I have used the tach signal in relay position 2. VW put it there for use
by certain fuel pump relays that use the tach signal as an rpm limiter
and as a fuel pump safety shut off when the engine stops turning. I
installed such a fuel pump relay into socket 2 for my 5 cylinder with
CIS injection.

The 4 cylinder hall puts out 4 pulses per distributor revolution. The
distributor turns at half crankshaft speed, namely the crank turns twice
while the distributor only turns once. There would be 2 pulses per
engine revolution.

Mark

tinker man wrote:
> Hello Volks,
>
> I have a 1990 2100cc Digijet engine (European model). I want to
> measure engine RPM without a strobe light, by connecting a frequency
> counter to the Hall control unit ignition coil drive output.
> How is engine RPM related to ignition pulse frequency?
> The motor is a 4-stroke, 4-cylinder, where every cylinder gets a spark
> once in a complete revolution and no two cylinders are sparked
> concurrently (1-4-3-2 firing sequence as per Bentley), so it seems it
> should be a 1:4 relation. Can anyone verify or correct this
> conclusion?
>
> Also, looking at the schematic, I see that the coil driver output goes
> in parallel to the dashbord to drive the tachometer, but also to the
> main fusebox to two vacant relay positions (2 and 4).
> Can anyone verify this, and has anyone actually used this signal to add a tach?
>
> Cheers,
> Tman
>