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Sudden slipping clutch
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dobryan
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PostPosted: Wed Jan 22, 2020 9:35 am    Post subject: Re: Sudden slipping clutch Reply with quote

No need to double clutch except maybe in very cold weather when starting out until the trans warms up some.
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PostPosted: Wed Jan 22, 2020 8:53 am    Post subject: Re: Sudden slipping clutch Reply with quote

jeremypbeasley wrote:



I'm also looking into "double clutching" as a way to better operate.



Double clutching is generally used to save syncros. I only really use it when the transmission is really cold (0F sort of thing). Maybe heaving downshifting. Just my personal preference.
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PostPosted: Wed Jan 22, 2020 4:21 am    Post subject: Re: Sudden slipping clutch Reply with quote

There is a natural tendency to associate failures with recent activity.

If the clutch disc was not contaminated with oil or brake fluid, it is just a worn out clutch.

Things that commonly get done incorrectly on a clutch job, other than cheap parts:

Cheap parts
Not resurfacing the flywheel in three places
Flywheel inspection
Pilot bushing
Cheap parts
Checking that disc slides cleanly in splines
Fingers on PP
Release bearing inspection
Cheap parts
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jeremypbeasley
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PostPosted: Tue Jan 21, 2020 11:42 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Hey folks,

We're back on the road! New clutch, new slave cylinder, new master cylinder, all new fluid.

When I flushed out the hydraulic lines, there was a ton of greenish black crap in there that looked like mold and rust combined. I'm sure this wasn't helping!

Test drove it for 30 mins tonight without issue. There's definitely a big different in how sensitive the clutch is. Maybe this whole time I was driving on a shit clutch and just didn't realize it was as bad as it was. Definitely learning a ton here.

I'm also looking into "double clutching" as a way to better operate.

Appreciate all your help and support.
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E1
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PostPosted: Fri Dec 27, 2019 6:51 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Once all's good, you shouldn't have to do a clutch for quite a long time.

Ours on our first bus chattered a fair bit upon purchase. I was able to (carefully) "burn" the glaze off and it went over 40,000 trouble-free miles before replacement. And I only replaced it because the tranny was getting rebuilt.

I know some/most think downshifting by using "engine braking" is fine, but really it's "clutch braking" and it ain't fine. Brake pads are cheap, and good technique is free.
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jeremypbeasley
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PostPosted: Fri Dec 27, 2019 3:32 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Whoops. I missed the whole second one of replies. Catching up...
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jeremypbeasley
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PostPosted: Fri Dec 27, 2019 3:31 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Right on. Appreciate all the great points and examples.

Flywheel came back resurfaced today. Iím gonna flush out those clutch hydraulic lines real good before installing the new master and slave. Since Iím in here, I might as well replace the felt seal, rubber seal, and pilot bearing. Loads of fun when youíre working on the street in the winter.

In lieu of a clear silver bullet, Iím going to acknowledge that thereís a huge margin for my own error here and do some research to clearly know I can sense if/when clutch slips slightly in the future. Iíve only been driving a Vanagon for a year so itís entirely possible that there was slippage before and I didnít notice. I donít recal any stink any any time.

Hoping when I put this all back together, I can get this puppy running till next summer without having to pull the damn gearbox off again!

Who knows, maybe Iíll be lucky enough to even have time for some body work!
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MarkWard
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PostPosted: Fri Dec 27, 2019 3:26 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Sodo, Then my vote is "Coincidence".
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E1
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PostPosted: Fri Dec 27, 2019 1:27 pm    Post subject: Re: Sudden slipping clutch Reply with quote

As I've said, this is a very intriguing case.

But if there's one thing I read on Samba over and over, it's claims of coincidental parts failures. I may not be a mechanic, but odds of such things are *extremely* remote.

I hope the result of this thread is definitive. Quite a mystery to be solved.
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Sodo
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PostPosted: Fri Dec 27, 2019 12:13 pm    Post subject: Re: Sudden slipping clutch Reply with quote

MarkWard wrote:
I'll emphasize this again. You did a major panic stop prior to the clutch starting to slip. The brakes must have stuck partially on and that caused the clutch to burn up.


Mark I think that would be noticeable driving the van. How would you even get into 2nd gear if the van decelerated (braked) whenever you press the clutch?

vwwestyman wrote:
Kinda along Wildthings' idea, I am wondering if the clutch had been slipping off and on for some time, but had gone largely unnoticed?


