Speedometer Magnets and Carrier Replacement

[David Inc.]

So while at a recent track day my speedometer suddenly started reading at exactly 7/8 actual speed (GPS confirmed). I first noticed it when shifting into 5th gear from the top of 4th at .... 95 mph????

Anyway, I've since confirmed something hinky is happening inside the differential as one, I can hear a magnet happily rattling around in there, and two, this is the lovely song of a speedometer sensor only seeing seven of eight magnets on the tone ring (ohmmeter on speedo contacts, passenger wheel spinning in air)

https://youtu.be/DFhy5DMakaU?si=VXWN7Fj4vVykXqdV

There are a few posts here about people finding magnets for replacement, epoxying them back in, and all that, but what I'm curious about is if I want to just replace the whole magnet carrier on the diff (if one magnet gave up how long until the next), how far do I need to go with this?

A few questions then. Can the diff be pulled from the car without dropping the engine and trans? If I replace the magnet carrier do I then have to expect to check backlash and pre-load after reassembly, or can I reasonably expect that the new carrier will be +/- 0.1mm thickness as the old and everything will still be in spec?

Pre-load I think I could check, but backlash? Nope. No way.

Edit: I do get a kick that the Bentley's only instructions on differential work are "don't try this, you don't have the right tools."

[Rosco_NZ]

I recently went through trouble shooting 915 Speedo output … yes the diff head can be pulled from the LHS without needing to drop the engine. The half shaft needs to come out first .. and drain the oil.

[Rosco_NZ]

A bearing needs to be pulled to remove the magnet carrier .. I’d do it sooner rather than later, lest it finds its way between gears. A good 2 part epoxy will hold the magnet in its correct place. Check the others while you’re in there .. they can come loose.

[Rosco_NZ]

Make sure you check the orientation when you pull it off so it goes back on the same way .. or the magnets may be too far inboard from the case pickup area when installed.