915 Trans Clunk problem - What could it be?

hugofirst · 11-25-2008, 05:27 PM

Hey all,

I've had a bit of a problem with the 915 box in my 911. I know its because I put a For Sale sticker on the back just 15 minutes before it let go, but that’s its way of saying Don't sell me!

Ok, Its a 73 911t with a Euro 3.2. Not sure what era the 915 is from though.

The gearbox was reconditioned about 3 years ago and has probably done around 20,000k's since. Trans fluid replaced about 5,000k's ago with no signs of metal. Actually looked a lot better than expected.

Anyway, I'd just changed into 3rd gear, (not giving it a hard time) and heard a loud clunking sound from the box.. I popped it out of gear and couldn't hear anything.. So tried 3rd again, same thing. 4th, same (this is all while still rolling to a stop - only travelled about 100m). So back out of gear I pulled over asap.

I've had a bit of drama as to who to take it to locally, as I couldn’t' get hold of my usual guy, so took it to a different workshop that I knew had a few Porsche customers. I now regret taking it there as I’ve since found out that the guy actually drove my car to see what the problem was. Now I’m not too technically minded when it comes to transmissions, but I would have thought that if you have a trans problem, you stop! The less movement in there, the better! Prop it up and turn the wheels manually. Drain the oil - check for metal flakes? I mean, when you just push the car, you can hear a continuous clunk in the box.

Now I can't really say that I’ve blown a Porsche trans before, but know what separate gearboxes / diffs sound like when blown. The CLUNK just gets me thinking that its not a gearing problem. All gears can be found just like it was new (while stopped). Like I said, its not a grinding sound, its a clunk each rotation of the driveshaft (?). Eg. if I was to turn the rear wheels, I would probably get 2 maybe 3 clunks per wheel rotation.

I've since got hold of my regular guy and its due to be towed (flatbed) there this coming Monday. I have no doubt about his ability, but was more curious to see if its a common problem. Does it sound like it could be something major / minor? Could the guy that drove it - made mince meat out of it? The guy actually said you couldn't really notice anything at all until you were above 30kph (said he only drove it a short distance), which I find hard to believe, as you can feel it just by pushing the car out of gear, but then again, it may be a different story when in gear?

Thoughts?

Thanks!

john walker's workshop · 11-25-2008, 06:07 PM

CV joint slop maybe? or maybe the CVs are lacking lubrication.

hugofirst · 07:32 PM

Are they they type of thing to be perfect one second, then just let go all of a sudden and make the clunk sound mentioned above? Or more the type of thing to start out small and gradually get worse?

It was actually driving smoother / better than ever just prior to it letting go.

Is there anything I can check myself that would point to that?

When I say, check myself, I should clarify, In a garage with very limited tools.

Thanks again for your help, I do really appreciate it.

EDIT: After reading a few CV threads, i just thought i'd ad that the car is actually still drivable. I wouldn't dare drive it, for fear of making something bad - worse. But it can still shift into gears and be driven (as mentioned above where the 2nd choice mechanic drove it). Most of the CV threads mention that they can't actually move - as in, rear wheels don't engage.

Jack Olsen · 11-25-2008, 08:17 PM

CV joints break, and sometimes one or more of the bolts that hold them in place come loose or let go. It can lead to a sudden, complete failure, or a lot of noise but normal function, or a gradual increase in noise that leads to a failure. I would have your mechanic check them first when he puts the thing up on his lift. If they're bad, you're probably going to want him to replace them with rebuilt or new axles. You could do it yourself, but the axle nut requires a lot of torque (big breaker bar) and might be beyond what you want to take on right now.

hugofirst · 11-25-2008, 08:33 PM

Ok, Thanks for the quick reply. Its something that is unfortunatly beyond what I'd take on right now, given the lack of tools where im at at the moment, but will still jack it up tonight and try turning the wheels and see whats turning whats not.

The clunk is more dominant on the left side, so hopefully it is a CV problem and not too much more.

Last question, If it is the LHS CV, when the car is jacked up, should I expect one wheel to turn - clunking, and the other (RHS) to spin freely? or it doesn't really work like that?

Thanks again!

Jack Olsen · 11-25-2008, 09:06 PM

Whether both wheels spin or one goes backward, or whatever, is all about your differential, not your CV joints. The CV joints (CV stands for constant velocity) are at either end of the axle, covered with a rubber boot. Inside, there's a metal cage and some steel balls and a driver piece. The axles can wear out, dry out, become contaminated with outside debris (or all of the above). If you jack the car up in neutral and rotate the wheel you can listen for funny noises from the joints.

There's also a ring of bolts that hold the inboard joints to the transaxle. You can check to see if any of them have backed out. As they loosen up, things can get loud. It might not be as pronounced when you turn the wheel by hand as it is when the engine is applying torque.

Be very careful under the car with the wheels moving. At a minimum, use multiple jackstands.

hugofirst · 11-26-2008, 01:31 AM

Thanks for the info Jack, and I have a bit of an update.

UPDATE:

Jacked up, out of gear, both wheels spin with the same clunk both sides. The clunk is louder when spinning the wheels forwards, but not a HUGE difference. The clunk occurs every half turn of the wheel - in the same spot. Each wheel spins independently of the other.

Ok, I also found that when the RHS wheel is turning, the clunk can be felt while holding onto the inner LHS CV. and same when the LHS wheel is turning, the clunk can be felt in the RHS CV. You can still feel a very slight clunk in the inner CV of the wheel thats spinning, however no where near the size of the opposite inner CV.

I'm not sure if its the CV itself, or something inside the box, that can be felt through the CV.

Thoughts?

john walker's workshop · 11-26-2008, 04:15 AM

next step, disconnect the CVs from the trans and turn the drive flanges on the trans by hand.

hugofirst · 11-30-2008, 04:12 PM

Found the problem.

1 tooth missing off the crown wheel. :S

Its a '74' cable box. And doesn't seem to be many CW/P's available at a reasonable price!

Just curious, is it a fairly major job to replace the Crown wheel & Pinion? Im told I have a rare ratio in there and have bucklies of getting the same ration again.. (Im in Aust.)

Hmmm Not happy hey!

Grady Clay · 12-01-2008, 04:38 AM

WOW!

That is unusual. Is this "... rare ratio....) not an original Porsche part? Who is the manufacturer? What ratio?

Replacing the crownwheel & pinion is properly done with the tools to set the pinion depth, taper roller bearing preload and gear backlash. You are correct; the gears are expensive.

Best,
Grady

hugofirst · 07:56 PM

Sorry, my mistake, Its not a rare ratio (7:31), but apparantly hard to get hold of unless you go genuine (here in Aust.)

It was actually 2 teeth, however I haven't seen it yet so don't know if its off the CW or P. And apparantly its not unusual for this to happen in the early 915 (mag) boxes.. Not common, but also not unusual. Im told thats the gamble with a Mag box.. They're a lot lighter than the newer 915's, however they're also weaker. Cake and eat it?

Thankfully no shrapnel went forwards into the actual gearbox, and all the gears still look crisp, so it could have been a lot worse.