I'm in this camp.
Clutches survive enormous amounts of abuse.
Such as kids (and now millennials) learning how to drive stick.
This abuse happens at the higher revs, when the engine is making torque.
--->Gasoline is being burned.<---- foot on throttle.
Rotational energy is departing thru the crankshaft, some thru the wheels, some being "wasted" at the slipping clutch.
When the heat adds faster than the clutch's ability to shed the heat, the pressure plate and flywheel get red hot, and it roasts the friction disc.

The braking incident does not remotely qualify as abuse, it's a very short event, at low RPM, low torque.
Well within the clutch's capacity.

I have a hard time believing the braking event is related ...... other than coincidentally at the same time as the clutch problem coming to light.
And I don't see any oil anywhere on the clutch.

Look for a 'slipping' fault.
Either clutch mechanism stuck, or driver error, or a pressure plate spring failure.
Maybe someone roasted this clutch awhile ago (like that feller I wrote about at Descend on Bend), and it still was able to move the van.
But it was damaged, slipped a lot and finally "gave out".

Can you post pics of the 'spring fingers' of the pressure plate?
If the 'fingers circle' is 'uneven' that suggests a broken diaphragm spring, which would certainly result in a slipping clutch, which would then roast the disc to look like your pic.
Press down on them with something 'round', how much spring pressure is there?
It should be "difficult" you have to "stand on it".
Much harder, like about six times harder than pressing the clutch pedal.
You have a new pressure plate, right? (I hope so)
Compare spring pressures between the old and new pressure plates.
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MarkWard
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PostPosted: Fri Dec 27, 2019 10:54 am    Post subject: Re: Sudden slipping clutch Reply with quote

I'll emphasize this again. You did a major panic stop prior to the clutch starting to slip. The brakes must have stuck partially on and that caused the clutch to burn up.

I worked for Audi in the mid 80s with the run away 5000. "The harder I pressed the brake pedal, the faster the car went".

It was determined the new owners, especially short women backing up, would get their feet mixed up, believing they were on the brake pedal not the accel pedal. Audi had so much bad press, they let it go. The recall was to add a shift lock to train owners how to "find" the brake pedal. There was nothing wrong with the car other than the brake pedal was close to the accel pedal to make room for the clutch pedal.

Anyhow I mention this because the brake are about 3x as powerful as the engine. There is no way the engine can overpower the brakes. I would suggest you examine your brakes very closely. It could be either the front or rear hanging up. In normal application, you may not notice this. It may need the "extra force" to cause this to happen.

If it's not the brakes as I suggest. It is just a coincidence that it happened after a panic stop and unrelated to the failure.
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vwwestyman
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PostPosted: Fri Dec 27, 2019 6:36 am    Post subject: Re: Sudden slipping clutch Reply with quote

Kinda along Wildthings' idea, I am wondering if the clutch had been slipping off and on for some time, but had gone largely unnoticed?

For example, I had an old Cabriolet that had a slipping clutch. It would mostly only happen if I were to try to power up a particularly steep hill. I let off and it would quit slipping. But frequently the slippage could be minimal and perhaps not be noticed if things catch up with themselves pretty quickly.

Another time, with a different Cabriolet, I had decided to change the clutch for one reason or another. I drove to my friend's house in Wichita (two hours away) and we changed the clutch with a part he'd gotten locally. Everything seemed fine, but when I was about halfway home, going up a steep hill on the interstate, the brand new clutch gave out. It was slipping so badly the car couldn't go up that hill and I had to get a tow. I swapped in a new one at home and all was fine.

So sometimes stuff is just bad off the shelf.

One other thought I just had is, you might want to really flush out the hydraulic lines before replacing the slave cylinder. There could be some kind of crud in there that was preventing the fluid from returning fully, which would show up as being like you were halfway pushing the pedal. (I don't recall, on a Vanagon are there rubber lines in the system? If so, I'd get a new one-they can fail internally.)
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Wildthings
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PostPosted: Fri Dec 27, 2019 12:57 am    Post subject: Re: Sudden slipping clutch Reply with quote

jeremypbeasley wrote:
For the record, Iím entirely open to this being improper driving/operating. Damage is done. I would just want to know what Iíd need to do differently.

I drove five miles home at about 20mph immediately following the heavy braking incident. I stopped 3 times along the way just to let it rest. Once home, a week later, I drove it around the block just once. Maybe Iím ignorant, but could I really fry a disc as bad as Iíve shown in under 6 miles of driving?

Besides what Iíve described just now, what would be improper use?

PS, I donít think thereís any malfunction in the cross shaft. It appears to operate flawlessly despite its age.


