Sean79 5spd

Posted a llittle while ago about this. One of the suggestions was to check the slave cylinder rod to make sure it was seated. I re & re'd but there did not seem to be any improvent. One thing I didn't do when I replaced the clucth was to have the fly wheel re-surfaced. Any possibility I could be getting poor grip between fly wheel and clutch disk? Anyone else have similar experience?

Thanks
Sean

worf928

Typically, when reversing, most folks do not fully engage/disengage the clutch. So, if the shudder happens during clutch engagement, check your release arm ball cup bushing. If it is fragged the arm can move at an angle relative to the plane of the flywheel/clutch thus causing uneven pressure on the flywheel. (At least this is the case in the single-disc S4s.) On an S4, you can see the top of the release arm once the airbox is removed. Don't know about an OB though.

Gretch

Forward or reverse should not make a difference....does the "clutch" shudder in first gear? If not then the shudder is likely in the tube or the tranny......

Kolbjorn S

When I first experienced the "out of driveway" shudder in reverse (and must I add, during low-speed low-rev turns in 1st and 2nd) on my '87 5sp S4 , I thought it was the clutch. So, new clutch in, re-machined flywheel smooth as silk - shudder still there. Turns out to be the LSD (having said that: opening it up, there is no metal spoon in the diff oil or uneven wear, so..it just adds soul to my shark).
Maybe you don't have a diff on your '79 5sp, but worth looking into?

Sean79 5spd

There is a little shudder in 1st. The worst shudder is when I back up my driveway. Backing up on a flat is about the same (or slightly worse ) shudder as 1st.

Before I did the clutch job, there was no shudder at all.

Sean

worf928

OB Double-disc clutch alighnment issue? (Search for threads on that...)

Sean79 5spd

I checked into the double disk alignment write ups. It appeared that proper alighnm,ent of the central plate only had to do with proper clutch release. I thought that when the pressure plate compresses the disks and central plate against the flywheel (clutch pedal up) alignment is not an issue.

Maybe I am incorrect.

Sean

worf928

Sean, can you describe more-precisely when the shudder occurs? Does the shudder occur when the clutch is completely engaged or does it occur during engagement? Or both?

Sean79 5spd

Dave, on the flat the shudder is only under engagement. Once the car is moving everything is fine. However, uphill, I can't ever get the clutch to engage as the shuddering gets too violent and I have to push the pedal in and then start to release again.

Question : can improper adjustment of the tabs on the central plate actually impede the pressure plate ability to squeeze everything together against the flywheel?

Thanks
Sean

worf928

These symptoms are totally consistent with uneven pressure from the clutch disc(s) to the flywheel. If you hadn't just done a clutch R&R I would place my bet on the ball-cup bushing. It's still worth checking.

What did you replace during your clutch job? T-O bearing?

I won't hazard a guess. You'll need council from someone who knows the OB 2-disc setup.

docmirror

Okay, I know this one, but you're not going to like it. It's actually two different syndromes but they are related. What you have is NRRO of the flywheel when mounted on the end of the crankshaft. NonRepeatable RunOut is simply a 'wobble' in the plate as it rotates around the shaft. The entire assembly is wiggling back and forth as it rotates(think of the old plate on a stick show, the flywheel is the plate). The root cause is either a slight Phugoid of the crankshaft, or the end of the crank where the flywheel bolts to is slightly uneven, or the flywheel mating surface is slightly uneven, or a mixture of all of them.

The point is, when rotating, the runout at the outer edge of the flywheel is excessive. You could get under there and measure it with a depth mic and wheel, but I know that's what the problem is. when the flywheel is surfaced, the registration point is not the mating surface of the flywheel, it's the ring of the highest point of the back of the flywheel. This is usually a flange around the mating surface. And the surfacing tool does not operate while the flywheel moves in a circle, but the flywheel is stationary on the table, and the tool moves around it.

