jhalsey

I bought a 1990 928 GT six months ago, 73k miles, black with tan interior, very original car. Have done intake refresh, motor and trans mounts, injector service, general service. The car runs great but there is one issue yet to be diagnosed. Need the combined expertise of this group to point me in the right direction, I have other Porsche's but this is my first 928.

There is a wah wah grumble type noise coming from the drive train, sounds like a bad wheel bearing, frequency increases with car speed. It is a pretty prominent noise but w/o any noticeable vibration, most noticeable in 2nd or 3rd gears and fainter at higher speed in 4th or 5th. If I push the clutch in and coast, the noise stops completely.

It would appear this is clutch related as it stops when clutch is disengaged but trans still in gear and drive shaft spinning. Throw out bearings usually only make noise when the clutch is disengaged in my experience.

Appreciate any guidance. I have attached the one crappy picture i have handy, will post more later as it was just detailed.

Sapientoni

There is a possibility the PO liked to spin the wheels. The drive shaft is a 19MM solid bar. There are 3 (if memory is correct) bearings that support this shaft. These bearings are inside the torque tube. There is a rubber piece supporting the outside of the bearings to the inside of the tube that looks like a timing gear when pressed out. The inside of the bearing ( a standard industrial ball bearing) also has a sheet metal/rubber composite piece that holds the inside to the shaft and absorbs the twisting/pulsating inputs. These bearings can migrate forward or backward, and if the power inputs are often and strong, can impart a permanent twist to the shaft. The rubber friction is the only thing that holds the bearings to the even spaced positions in the torque tube. They can be replaced if you are careful and have some PVC tubing to push the parts in and out.

jhalsey

anything is possible, but po was an elderly anesthesiologist. Thank you for your reply. My understanding of the torque tube design is sketchy but if the car is in gear and in motion with the clutch disengaged, isn't the drive shaft within the tube still spinning? If so, that seems to eliminate the torque tube components as the source of the noise as when i disengage the clutch the noise stops. I might be way off base though.

jeff spahn

You still may be looking at either the carrier bearings in the torque tube. they can make strange sounds at strange times. IIRC the manual TT's have three bearings vs the two in automatic (they have a dampener as well) Constantine from Black Sea R&D is the guy to ask on this subject. Look for Constantine in the posts and you'll find him. Post a PM to him and he'll get back to you. He built my new TT, it is a work of art.

jhalsey

i will see what Constantine says, thank you for the suggestion. However i am unclear as to how a noise that goes away when the clutch is disengaged can possibly be related to a shaft which continues to spin inside the TT. If the TT bearings were the source, why would the noise stop if the shaft is still rotating?

Can someone confirm or correct me regarding whether the shaft within the TT continues to turn when car is coasting in gear with the clutch disengaged?

Thanks for any insight.

Black Sea RD

If the noise can be immediately changed by modulating the clutch pedal, it is probably with the clutch throw out bearing.

Another test is to get on a highway, go up the to the speed limit, push in your clutch and coast for a while. If the noise starts to slowly wind down it is probably in the torque tube.

Your symptoms seem to point to the clutch throw out bearing, so we would start there first.

And Jeff, thank you for the very kind words!

HTH,

jhalsey

when i disengage the clutch while on the highway in gear, the noise quickly disappears. i cannot coast with clutch in and hear the noise at all let alone hear it slowly winding down.

so it appears to point to throw out bearing, usually they only make noise when loaded during disengagement, this one seems to make noise with clutch engaged and no load. clutch operation is smooth and normal in all respects.

i assume the TT has to come out to access clutch and throw out bearing? then it would make sense to replace the TT bearings at the same time?

thanks

Rob Edwards

The clutch can be replaced by disconnecting the cat from the exhaust manifolds and letting it drop 2-3" (careful of the O2 sensor wire going taut), undoing the starter (battery disconnected...) and slave cylinder, dropping the lower bellhousing cover, and the clutch pack is right there. Undo the pinch bolts in the shaft coupler, slide the coupler rearward on the TT shaft, undo the guide tube bolts, rotate the motor (clockwise only) to undo the 9 pressure plate socket head cap screws. Pop the release arm off the mounting ball, slide the intermediate shaft out of the pilot bearing, and as Emeril would say, BAM- clutch in your face.

jhalsey

thanks everyone, looks like we'll be dropping the clutch and replacing at least the throw out bearing, pilot bearing and ball cup bushing. Will post if that cures the noise.

Black Sea RD

Mr. Edwards is spot on, of course.

The beauty of the 928 5-speed transaxle is the ease of doing a clutch job. Unlike the 924 and 944 owners where they have to dismantle the car to do a clutch job, much like the 928 automatic owners do when replacing their torque tube.

Good luck with your diagnosis!

Wisconsin Joe

As Rob very clearly described, the clutch on a 928 can be dropped without removing anything else major. You just have to get the exhaust out of the way. He also isn't kidding about dropping it on your face. It goes about 30 lbs or so and will drop free once it's loose. Be careful.

The TOB on the 928 spins all the time. It's a rather unique setup.

BUT...

The stresses on both the TOB & TT bearings are very different when the clutch is engaged or released, under load, reverse load (engine braking) or just coasting.

And a second BUT...

You said "speed related." Are you certain? Not RPM related?

As in, does the noise stay the same at a given speed no matter which gear you are in?

Or is it the same noise at the same RPMs no matter what speed you're going?

jhalsey

Wisconsin Joe: I will attempt to confirm whether the noise is speed related ( as i originally thought) or rpm related tomorrow. so far i have been listening in gear to the noise as i accelerate with both speed and rpm increasing, the frequency of the sound increases, as i coast in gear, clutch engaged, frequency decreases. what would speed related vs rpm related lead you to suspect ?

Wisconsin Joe

I wouldn't "suspect" anything in particular. But it would determine where I would start to look.

RPM related would be basically "in front of" the transmission. Motor, clutch, TT, input part of trans.
Speed related would be "after" the transmission. Output part of trans, differential, CV joints, wheel bearings.