The hall effect sender generates its pulse by way of the "hall effect"
which is the property of a material which is conducting current to
induce a voltage while in the presence of a magnetic field.  Basically,
when a conducting material moves past a magnetic field (or vice versa) a
voltage is induced.  This voltage can be used to generate timing signals
for motor synchronization and/or speed control, etc.

I assume if you take apart your hall sender you'll find a magnet(s)
mounted on the shaft or housing.  As this magnet(s) moves past a
conducting wire(s), a voltage spike is generated in the wire.  The
frequency of the voltage spikes is used by the ECU to determine rpm.
For a good theoretical discussion see:

http://www.honeywell.com/sensing/prodinfo/solidstate/

Have fun.

JN

Dave Bayer wrote:
>
> > John Russ wrote:
> >
> > John R. 86 Vanagon
> >
> > What is ,and where is and what dose it do ? This so called Hall effect
> > sender.
>
>         The Hall sender is part of the distributor. It more
> or less replaces the old points and condenser. Basicly as
> the distro spins, this thing generates a sharp electrical
> peak once a revolution. I'd have to take it apart to tell
> you just how it does this. Anyway, the hall sender unit
> consists of a collar on the distro shaft and a plate in the
> distro with some electronics and then three wires connect
> it to the outside system. A positive 10 or 12 volt wire,
> a ground, and a signal wire. These three wires come out
> of the distro thru a plastic elbow near where cylinder
> 4 will be sparked. This elbow is a connector socket as
> well and so there are three spade connector type things
> in the elbow. It just so happens that these spade connectors
> attach to the wires of the hall sender plate over the metal
> bodyof the distro. Thus, when the elbow ages and cracks with
> use, the three uninsulated spade connectors are free to contact
> the distro body which grounds out at least the signal wire. Without
> this signal, the ECU and tach do not get a timing/rpm signal and thus
> think that the engine is not turning and never getting to a point
> where a spark should be delivered. Thus, you get no spark and no
> fuel delivery...
>
> dave