Next the fault is exacerbated by what we call "PIO" for Pilot Induced Oscillation. Consider your foot and leg as you release the clutch in reverse; You let up on the clutch in a backward direction, the clutch starts to grab and move the car back, but your body, and your leg tend to move FORWARD, thus slighty depressing(engaging) the clutch, and relieving the grab. This motion, in turn, slows the car, it moves less backwards, your foot moves more backward in the seat, and the cycle begins again. That is why the symptom gets worse and worse, because it is a motion induced cycle. Each time the judder occurs, it induces an even stronger motion of your foot on the clutch.

You cannot control it because your reactions are ALWAYS behind the coupling time of the oscillation. The judder starts out in the clutch, but your manipulation of the clutch actually intensifies it, and feeds on the cyclic motion of the car, body, leg, clutch pedal, disk, pressure plate. It is noticeable in forward gears, but when the clutch is released in the forward motion direction the natural tendency is for your body, and leg to move backward in the seat, thus further engaging the clutch, and relieving the cyclic motion on the pressure plate. QED.

How to fix it. Remove the NRRO of the flywheel asm. The flywheel mating to the end of the crank must be perfectly flush. The end of the crank must be a perfect plane, perpendicular to the shaft, with no phugoid in the shaft. The flywheel disk surface, and mounting surface of the pressure plate asm (single or double) must be perfectly parallel to the flywheel.

It may be as easy as having the flywheel correctly surfaced by registering it at the MATING surface. It may be as hard as removing the crankshaft to check it's true, and check the runout of the flywheel mating surface. The pressure plate may also have NRRO, as it relates to the flywheel. I've seen PPs that were not setup correctly cause the same symptom.

Doc 90GT in resto

Sean79 5spd

Doc, thanks for the comprehensive explanation. Two questions :

1. This probelm did not exist prior to the engine rebuild (top end only) & clutch replacement. I did have the flywheel off the crank to replace the main seal. (FYI, the release arm ball cup and pilot bearing are also new). How could this have caused a Phugoid misallignment to occur. The flywheel bolts were torqued to a proper tightness. I believe that I did not mark the relaitionship between flywheel and crank, if they went back together differently, could this cause the "wobble"

2. How can a pressure plate be set up incorrectly?

Thanks
Sean

hupp

Sean,

One thing that I have learned is that the "pry the intermediate plate tabs all the way back and let clutch self adjust" method does not always work. I recently replaced my clutch and it continued to drag and shudder. What I found was that the IP tended to remain more on the pressure plate side of center and I had to adjuct back toward the flywheel. If this is true in your case, the flywheel disc may not be engaging soon enough when you ease the clutch pedal out. Here is what I eventually had to do to properly adjust:

- Remove lower bell cover and pull lever back to spec. travel (17.4mm if I remember correctly). I used a winch strap and attached to the rear cross member to do this.

- Once the clutch is released, center the pressure plate by adjusting the brass tabs. Check the gap between the each disc and respective plates to ensure center.

- Release clutch and re-check.


Also,

The flywheel can only go on one way. If you look closely, two of the bolt holes are slightly closer together than the others.

AJK

This will probably only confuse the issue, but what the heck. When I first bought my '79 (in '81) I looked at and drove many 928's. All of them had a certain amount of clutch judder if I tried to engage the clutch very easy, which is my practice. If I built up some revs and was more aggressive with the clutch, the judder went away. I have always thought that just the way the early 928s were.

Garth S

A dilema ....
What else could cause a 'shudder'?
- burnish patches on one or more of the four metal contact surfaces ( scratch the PP, as it is new).
- oil contamination on the disc(s).
-left one install spacer in the PP ( sorry - just digging for possibilities )
- dragging on the splined surfaces of the intermediate shaft.


FWIW, the kelvar clutch in my truck (T100) judders like a sumbiatch unless it gets 10% more throttle than your best instincts dictate ... some things are happiest when whipped