william_b_noble

so, a few months ago, I changed the clutch on my 993 - at about 142K miles - first ever clutch change. Anyway, I followed the instructions in the Porsche service manual, dropped engine and trans, separated them, put in the new clutch stuff, and put it all back together - all seemed good for a while, but now I am getting what sounds like a differential whine - quite strong, maybe getting stronger, it's hard to tell.

The whine is speed dependent, pitch does not depend on the currently selected gear, volume depends on torque applied, etc - so it sure is acting like differential whine. I changed the trans fluid and there was very little metal on the magnet - one chip, some fuzz. Refilled with synthetic per recommendations - I thought that fixed it, and for one or two drive cycles it was seemingly better, but not so now.

I can't imagine anything I could have done to affect the rear end while changing the clutch, but it's not impossible that I made a mistake - I have been under the car and everything is tight - The sound is quite audible bouncing off a freeway wall too, so I don't think it's just conducted sound from a bad motor mount or something, though that is not impossible. I don't think it's a wheel bearing or axle because it goes away when I take my foot off the gas.

Any suggestions?

chris walrod

One thing to double check is that your gbx oil level is correct.

william_b_noble

indeed, I did check that, per the manual - in fact I drained the oil since I hadn't chaned it and refilled with the 3.6 liters it asks for - I added just under 4 quarts - I can I suppose add the last of the 4th quart to it and see if that matters

AOW162435

I replace the gear oil annually with Mobil1 Delvac and go about it a bit less scientifically. Rather than measure the required fill, I just empty the jug and then let the excess drain from the transmission case while I work on something else.


Andreas

william_b_noble

I'm starting to wonder, is there any situation in which the whine I describe could be caused by a bearing rather than a gear? it isn't that hard to get at the bearings, at least the axle bearings/wheel bearings - I just haven't figured out how torque could change the sound if it was a bearing.

any ideas?

Rick Lee

Did you change the pilot and throw out bearings?

william_b_noble

I changed the throwout bearing, not the pilot. But, with the clutch engaged, the pilot bearing isn't rotating, so that can't be the source of the noise. And pressing the clutch doesn't change the noise if I do it at a "no torque" load, so that rules out (in my mind) the throwout bearing and the transmission itself leaving only stuff in the rear end. Similarly, shifting into neutral and reving the engine does not cause any unusual sound with or without the clutch depressed. this is why I worry about the differential - or some part of the power train aft of the transmission. Can bad CV joints whine? maybe it's CV joints, but I've always heard them clunk going around turns.

This car was hit in the left rear wheel about 640,000 miles ago, so the axle and bearing on the left were changed - that means that the axle and bearing on the right have more miles on them.

Steve Weiner-Rennsport Systems

Hi,

Assuming that the gearbox oil is the correct viscosity and at the right level, whining sounds are generally due to bearings (aftermarket gears can whine, too). CV's don't make those kinds of sounds, as you already know.

At 640K, those wheel bearings (and others) should probably be replaced since you've clearly gotten good life out of them [a joke].

Seriously though, I'd looking into the possibility of wheel bearing and possibly differential (carrier) bearing noises using a stethoscope while the car is running on a lift (in gear).

william_b_noble

thanks, I'll check bearings - I can run the car on jackstands and see what I can hear with the stethoscope - that can be next week's project. the car is 100% stock, I don't think there is any aftermarket stuff on it except for carpet covers, so bearings makes most sense - my usual experience with porsche transmissions is that they are good for a lot of miles - over a million on my 356 before the differential carrier cracked, the only problem trans was on my 944, where gear teeth fell off and really did a number on it - my theory is that the 944 failure was due to the rubber clutch donut getting soft and imparting impact loads to the gearbox - but there is no rubber center on the 911 clutch. that's a long winded way of saying it will be a great relief to find a bad bearing or two

william_b_noble

following up - I crawled under car and discovered bad axle boot on LH side, so removed both axles (right side is a real pain - really), then pulled diff cover off trans and removed differential - there is some brinelling on the carrier bearings (hence the noise), but the great oddity is that there is metal in the oil caused by the spider gears flaking apart - this makes no sense at all - those gears rarely do anything, and I am not racing or anything, just driving around - I've ordered new bearings, seals, and a used carrier that looks to have good spider gears ($800 for gears from dealer, new, hence going used). Any idea why the spider gears would fail? And, anyone know if I can just transfer the ring gear to the new (to me) carrier and use the same shims - I don't have the super fancy micrometer tool to measure for reshimming, so I can either move the new (to me) gears into the old carrier, or move the pinion to the new carrier - the latter seems like a better idea if the carriers are accurately machined - I guess i can measure (that's what a surface plate is for, right?) but I thought I'd ask here.

Steve Weiner-Rennsport Systems

I have seen improperly heat-treated differential gears in these things and its not unheard of.

These are lightly loaded in these applications so failures are generally due to improper heat treatment at time of manufacture or the use of the wrong transmission oil. These MUST have a GL-5 rated oil.

JMHO, but these installations and setups are not DIY-friendly and you must have all the proper tools and shims, otherwise you run the risk of destroying the replacement parts as well as the R&P. Get an experienced pro to do this critical task.

william_b_noble

is there any reason then if I use the original shims and original carrier that there should be a setup issue? the bearings are accurate to .00001 or so in width, far better than the accuracy of anything else, but I can measure the new bearings when they arrive to be sure, though I only trust my micrometer to maybe 2 or 3 tenthousandths (of an inch).

I'm inclined to assemble the trans and leave the axles disconnected, and fill/flush with kerocene to get any residual metal from the gears out - there used to be some stuff called "flushing oil", but I haven't seen that in decades. what are your thoughts on this?

Steve Weiner-Rennsport Systems

If you change bearings, the backlash should be checked and adjusted as needed.

I do not recommend the use of any type of "flushing oil". If there is metallic debris, then the transmission should be removed, disassembled and cleaned.

william_b_noble

well, things get worse and worse - I disassembled the differential - the shaft on which the small spider gears ride is pitted, some of the rollers are broken, the inside of the housing is galled where the gears rub against it - very much not good. So, I'm going to replace the whole housing. For a first step, when I get a new diff housing, I'll set it up on a surface plate and meausre the three key surfaces (two bearing seats and the ring gear seat and see how close they are to the locations of the same features on my original.

The Porsche manual, page 39-13 says that if I replace the differential housing, then I only need to adjust the ring gear (S1+S2). It appears that they want the adjustment to be to .05mm, which is about .002 inches - I'm sure I can read these features to .002, so it seems to me that if they are within .002 overall, I should be able (at least initially) to reuse the old shims and that way I can drive to a shop that has the right equipment if the sound isn't right. Agree? I can put a dial indicator on the trans after assembly with the original shims and measure the side to side slop and see if it is around 1 to 2 thousandths??

actually, at the end of the section (on page 39-21) it says "permissible deviation 0.02 mm (per shim) across the shim itself, so maybe even a little more slop would be OK? What is a little confusing is step 6 that says "fasten uiniversal dial gauge ... and set to 0 with 2mm preload" I presume what they are saying is to have at least 2mm of travel in the negative direction past zero on the indicator, rather than something related to the preloading of the bearing - agree?

Could this have been caused by the old oil going bad? I didn't see a change interval for the trans oil, but it was original - the level wasn't low but maybe it lost its lubricity? or is there some other potential cause for this kind of wear? The ring and pinion gears look perfect, by the way. I haven't disassembled the trans.

chris walrod

Carrier bearing preload is an interference fit between the diff assy and the maincase/diff cover castings as assembled. This is important to acceptable carrier bearing life!