clutch slipping mystery

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dingo
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clutch slipping mystery

Post by dingo » Thu Sep 17, 2015 8:16 pm

Slipping clutch on Type 1 Bus....pulled engine..clutch looked 'ok' as did pressure plate...replaced them both with 'very good' clutch and 'good'; pressure mplate. Exact same symptoms despite both components being upgraded...so what else is going on ??

This coincides with clutch pedal activation point being strangely high up on the arc
'71 Kombi, 1600 dp

';78 Tranzporter 2L

" Fill what's empty, empty what's full, and scratch where it itches."

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Amskeptic
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Re: clutch slipping mystery

Post by Amskeptic » Fri Sep 18, 2015 8:33 pm

dingo wrote:Slipping clutch on Type 1 Bus....pulled engine..clutch looked 'ok' as did pressure plate...replaced them both with 'very good' clutch and 'good'; pressure mplate. Exact same symptoms despite both components being upgraded...so what else is going on ??

This coincides with clutch pedal activation point being strangely high up on the arc
This is counter-intuitive . . .
Normally, slip occurs with an old clutch after the disk is worn skinny. The pressure plate is designed to lose clamping force rapidly as it gets closer to the flywheel friction surface. On the other side of the fulcrum ring, the release bearing gets further and further away from the flywheel, eating up free play.

I think you might have incorrectly surfaced flywheels where the friction surface was turned down, but the pressure plate mounting surface was inadvertently left out of it. I can find nothing in the VW manuals about turning flywheels. My assumption would be that the flywheel can be safely turned down .010". Therefore, you are looking for a .010" deviation between a good flywheel and an incorrectly turned flywheel. This is subtle, a nice depth micrometer will be useful. If the depth between the pressure plate mounting surface of the good flywheel and the friction surface is, let's say 45.28mm, the incorrectly turned flywheel will show 45.53mm or even 45.78mm. These are made-up numbers to illustrate . . .

You can see the fingers draw IN towards the flywheel as you bolt down the pressure plate. This is your proof of clamping force. Find a known good flywheel or seventeen, and start measuring the depth between the pressure plate mounting surface where the 6 bolt holes are, and the friction surface.
Only .25mm to .50mm too much depth will take out a couple hundred pounds of clamping force.
Also, check thickness of clutch disk. A 7mm open-end wrench should not fit over the edge, it would better still if an 8mm open-end wrench cannot fit over the edge.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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dingo
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Re: clutch slipping mystery

Post by dingo » Sat Sep 19, 2015 8:23 am

Thanks. I too found nothing in any of the manuals on the specs of flywheel depth. Called the local machinist with decades of VW machining under his belt..and he gave me a range of acceptability...it didnt seem that narrow. Ill post the numbers when i find them. This particular flywheel seemed to fall right in the middle of the range..so we moved onto other possible culprits..namely pressure plates..the two i had tried measured the same but much thinner than the beefy fresh outta the box Sachs...once again ill post numbers
'71 Kombi, 1600 dp

';78 Tranzporter 2L

" Fill what's empty, empty what's full, and scratch where it itches."

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dingo
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Re: clutch slipping mystery

Post by dingo » Sat Sep 19, 2015 7:16 pm

For future reference....acceptable range for flywheel 'depth' is .830 to .835 and the machinist said up to .845 is about the limit of useability. So i discovered if youre near the edge of acceptability and add a worn but 'visually decent' pressure plate plus an 'ok' clutch plate , then the thousandths of inches add up to insufficient 'grabbage'.
My only other obstacle was the metal ring mounted on the fingers of the Sachs press plate. It would not clear the rim around the spline shaft...dremelled off the retaining washer and removed the ;ring; altogether...not sure what its function is/was. But i now have good clutch action.
'71 Kombi, 1600 dp

';78 Tranzporter 2L

" Fill what's empty, empty what's full, and scratch where it itches."

kreemoweet
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Re: clutch slipping mystery

Post by kreemoweet » Sat Sep 19, 2015 9:17 pm

dingo wrote: ... not sure what its function is/was.
Methinks you could benefit from looking at this very popular topic: http://www.thesamba.com/vw/forum/viewtopic.php?t=148307

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dingo
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Re: clutch slipping mystery

Post by dingo » Sun Sep 20, 2015 8:41 am

yeah...pretty damn embarrassing for having owned and worked on so many busses over the years...but I never owned anything'70 or earlier...me like disc brakes and alternators
'71 Kombi, 1600 dp

';78 Tranzporter 2L

" Fill what's empty, empty what's full, and scratch where it itches."

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