Speedo sender output voltage varies - is this normal?

[petepetrolhead]

My speedo died (it was a second hand one from a Cerebra) and I've replaced it with another second hand one (also from a Cerebra). The Speedo doesn't register anything until I hit around 40mph, then it springs into life. I've managed to look at the output from the sender on an oscilloscope, and find that it is sinusoidal (which I think is correct) and the frequency varies with speed as it should.

However, the voltage of the signal varies with speed, and I think this may be the problem. Accelerating from rest, the voltage is about 2V peak to peak initially which increases with speed until it reaches about 4V peak to peak at 40 mph when the speedo starts working.

I can think of three possibilities (there may well be others!). Either the sender ought to be putting out a constant voltage level, in which case there may be something wrong with it, or the speedo ought to work with an input signal of variable voltage in which case there may be something wrong with it. The third possibility is that the 'new' speedo requires a different type of sender. (The speedo is of the type with the optimistic 220mph full-scale reading).

All advice gratefully received.

[blitzracing]

Thats totally correct- the sender has a magnet down the centre, and the toothed wheel passes the magnetic field to the surrounding coil as the teeth pass by. The faster the teeth go, the faster the magnetic field moves so the voltage increases. Try getting the gap as small as possible as this will increase the voltage.