Clunk - PeachParts Mercedes-Benz Forum

PanzerSD · #1 02-26-2006, 04:18 AM

When shifting from P to D or from R to D, there's a Clunk in the rear of the car, followed by the obligatory sack down..Like hydraulics

is that flex discs? or something internal with rear diff?
also when you park on a slight incline after you put it in park, it rolls back about a foot before it actually parks. does this on both forward and rearward inclines. Also, on the same token, when you're driving, lets say in 4th, and you take your foot off the accelerator slightly and then regain throttle, the clunk rears it's head as well......it's driving me nuts.....

Brian Carlton · #2 02-26-2006, 04:48 AM

This definitely points to excessive clearance in the diff. You can do a check and confirm it by jacking and supporting the rear, blocking the front, putting it in neutral, lock the parking brake, and attempt to rotate the drive shaft. If the diff is operating correctly, you won't be able to turn the shaft more than about 1/8"........maybe less. I suspect that you will find that you can rotate yours by about 1" or so. This would confirm the problem with the diff.

Ken300D · #3 02-27-2006, 10:49 AM

Most likely you have the differential wear issue, maybe similar to the description below or maybe some other component in there:

Excessive W123 Differential Wear - Pictures

But you should also take a quick peek at your flex disks to make sure they are not almost gone.

I would do the flex disk inspection right away. While you are under there, rotate the driveshaft by hand and see how much "slop" you have back and forth. When the driveshaft turns significantly back and forth (i.e. the input to the differential) but the wheels don't turn, you obviously have a worn differential.

Flex disks are critical because they can do a lot of damage when they fail. It may be possible to hold off on the differential replacement, if required, until warmer weather.

By chance, does your car (or did it) have a harsh transmission shift? Particularly 1-2? That's what caused the differential wear you see in the linked posting.

Ken300D

TheDon · #4 02-27-2006, 10:53 AM

soo that explains everything.. the rolling back when parking the clunks .. damn it.. so what would be a good rear diff to put in that would optimize driving .. like a different ratio? and from what vehicle and how much does it usually run to buy?

t walgamuth · #5 02-27-2006, 01:09 PM

that i would suspect is the diff. they are extremely long lived.

the other items should be checked as noted above.

the clutch pack in the tranny (i think maybe the reverse clutch pack) may also be going bad. this occurrs if folks put it in reverse while rolling forward and vica versa.

if not too bad this can be lived with for years.

tom w

Ken300D · #6 02-27-2006, 01:15 PM

The ideal situation would be to find one in a reasonable junkyard where they would let you pull it for a flat rate - then you can test them all and pick the one with the least play. However, doing the actual pull is not an easy job due to the part location, etc.

Next best might be a used parts retailer or EBay. My replacement came from Potomac German Auto and they had several at the same price. But the winner was a low-mileage 1985. The 1985 W123 differential had what is generally thought to be a better highway ratio. 2.88:1 instead of 3.07:1

So I lucked out on the convenient local supply and didn't have to pay shipping. Inflation being what it is, you shouldn't rely on my past price - $135.

Ken300D

Hatterasguy · #7 02-27-2006, 04:16 PM

My SD does this as well. It botherd me at first but I am learning to ignore it.

PanzerSD · #8 02-27-2006, 11:47 PM

Yeah, I ran some trials today on a deserted road and i can almost visualize that pin slopping around in the carrier...damnit...

Are all 300SD rear diff's interchangable?? or is it 116 chassis only?

The instant I saw that pic, the wheels in my head started turning, you see, and I came up with this idea.
Lets say I take my carrier assy. to a machinist. I have him make the slots (assuming there would be two of them) identical, then make a plug of HSS steel and press the plug into the reformed slots and drill a new hole for that pin...ya' suppose that'd work?

Brian Carlton · #9 02-28-2006, 12:14 AM

I have a 3.07 from the SD that you can have at a reasonable price. It does have a touch of pinion whine........so........you would probably prefer to have someone knowledgeable set the gear lash before you use it. Or, if it doesn't bother you, it can probably run forever with it.

pawoSD · #10 02-28-2006, 12:32 AM

The one in our 83 clonks when it goes into 4th gear with no load on it. Not much when engaging the gears though, it does it gently. Mine would clonk it if it was my tranny and that diff, thankfully my differential is in way better shape...

PanzerSD · #11 03-03-2006, 12:37 AM

Finally had time to get underneath the car, and everything is nice and tight, I rotate the driveshat barely .25" in rotation and the axles begin to move, so I'm pleased with that, so this "clunk" that I feel must MUST be either flex disks or internal transmission wear..

Pete Burton · #12 03-03-2006, 08:43 AM

Well, mine has clunked for the 9 years, 90K+ miles I've put on it. It's not the transmission, as I rebuilt that last year and it's exactly the same. It's an older car and that's part of its character as I see it. Yeah, I'd like to fix that, but it's not on the top of the list.

Ken300D · #13 03-03-2006, 10:02 AM

Just to be sure - You did turn the driveshaft in both directions didn't you?

This test is for back-and-forth free play, so that means rotating the driveshaft back and forth to estimate how much free play there is.

If you are turning the wheel(s) slowly in one direction, then stop, turn the driveshaft back in the other direction, and if the wheels start turning again in the other direction after only 1/4" turn on the driveshaft - that's quite good as far as differential slop.

Ken300D

PanzerSD · #14 03-03-2006, 11:01 PM

yeah, 1/4" in either direction, so I guess that amounts to a half inch total, which still isn't bad.
are there splines on either end of the drive axles? or is it a flange mount? I never noticed....

Ken300D · #15 03-04-2006, 07:27 AM

Yes, the axle half-shafts are splined at both ends. In fact I'm trying to remember if there is any difference between the ends. I think they might be reversible. I know you can swap left for right, but that's not necessarily reversible as you keep the end closest to the differential that way, but move the axle around to the other wheel. But it may be that the axles are totally symmetrical and reversible in place - just can't remember.

Some people swap their axles to get more wear life. But for the most part the wear life is determined by the boot integrity. When a boot fails the axle gets dirt in it and that's often it for that joint.

Ken300D