thump or clunk in the rear end

[benoit454]

Hey guys, I have another question for all of you. Well we have an 84 c10 with a 305 ho, and th350 auto, and the sticker said 2.73 locking differential.

Well ever since we got it when you'd be going slow, like from a stop and 1s take off you'd get this thump, like somebody kicked your rear tire while you started moving. it will do it more when you are cornering, and going slow, and it seems to be doing it alot more lately. My dad said it acted like my monte carlo when I get it, because they had a mini spool in it and he said mine clunked and all when it did.

Well would it be the locking differential doing it? Bad bearing in the rear end?

Brakes maybe? I know I feel it on the passenger side but I've never ben on the driver's side to feel it, so not sure if it's side specific or not.

Rear end grease is good, right to the bottom of the fill plug.

Brakes are new, drums turned, and new wheel cylinder on my side.

Only time it does it is from a dead stop, or going very very slow and stopping, like back and forth, go and stop, stop and go. Mainly anytime you stop and start going again or are going very very slow, like maybe 1 mph or so, it does it, after that, it's fine doesn't seem to thump or vibrate or nothing.

Tires and wheels are new and balanced, front end is fine, trans. and motor mounts are good. I have no idea what it is, thinking a bearing is going down in the rear, maybe carrier or axle bearing?

Feels just like when it happens, somebody is kicking the back tires while it rolls, feel it in the whole truck, doesn't make a noise, just feel it.

[f.monroe]

If it is a clutch type posi you probably need to add a bottle of posi lube to the rear . If it is a centrifugal locker, that probably isnt the problem . The rear brakes stick sometimes and will do the exact same thing you are talking about . I have been told the springs get weak and dont pull the brake shoes in when you let off the brakes . If that happpens , they let go once you roll out a little and the truck jerks as the brakes release . Just a thought . Frank

[68MoneyPit]

if it is a single thump I have no clue, but if it is a rythmic thumping that goes away when you pick up speed you should check for slack in the drive shaft. I had the same problem on a two piece shaft on my 2000 s-10. It only did it at low speeds and went away when I got above 20 or so. found out it was wearing at the pinion. Something tells me that yours won't be covered under the factory waranty though.

[Blackstone]

My '73 does the same thing. For what it's worth.... an old-school racing guy told me to drain the diff, dump in a quart of Kerosene, jack up the rear, start it and put it in drive for three minutes. Next, drain the Kero and replace with "special gear oil for positraction rearends" from the GM dealer. Sounded scary to me but I don't know jack about differentials.... The word is he's the sharpest guys in the state in regards to building race cars.

[N2TRUX]

Originally Posted by Blackstone

My '73 does the same thing. For what it's worth.... an old-school racing guy told me to drain the diff, dump in a quart of Kerosene, jack up the rear, start it and put it in drive for three minutes. Next, drain the Kero and replace with "special gear oil for positraction rearends" from the GM dealer. Sounded scary to me but I don't know jack about differentials.... The word is he's the sharpest guys in the state in regards to building race cars.

What he is asking you to do is flush your rear end. The Kerosene will wash the old gear lube out, and get it ready for fresh lube. You must add the limited slip additive, or the clutches will not grab.

[dooce]

It may very well be a u-joint. If one of the caps pf the u-joint have fallen off, it will make a loud clunk everytime the u-joint comes in contact with the yoke. I've only seen it when putting it into gear, but you never know.

[Blackstone]

Originally Posted by N2TRUX

What he is asking you to do is flush your rear end. The Kerosene will wash the old gear lube out, and get it ready for fresh lube. You must add the limited slip additive, or the clutches will not grab.

I'll add that any more than a few minutes can destroy the bearings.

Just for comparison's sake, here's what my truck does... Only on takeoff from a dead stop, there is a "chunk, chunk" type sound that will actually create a slight side-to-side jolt to the rear of the truck. It would only do it 1-2 times going straight, 3-4 times while turning in either direction. He was 100% sure that the clutches were siezed, and that the kerosene trick would free up the unit.

Of course, check the obvious items first, such as u-joints and brakes before introducing kerosene to your bearings.

[jimfulco]

'80s Suburbans are famous for having a clunk like that, and I imagine a pickup could do the same thing. It's the driveshaft yoke splines binding when you stop, and then letting loose suddenly when you start to move again. Sometimes it clunks loose just as you come to a complete stop. The Chevy place told me their fix is to pull the driveshaft and clean the splines, then slather them with anti-seize compound and put it back together. It worked on a friend's '87 Sub for about a year.

[wedgemon]

I've posted my history on the "Click/Clunk" before, But I can't find the post.

So here goes. (Short version summary)

Had a 90 Caprice, clunked on stop and start. Turned out that I cracked and almost spun the center of the flex plate (where palm of crank bolts thru) completely separate from the rest of the plate. It had just enough interference that it would spin the T/C and move the vehicle. But, The clunk got progressively worse and would have eventually spun free.

Pull the inspection cover and shine a light up in there.

This problem can act a LOT like U-joints!!

- Dave