Seems like there may be some confusion between the vehicle speed
and the engine speed.  The tail of the transmission usually has
a speed sensor for the speedometer.  If using an electronic speed
sensor, these usually output a square-wave signal.  On newer cars
there is usually a VSS (vehicle speed sensor) that the ECU needs
and is also sent to the cruise control.  These usually put out a
sine-wave signal whose amplitude increases as the speed increases.
I've cleaned this up before with a LM339 comparator and used it
to drive and electronic speedometer.

But that is *vehicle* speed.  Even if you could easily determine
which gear you're in, you don't want to use this to derive a tach
signal as the engine rpms can vary outside of that, while idling,
etc, and especially if you have an automatic transmission.

For the tach signal, most cars in the last decade or two will have
a distributor with a central coil.  The tach signal will need to
come from the negative of the coil and cleaned up due to it's
unclean and spikey waveform.  Using an optoisolator is recommended.
Much newer cars use ignitions with no distributor, that have a
coil-pack directly over each spark-plug.  To get a tach signal off
of this, you will probably have to tap the ECU signal going to any
one of these coil-packs and calculate the engine RPM's with the
appropriate factor (1 pulse every 2 rotations).

These newest cars should also have a crank-position sensor to tell
the coil-packs when to fire, and also control the ignition timing,
so perhaps you could find that signal and use if for the tach
signal.

Cheers,
-Neil.




> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Alan B. Pearce
> Sent: Friday, November 22, 2002 7:23 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]:Where to tap tachometer signal?
>
>
> >On some cars I've seen magnets on the drive shaft
> >with a  pickup for the cruise control, but the
> >tachometer seemed to be driven from elsewhere.
>
> Well for cruise control, you obviously need to have the pickup after the
> gearbox. Same as for a speedometer.
>
> Electronic tacho is generally generated from whatever signal is
> used to fire
> ignition pulses in a petrol car, do not know how it is done in a diesel,
> unless there is electronic control of the injectors. If the tacho
> signal is
> used for cruise control, then there needs to be some interlink with the
> gearbox to know what gear it is in, but there is still a problem
> if there is
> a torque converter involved.
>
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