the same? I really don't think you have anything to worry about.

If you had a broken ring gear you'd get a clunk with every rotation of the tires.


Jim

[Clive]

A friend's truck had a thunk transmitted through the drive train when his free wheeling hubs went west....just a thought.

[Tim Smith]

Thanks Jim. I think you are on to something. Since I've never had the truck in 4wd at speed, I guess I was bound to learn something new about the truck. It just sounded terrible!

2wd was fine this morning, so either way I'm going to ME this weekend. Keep an eye out for a stranded lightweight.

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Clive. I swear, if it's these dang front hubs acting up again then they are going into the trash heap! I've spent enough time on them that they had better be working right by now.