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clutch / flywheel diagnostics

Posted by jaffar 
May 29, 2009 03:25AM
hi all

how can i check my clutch and flywheel without taking out the gearbox ?
a mechanic told me that i could take out the clutch slave cylinder and measure something. any details ? any other way ?

thanks

--
A physics truck just turned over outside. There's physics everywhere!
rkj
May 29, 2009 06:25AM
This is on your E36, right Jaffar?
May 29, 2009 06:59AM
The Bentley manual shows a technique for checking clutch wear by sticking a special tool between the transmission and the slave cylinder and measuring how far the tool sticks out after touching the slave cylinder pushrod. (It's on p. 24.) This was news to me -- I've never heard of such a technique before.

Why do you want to check it? Is there some symptom it's exhibiting now? Most people just change the clutch when it starts slipping. It will give you warning before it goes completely.

__________
Dave
'91 325iX
May 29, 2009 09:46AM
@rkj: yes - but it's really 100% identical with the E30 (except maybe for the damn dual mass flywheel)
@Dave: i have some symptoms, but I'm not ready to buy a clutch and a flywheel so i prefer to fool muself that maybe not both of them are broken smiling smiley

syptoms:

- if i rev the engine a lot (4000+ rpm) and try to make a launch with clutch drop, I get a lot of slipping for 1-2 seconds, then everything is normal
BUT
- during normal driving, there's no slipping at all. even if i make hard manouvres on the clutch, like high rev/high speed + sudden engine break; high rev, shift down, drop clutch etc.

As I have no idea about the history of the car, I am not very happy to blindly invest high $$$ in a new flywheel + clutch set. But if I do have to change them both, I'd rather know before, so I can get a different kit (also a lot cheaper than the OEM parts).

On page 24 of my (electronic) Bentley service manual I have some info about inspection I and II, oil service and service indicator. But from the diagrams I understand that the clutch disc is sandwiched between the flywheel and the pressure plate so I don't see how one could get there and measure it (I did not notice any holes in the pressure plate).

Thanks.

--
A physics truck just turned over outside. There's physics everywhere!



Edited 1 time(s). Last edit at 05/29/2009 09:47AM by jaffar.
May 29, 2009 02:27PM
Your 'racing starts' (high rpm and dumping the clutch) put a lot of wear and stress on the clutch, actually, you should avoid it as much as possible if you want a long clutch life. Your dual mass flywheel won't like it either. Actually, to make a good start, you shouldn't have the cluch slipping at all, it should be your tyres. Let them spin for max 1 second. If the clutch or the tyres slip too much, lower the launch rpm, such that engine rpm stays steady until the tyres grip completely and then apply more throttle. If the engine rpm drops significantly, your launch rpm was too low.

You will notice clutch slipping when applying engine power rather than engine braking, as the engine torque is higher than your engine braking torque. As long as you don't notice clutch slip or vibrations when engaging the clutch, your clutch is ok.

A worn dual mass flywheel wil go clonck clonck at very low rpm (eg near idle in 4th). Driving at too low rpm is killing the dmf, as it has to work too much to compensate for engine vibrations - mostly in 4 cil or less engines. Hard torque changes (such as dumping the clutch) will also damage the dmf. (a dual mass clutch has a steady flywheel to the crank shaft and on this flywheel there is a flywheel connected to it with springs working in the direction of rotation - this second flywheel also has the clutch surface)



Edited 1 time(s). Last edit at 05/29/2009 02:34PM by Michiel 318iS.
May 29, 2009 03:10PM
Quote
jaffar
- if i rev the engine a lot (4000+ rpm) and try to make a launch with clutch drop, I get a lot of slipping for 1-2 seconds, then everything is normal
That slipping sounds like your clutch is on its way out. Usually you will notice slipping in the higher gears first. (I'm actually in the middle of replacing the clutch on my other car after it started slipping in 5th gear.) Often you will notice the engagement point getting very high as well. However, as Michiel said, your 4K RPM clutch drop starts aren't doing you any favors either. That's the kind of thing I would do if I was trying to fry my clutch.
Quote

- during normal driving, there's no slipping at all. even if i make hard manouvres on the clutch, like high rev/high speed + sudden engine break; high rev, shift down, drop clutch etc..
I would not expect a worn-out clutch to slip under those conditions. Again, it will slip most under hard acceleration in high hears. (I first noticed mine when I had the accelerator floored and shifted from 2nd to 5th. Instead of bogging the engine, the clutch slipped.)

