Hello everyone!

Since a couple of weeks I'm experiencing a drivetrain issue and I cant locate the source of the problem.
Perhaps some of you have had the same problem and can help me with my research.

The issue is as follows:
In second gear the following happens.
1. When I release the gass I hear a clunk
2. When I put my foot down on the gass I hear a clunk
3. When I switch gear from 2>3>2 there is a small hiccup
4. When I am breaking heavily I hear a clunk when the gear reaches second gear.

I have no error messages.
All fluids have been recently changed (2 months ago).
The clunky sound I hear seems to be more prenounced when the fluids are cold.

- I am told that the transmission is bullet proof (mercedes trans).
- I was thinking maybe the torque converter might be the problem but torque gets converted perfectly in every gear, instantly.
- Maybe not enough oil in a part of the drive train?
- Could the problem be in the rear differential?
- Maybe play on the Cardan shaft?
The last two could give a clunky sound but it wouldnt explain why it is only in second gear, or would it?
I personally lack the knowledge or expertise to figure out these things

I would really appreciate the help and insights into how the drivetrain is setup and I sincerely hope some of you have had this problem.

Thanks in advance!
Jesten

 

Does it sound like it’s coming from the front or the back of the car?

What year and how many miles does it have?

The transmissions are pretty bulletproof in the R cars but it’s not invincible. I had to change one out in my 98’ XJR about 4 years ago and it had about 245K on it then and it was supposedly rebuilt but hard parts break. From what I gather the earlier Mercedes boxes had a few issues anyways. I’m not sure what year they resolved them.

The differentials are a weak point in these cars. A lot of slop can be had between the ring and pinion contact areas. Also the splines on the pinion where the driveshaft hub bolts to wear out and fluid will leak out the front of the differential. A new pinion hub won’t resolve the leak. I have a repair procedure for the leak if anyone cares.

Check the condition of both the Jurid couplers on either end of the driveshaft.

The center driveshaft support can go bad but that’s not always an easy diagnosis.

If I were you, I’d start by checking the condition of the motor and transmission mounts. Then the Jurid couplers, driveshaft support bearing, then differential. Maybe check the fluid level in the diff as well.

 

blown transmission mount also causes clunks

 

Does it vibrate more in gear than in neutral? If so it may be the transmission mount as mentioned above. I had the same clunking issue and new trans and engine mounts fixed it. Idles smooth as glass now in gear and at startup.

 

@Addicted2boost
Thanks for your reply,
My XJR from "99 has about 200k Kilometers on it which is about 124k miles.

I have checked the mounts. They seem to be fine, rust free and strong. No play

No leaks whatsoever in the entire drivetrain. I do care for the procedure to fix the leaks, all info is welcome and to be consumed.

Would play on the differentials explain why it only happens in second gear?

How do I check the fluid level in the diff?

I will check the points you stated above.

 

@xalty
Would that also cause clunks only in second gear?

 

@anduha
Thanks for your reply,
It does not vibrate in any way. Its completely smooth throughout the entire range of gears except second gear.
Fluid seems to be normal, no burnt smell or metal dustings (checked with magnet)

 

Underneath the car, in front of the spare tire well. There’s a 1/2” square. On a short wheelbase car, it’s harder to check the level. I’ve even heard of somebody taking a holesaw from inside the spare tire well to access it.

You'll need a 3/8” to 1/2” short adapter, another short 3/8” extension preferably with a wobble tip and a 3/8” ratchet.

 

@Addicted2boost
A holesaw is out of the question
Once I am near that 1/2" square, how do I check it? just put my finger in it and if there is some in there its fine? Or is there a level pin?