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