915 diff speed sensor mystery

[Coultl]

New to the forum but have been learning a ton! Love it.

My speedo reads about 15% too slow. After some reading here I figured it was a dropped magnet on the disc mounted to the diff. Hooked up a volt meter to the speed sensor tonight and I only get 4 equally spaced connections per revolution of the wheel. Video here: https://youtu.be/hbA0JwQErqk?si=aVk-mwrEeKtV0CkO

No way I’m missing half my magnets. Any ideas? This was done with half the car jacked up and the trans in neutral. The car has an LSD but I don’t think that matters.

[Coultl]

I think I answered my question… With on wheel on the ground, the diff will spin the wheel faster (2x faster) than the diff ring gear. So, when I turned the wheel through two revs the diff ring gear only made one rev. Looks like my magnets are all in place!

[gled49]

Use an ohm meter to check magnets. Open the rear tunnel cover, disconnect wires in the 2 position plastic wire holder and check wires from trans with the ohm meter. In neutral with both wheels off the ground.

[Dan J]

Half of them could be missing. They're probably on the drain plug
You can remove the diff with the trans in the car and replace them
Epoxy them back in place

[Matt Monson]

Originally Posted by Dan J

Half of them could be missing. They're probably on the drain plug
You can remove the diff with the trans in the car and replace them
Epoxy them back in place

This.

[Coultl]

Thanks all. Just to confirm for anyone with the same question. If one wheel is on the ground, the diff body will spin at half the rate of the wheel that's off the ground. So, you will only get 4 closed connections on the speed sensor when you rotate the wheel one revolution. You need to rotate it through two full rotations and make sure you get eight closed connections. If so, you have all of your magnets where they should be.

You can also check to see if you dropped magnets by seeing if you odometer is accurate. If the odometer is accurate and the speedo is not then it's the speedo and not an magnet issue. That was the case for me.

[kevingross]

In this case, the dropped magnets created damage in other parts of the transmission. This "tone wheel" became available once more via Porsche Classic not too long ago.