I would have expected the clutch to stink like crazy if you suddenly got it hot enough to do that kind of damage. If no stink, I would be inclined to say it happened previously when pulling a hill with the pedal down and for some reason you just didn't notice the stink. Maybe you were having a strong cross wind or something at the time the damage was done.
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E1
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PostPosted: Thu Dec 26, 2019 11:05 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Right attitude, good. 👍🏼

Per the sudden braking, did you bring it to a complete stop? In gear? Clutch in or out? Did it stall?

It's okay if you're not sure, recording such details in one's memory aren't exactly priority compared to safety considerations "in the moment." Cool

Intriguing issue all-around...
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jeremypbeasley
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PostPosted: Thu Dec 26, 2019 10:43 pm    Post subject: Re: Sudden slipping clutch Reply with quote

For the record, Iím entirely open to this being improper driving/operating. Damage is done. I would just want to know what Iíd need to do differently.

I drove five miles home at about 20mph immediately following the heavy braking incident. I stopped 3 times along the way just to let it rest. Once home, a week later, I drove it around the block just once. Maybe Iím ignorant, but could I really fry a disc as bad as Iíve shown in under 6 miles of driving?

Besides what Iíve described just now, what would be improper use?

PS, I donít think thereís any malfunction in the cross shaft. It appears to operate flawlessly despite its age.
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E1
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PostPosted: Thu Dec 26, 2019 7:57 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Kinda did mention improper use. Wink

Any downshifting without matching revs is improper technique, as would be stopping in gear without dumping the clutch at some point.

Just didn't want to come off as accusing the driver. Wink
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PostPosted: Thu Dec 26, 2019 7:52 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Clutches are not damaged on the low end of the torque curve.

That clutch is burned up from slippage while the engine is turning fast and putting out max torque..

It could look like that from either a malfunction such as a stuck cross-shaft bushing, or improper use (which you have not mentioned).

I saw some van-folks at Descend on Bend trying to tow-start a diesel van.
At some point, the tow-van clutch belched out a thick puff of smoke.
His disc looks like yours (like a piece of carpet) but he didn't know it (yet) because he didn't take it out and look.
But when we left Descend on Bend his van was still there, almost the last one in the camp.
Poor guy had a California license plate... (600 miles?)
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E1
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PostPosted: Thu Dec 26, 2019 7:42 pm    Post subject: Re: Sudden slipping clutch Reply with quote

I'm curious if during your panic stop the engine went so low in revs as to stall with the clutch engaged -- and/or too much was demanded of the parts while working the clutch.

Just a thought. I've mentioned heel-and-toeing here several times but have yet to get a reply from others knowing or using this (and Mark Ward surely does).

Also, the pressure plate seems to have a chunk of clutch on it as if ground off by a ton of stress. It looks very similar to a clutch drum after toasting a clutch on a small race motor when caused by heat and/or chatter.

Glad you nailed it regardless.
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Last edited by E1 on Thu Dec 26, 2019 7:53 pm; edited 1 time in total
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jeremypbeasley
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PostPosted: Thu Dec 26, 2019 7:03 pm    Post subject: Re: Sudden slipping clutch Reply with quote

Thanks for checking in on me, folks.

Today was the first day in a month Iíve had to work on the van. I pulled the transmission off shortly after the original post. I also removed the master and space completely. I have since received my replacement master and slave cylinders. Even if theyíre not the culprit, I figured Iíd replace them once and for all.

Today I removed the pressure plate and disc from the flywheel. Theyíre toast. Photos below. I also removed the flywheel and dropped it off for resurfacing.

Thankfully Iíve got a brand new SACHS clutch kit in the garage that I can throw on, but I remain perplexed as to how this happened.

Iím donít really care about finding the mystery as if itís some map on the back of the Declaration of Independence. However, I can only assume this will happen again if something is left in the same way that caused this in the first place.

Checking boxes: The wheels spin freely and I have no reason to believe the brakes have ever seized to the point that it would overwork the engine. The flywheel end play is just above spec at .006Ē, but that shouldnít be a big deal. Everythingís been torqued to spec.

The only suspect thing here is the leaky master cyclometer. However that doesnít explain a slipping clutch at all.

Any help would be much appreciated. I really need this van back up and running again for a trip with the kiddos soon.


Image may have been reduced in size. Click image to view fullscreen.


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Image may have been reduced in size. Click image to view fullscreen.


Lastly a comparison between the disc I just pulled and a brand new one, just for kicks.


Image may have been reduced in size. Click image to view fullscreen.
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Wildthings
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PostPosted: Thu Dec 19, 2019 1:16 am    Post subject: Re: Sudden slipping clutch Reply with quote

I am going to guess that for some reason when you did your panic stop you got lubricant all over your clutch, either engine oil or tranny fluid. Can't ever remember hearing of this happening before though.
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