Does anyone know the number of teeth on the tone rings for the speedometer/ABS, year by year? I have a 3.31 rear axle from a 90, and am trying to find out what years of car it is compatible with. I know that it has half as many teeth as an 88---I think the actual numbers are 48 and 96. I am particularly interested to know what the number is for later cars with the 3rd generation ABS--92 and later.

I am looking to buy a car, which will most likely be automatic, and if so I will want to convert it to M46/3.31, so I need to know what years to look for.

Thanks in advance.

I used to make tone rings for banjos and have a pile of them under my stereo.
Mostly made of cast bronze.
What does the tone ring in a speedometer do?
--
George Downs, Bartlesville, Heart of the USA!

The tone ring is a steel wheel with squared off teeth on it, about 5" diameter, in the rear axle, that rotates in close proximity to something that resembles a single-pole electric guitar pickup. This generates a tone, which varies with speed and is interpreted by the electronic speedometer on 740s (and later 240s) If the car has ABS, there are additional tone rings on the front wheels. The ABS computer compares the tone produced by all three sources, and if one of them is flat by more than a certain threshold, it means that wheel is rotating too slowly (locking up, or about to) and it triggers the modulator to pulse the brakes rapidly. For whatever reason, the number of teeth on the ring was changed at some point, so if you swap an axle from a different year car, your speedometer may be way off, and the ABS (if equipped) won't work at all. This change occurred between the 88 and 90 model years, and I am trying to determine if there was another change on later models. I am considering buying a 940, all of which are automatics, and I would want to convert it to an M46 and 3.31 axle. The parts I have are from a 90. I found out about tone ring incompatibility the hard way, when I put these components in my 88, and want to avoid a repeat.

I build instruments too. One of them is an all-metal bouzouki, the body of which is a 140 bellhousing.

The 1992 960s have a 48 slot tone ring. I expect 48 slots were used through 1995.