Don"t Let This Happen To You! (long

[PORSHIN]

If you notice your speedometer accuracy has suddenly changed, like about an 8 mph drop in indicated speed at 60 mph, don't drive the car! It can result in disaster! Pull the differential out and inspect the tone wheel on the differential immediately. The tone wheel (or exciter ring) whichever you prefer to call it, is an aluminum plate attached to your ring gear carrier. It has 8 small hardened steel magnets pressed into it which induce a magnetic pulse into your speed sensor. These pulses are counted and compared to time in order for your engine control computer (DME) to determine vehicle speed and then transmit this info to your speedometer. The problem is, if one of these come out of the ring, as one did on mine, it can stick in your ring gear and the next time that tooth of the ring gear gets to the pinion gear, your rear wheels may lock suddenly and completely. If this happens at a critical time, the vehicle can go into a sudden and uncontrollable spin which we all know is no fun. I noticed my speedometer was off considerably all of a sudden. It wasn't reaching the same speeds in each gear as it had previously. The car was running fine and the rpms were still there. That was when the magnet came out and began floating around in the housing. After inspection I found that the cage on the pinion bearing had also started falling apart leaving pieces of steel laying around in the alloy case. This little magnet obviously bounced around in the case collecting pieces of the bearing cage for about a week. Then one fateful Sunday morning while on a rather "spirited" drive, when we came over one of those quick drops in the road that make you kind of weightless, it decided to attach itself to the ring gear. The ring gear is steel and since the case is non-ferrous alloy, the magnet decided the ring gear is where it would rather be. When that tooth reached the pinion gear the car made an instant, uncontrollable left turn into the woods. The car is history but what happened is no longer a mystery. When you look at the picture, you can see where one of the eight magnets in the exciter ring is missing. just below and to the left of that, you can see where it was deposited in the ring gear. I would post pictures of the car but they are too painful. See picture in next post.

[PORSHIN]

[Gordo2]

Porshin,

Glad you figured it out. The rest of us were at a loss for your loss.

Hope you were able to salvage as much as possible and on your way to your next car.

What a freak thing. You should work for the NTSB, an amazing find.

Gordo

[PORSHIN]

Thanks Gordo! I thought there might be some other people who may benefit from this and some who would just like to know what happened like me.
Rob

[APKhaos]

Rob,
I'm in awe of your analysis, and the fact that you found the evidence. Both enlightening and a little spooky that such a minor failure could [would - its obvious now] have such terrible consequences.

The how of the events have been on my mind since it happened. Good that we all know that a mechanical failure was the cause, rather than any of the theories. Really appreciate you having the stones to share this after the loss of your gorgeous car.

Thank you.

[carreradpt]

Porshin, you are one lucky fellow. I am amazed that such a seemingly small part could cause what sounds like a very serious effort on that failure to try to kill you. How you ever figured that out is really cool. Sorry for your loss though.

[widebody911]

Since my rebuild, I've noticed that my speedo jumps u at speeds above 60. For example, @ 60MPH, it will jump to 64 for a second, and then fall back. No idea why.

[jtf914]

That's an amazing find (pretty scary as well). Once again I'm sorry for your loss. She was a beautiful 911.

Justin

[Doug E]

Nice detective work. I'm sure the peace-of-mind was worth the effort it took to figure this out.

Thanks for sharing and hopefully we'll see you back in a P-car soon.

[PORSHIN]

I again thank all of you and maybe someday I'll have another one. ...Maybe one that has a cable-driven speedometer! Just Kidding. This seems to be something similar to winning the lottery in reverse. I doubt it has happened many times before and hope it never happens to anyone else! I'm posting another pic in which if you look closely, you can see the rollers in the pinion bearing are not separated as they should be which creates a gap in the top of it where there are no rollers. That is because the cage that holds them in their proper place has disintegrated and fallen down into the trans case. I don't know if the magnet would cause this by itself but after collecting pieces of the cage, it obviously can!
Rob

[Jim Richards]

Rob, sorry to hear about your car but I'm glad you're OK. What a wierd problem to have, and at such an inopportune time. So, what's the plan going foward? Best wishes and Happy Holidays!

[PORSHIN]

Thanks Jim!
I'm just sorting all this out right now and once I'm through it, I'll decide which way to go from there. I have a lot of other irons in the fire right now too so P-cars have to be on the back burner for a while. Congrats on the new addition to the family. Looks like you got a nice one. How are things going with it? My family and myself hope you and your family have a wonderful Holiday and wish for you a healthy and prosperous new year!
Rob

[Doug Zielke]

Rob,
This is the most interesting (and discouraging) thread seen here in a while. Sorry you had to be the one to post it.

Those lucky people who have their 915 and G-50 transaxles apart now, can check the sender for loose magnets.

[scphelps1]

Hi Rob

Glad to hear that you have sorted out the "why" as to what happened. I'm sure it is nice to know what caused that tragic turn of events. Any time you want to "tweak" something on a p-car, know you have a turbo at your disposal (still a lot of tweaking to go!). Again, sorry for your loss, but glad you are ok. Wishing you and your family a happy holiday!

Clayton