TACHO PROBLEMS...YET AGAIN

[Nick]

Hello.
I realise that the weak spot of this system is the tacho circuit and every one is bored to sobs with the subject but....( and, yes, I have read the installation notes and most of the forums relating to this tedious matter!!)
I've built and fitted a MJLJ ignition system to a 4 cyl. Zetec engine. All works perfectly EXCEPT the tacho. The tacho is a VDO and worked perfectly when reading the Coil Neg in the original installation.
I've tried:
1.) the TACH take off from MJLJ pin (same side, opposite end from 12V live)
2.) the coil-pack negs with MJLJ dongle (both with & without final zener diode). This contact gives a pretty clean, square 12-14V read out.
The individual coil-pack negs., naturally, show all the fly-back and bounce clutter. As the tacho refused to work when both sides of the pack were used with the smoothed, square wave, I haven’t bothered to try just one half…maybe I should…
Nothing has the slightest effect.
The tacho 'ticks' upward if the signal wire is brushed over the positive battery terminal, indicating to me that it is a 12v signal that’s required.
The TACH terminal from MJLJ gives a regular, speed variable, 12mV spike when quizzed by our oscilloscope, this I assume to be too small for the instrument to read, and I cannot seem to get a regular square wave out of the PIP signal. It starts square for a few seconds and then seems to decay to spikes; in any case this is again on the 12mV scale (selected automatically).
The tacho responds to being attached to the 'W' terminal of the alternator but reads high.

Where else can I get a 12v signal of sufficient strength to kick it into life; is the drilling and tapping of pin 11on the EDIS likely to give me this?
All help gratefully received.

[Nick]

Just tapped pin 11...no result. A signal at +/-0.2 V and no movement of the needle

[Nick]

UPDATE:

Just tapped pin 11...no result. A signal at +/-0.2 V and no movement of the needle

[MartinM]

2.) the coil-pack negs with MJLJ dongle (both with & without final zener diode). This contact gives a pretty clean, square 12-14V read out.

Try a 1K resistor between tacho and earth - the theory is that without the resistor the signal alternates between +12v and open circuit, which the tacho can't do anything with. With the resistor the signal alternates between +12v and ground which makes the tacho much happier. I was involved with a 205Mi16 install and this worked a treat (ie went from not working at all to working perfectly),

The individual coil-pack negs., naturally, show all the fly-back and bounce clutter. As the tacho refused to work when both sides of the pack were used with the smoothed, square wave, I haven’t bothered to try just one half…maybe I should…

You should do this, just to show you get a half-rpm reading and that everything is basically OK

The TACH terminal from MJLJ gives a regular, speed variable, 12mV spike when quizzed by our oscilloscope, this I assume to be too small for the instrument to read

TACH output wouldn't be much good if it was 12mV! Should be a very clean 0-5v pulse train. You need to find out why it isn't

and I cannot seem to get a regular square wave out of the PIP signal. It starts square for a few seconds and then seems to decay to spikes; in any case this is again on the 12mV scale (selected automatically).

PIP is a 0-12v square wave, all the time.

You sure your autoranging/scale reading on your scope is correct?

[Nick]

Thanks MartinM. We'll try these suggestions.

[Nick]

FINALLY!!! But I had to change the instrument to a programmable multi input gauge. Now it works perfectly from the 'tacho out' source....as expected.