At 10:22 PM 7/26/2011, Rocket J Squirrel wrote:

>>The normal idle stab on the 1.9l engines is an electronic module that
>>advances ignition timing when idle falls below 750 (Federal) or 850
>>(California) rpm.
>
>But how does it know? Where's the clock -- timer -- in that module?

It gets rpm info from the Hall sender.  I haven't had one apart but
clearly there's an oscillator or counter that the Hall signal is
compared to.  As long as it's faster than the internal oscillator it
gets passed straight through to the output.  If it becomes slower,
the circuit advances the phase of the output, advancing more as the
input continues to slow.  Obviously it has to compare at least two
Hall pulses and can (I believe) at the earliest advance the third and
subsequent ones.  It does react pretty fast; if you put a timing
light on it you can see the timing jumping around rapidly.

Yours,
David