915 Speedo magnet carrier ring issue

MuffinMan · 07-27-2013, 07:40 PM

I have an erratic speedometer in my '77 911S, and have taken steps to get it working correctly. It is a little bouncy, and reads lower than my actual speed. First I replaced the sensor, and saw no change. I then sent the speedo to Hollywood Speedometer, and it came back with a clean bill of health. The only thing left is the carrier ring in the transmission that houses the magnets. I figured one or more magnets lost their magnetism.

A couple of weeks ago, I pulled the differential with the help of a few friends, and checked the magnets on the carrier ring. I don't know how strong the magnets are supposed to be, but some were definitely a little stronger than others. They also seemed to flop back and forth with the north and south poles with each sequential magnet. We spent quite a while running another magnet over the current magnets in an effort to increase the magnetism, and it seemed like we increased the magnetic strength, at least somewhat.

We buttoned everything up, and now the speedometer essentially sits at "0". We clearly had an affect on the magnets, just the wrong one. I tried to order a new carrier ring, but I can't find them available anyplace. Has anyone run into something like this before, and do you have any words of wisdom?

jm58 · 05:54 AM

Hi Rob

I suffered the same problem, I bought a new disk (now it's NLA) then I test it with the sensor out of its barrel, the "new" disk installed on a driller machine...doing this ,I realised that the sensor was not installed in proper position !!
I turn back the sensor into the barrel and speedometer is now running well.
Maybe not the same problem for you but nobody adviced me before that it could happen.
JM

MuffinMan · 07-28-2013, 09:40 AM

Hi JM,

Would you better describe how your sensor wasn't properly positioned? I played with the position, but seemed like there was really only one way it could go.

Thanks!

yelcab1 · 07-28-2013, 10:06 AM

It only fits in one way. Install the sensor with the longish protrusion into the hole, and the end near the wires sticking out. Then the clip goes on top with the little M6 bolt. That is it.

schumicat · 07-28-2013, 10:47 AM

the sender is installed with the brown plastic side facing towards the trans, the waxy side facing out. if you had it facing the wrong way it would probably not work at all rather than sporadically.

MuffinMan · 07-28-2013, 11:24 AM

I'm relatively confident the sender is in correctly and not faulty, but I figure it was worth asking the question in case I'm missing something obvious.

Does anyone know where I can get a hold of a good carrier ring? The magnets are epoxied and swapping them out looks like a feat not worth tackling.