From: Wally Plumley <>
On the pre-'90 (except for John V's '90 GT) cars, the speedometer/odo are driven by pulses from a reed switch in the  differential cover, triggered by magnets in a ring mounted to the right axle drive flange.

FYI-There are 8 magnets on the diff. ring.
Speaking of this, I'm trying to find out if mine is an odd ball or it's Wally's car that is the odd ball ;) Anyone with a 90 please let me know if you have a magnetic diff.

John Veninger
'90 928GT
'80 928EuroS (#93 GT-2S)


>James R (keener) reports to me that the dif has 8 magnets on it - which hopefully implies 8 freq blips per 360 rotation. Thanks James!

A brown/red wire runs from the speedometer thru Terminal H1 to Terminal T1 to the speedometer sensor. A leaf switch in the sensor is operated by eight magnets on a ring attached to the ring gear of the differential. From the  leaf switch, a brown wire runs to ground thru Terminal T2, then H2, then  Terminal 11R, then 7R on the instrument panel.

No indication on the wiring diagram of what the voltage level from the speedometer is, but you should be able to tap a pulsed signal at H1 or T1. The signal, as you surmised, will be eight pulses per rear wheel revolution.

Wally Plumley
928 Specialists   


> Does anyone know the rate the pulse sensor for the speedo fires at? Is it picking up the teeth in the differential?
> Also, is the flywheel position sensor firing at TDC by any chance?
> I need VSS (vehicle speed sensor) and TDC inputs for a new ECM I'm working on.
> Thanks,
> Bill
> '84s 5 sp.

> the speedo drive 78-89 the micro switch on the back of the dif housing is counting magnets imbedded in an aluminum ring there are 8 of them so it pulses 8 times for one revolution of the axle . The 1984 has no timing ring no tdc gap unless is is a Euro will need to use a trigger wheel on the front of the crank or mount one where the current distributor is fitted .
> Jim Bailey
> 928 International

Thanks Jean-Louis and Jim....

That works out to about 6500 pulses per mile, a little higher than I need but I think it'll work for the VSS input.

As to the second part of my question, I found the answer in the manual. From '84 on a TDC sensor is fitted in the bell housing and triggered by 2 pins in the intermediate plate. I can probably remove one pin and stimulate the sensor to provide the TDC pulse for my ECU. Anyone know what this sensor is (hall effect?) and what it takes to drive it?



The earlier differential "sensor" is a reed switch, operated by magnets set in an aluminum ring attached to the differential. Not sure, but I think that there are about eight magnets. The switch flips a 12 vdc circuit on (to ground) eight times per wheel revolution.

The later system uses the pulses from the ABS sensor - many more pulses, at a much lower voltage.

BTW - they only used the front wheel for a couple of years, then went to a rear wheel ABS sensor.

Wally Plumley
928 Specialists


MY 92 Wheel sensor for the speed is on the left front wheel. MY 93 and after its on the rear Left.


There is a logical explanation for this. It was decided to use the ABS sensors on the rear wheel axle to see a spinning rear wheel and respond to that visually, seeing the speed at an unreasonable level, basically lost grip.  If you hit the gas the wheels start spinning, and a 1992 will show a PSD active light n the dash, but a 1993 will also show a sudden increase in speed by the spinning left rear wheel. 

So summary:
up to MY 1991: impulse sender on the differential case (reed contact)
(..look on the schematic 1990 position G30.)
Starting MY 1991 and thus also 1992:  impulse sender over the ABS sensor front left wheel (FCU freq converter relay XIX)
Starting 1993:  impulse sender over the ABS sensor rear left wheel (FCU freq converter relay XIX)