Tachometer input signal... CONCLUSION.

[btpeet at effervescent.com (Bradley Peet)]

Apparently the VW tach (on pre-ODB II cars, say 95 and earlier) expects a
high voltage spike somewhere in the neighborhood of 100 to 300 volts from
the "low" side of the coil.  Hence products like this "tach adapter" that
artificially produce the needed spike:

http://www.msdignition.com/fuel_10.htm

The only thing I could get to work was combining the signal from both coils.
This requires using diodes (part #1N4005) to isolate the coils from each
other, as they fire 180 degrees out of phase.  The previous diagram I wasn't
workable because the ECU ignition outputs drive amplifiers, not the coils
directly.  My wiring looks like this:

                         DIODE
[COIL-1/4 neg terminal]------>|-----\___________ Tach Input
[COIL-2/3 neg terminal]------>|-----/
                         DIODE

It's just a PITA to have to Y those together and run and extra wire in from
the engine compartment, but at least it works.

Bradley
88 S16V 2.0 EFI
02 S4 Avant Biturbo

> -----Original Message-----
> From: Bradley Peet [mailto:btpeet@effervescent.com]
> Sent: Saturday, May 24, 2003 3:28 PM
> To: Marc Scirocco Qu?bec; 'Scirocco List'
> Subject: RE: Tachometer input signal.
>
>
> I don't know if you're familiar with transistors, but the ECU
> outputs are open-collector switches that sink current to ground.
> +12V is supplied to one side of the coil at all times, the other
> end of the coil goes to ECU (or an ignition amplifier) that
> switches it to ground.
>
> I'd have to assume that the tachometer has it's own 12V source
> and needs the "input" pulled to ground, as is done with the coil.
>  I'm going to head out to the garage now and perform a few tests
> to try and verify that theory.
>
> Bradley
>
> > -----Original Message-----
> > From: Marc Scirocco Qu?bec [mailto:marc_scirocco@sympatico.ca]
> > Sent: Saturday, May 24, 2003 1:51 PM
> > To: 'Bradley Peet'; 'Scirocco List'
> > Subject: RE: Tachometer input signal.
> >
> >
> > Looking at your diagram, what is input, and what is output?
> > Because it looks
> > as the ECU outputs are facing the wrong end of the diodes. What is the
> > intended signal flow? From the ECU to the tach? If yes, perhaps you had
> > better reverse the diodes.

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