Finally, unless your flywheel is worn out for some reason, it should not need replacing. Just do the clutch disc, pressure plate, throwout bearing, and pilot bearing. (I'm speaking from experience with other cars here. I've never done an E30/E36 clutch, but I assume it's the same.)

Oh, and you are correct: there is no way to examine the clutch disc without dropping the transmission and removing the pressure plate. The test in the Bentley manual is doing something with the position of the slave cylinder piston, but I don't know how that works, and I would be skeptical of it anyway.

__________
Dave
'91 325iX
June 01, 2009 04:02AM
i am not doing "racing launches" often, but i don't like that this type of slip exists, so clearly something is wrong. i have no idea what the previous owners did to this clutch - but i know that if you drop the clutch at 4krpm, you should leave a very long tire trace on the road, and not just watch the rmp meter go up and speed meter very, very slowly start to move.

i did more tests: there is no slip whatsoever after the clutch is fully engaged and the car is moving. even the top speed is correct - and a slipping clutch will never reach it. i tried all kinds of flooring (uphill too, lots of gear and rpm combinations) - they also do not produce any slip.

--
A physics truck just turned over outside. There's physics everywhere!
June 24, 2010 09:15AM
the end of this story goes like this - i decided to replace the clutch set anyway, because i'm planning a long mountain drive vacation smiling smiley
the old clutch disc was 8 mm thick:



the new one is 10mm:



in my previous car, the disc was completely worn out but still not slipping at all, so i REALLY have no clue why the above problems happened. check out this completely shot disc what was working FINE until the release bearing cracked:



there's no oil spill or other problems around. oh well... at least now i have a new clucth and no worries. it starts to engage when i lift the pedal 1 mm from the floor, so it's difficult to drive compared to the old one smiling bouncing smiley

--
A physics truck just turned over outside. There's physics everywhere!
June 24, 2010 05:59PM
Quote
Dave_G


Oh, and you are correct: there is no way to examine the clutch disc without dropping the transmission and removing the pressure plate. The test in the Bentley manual is doing something with the position of the slave cylinder piston, but I don't know how that works, and I would be skeptical of it anyway.

As the clutch disc gets thinner, the pressure plate has less material between it's face and the flywheel, which causes the surface that the throwout bearing works against to move back away from the flywheel. I don't see anything about that measurement in the E30 Bentley, but I guess they must be giving you a dimension in the E36 Bentley for the position of the end of the release lever that corresponds to a worn out clutch disc. That does seem like a lot of work when you can tell pretty easily whether or not the clutch is worn out by seeing if it slips in high gear.

John
June 29, 2010 09:12AM
Well... goinng back to the start of this thread, my clutch did not slip in high gear smiling smiley
In fact, it did not slip pretty much in any conditions, short for a clutch-drop start on dry and clean tarmac.

My E36 Bentley doesn't tell me how to measure a disc without removing it. I find the entire design on the system pretty stupid - I don't see why they won't include some sort of checking window somewhere ? You need lots of work hours just to check your clutch... you might as well change if while you're there smiling smiley

I think the position of the release lever is not a significant measure of the disc status - the center end of the sun rays of the pressure plate bend in time so they affect that position, too.

--
A physics truck just turned over outside. There's physics everywhere!
October 15, 2010 03:15AM
Forgive me, but you may not have solved the problem. Your clutch slave or MC may be on its way out...
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