And thanks ALL for providing the answer regarding ratio of coil pulses to RPM.
TM

On 9/14/07, Dennis Haynes <d23haynes57@hotmail.com> wrote:
> A 4 cylinder engine will give you two ignition pulses per revolution.
> Every other pulse fires the injectors so they fire once each revolution.
>
> Dennis
>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
> tinker man
> Sent: Thursday, September 13, 2007 8:44 AM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Measuring engine RPM based on Hall sender/coil signals
>
> 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
>