has anyone got the tacho to work from the MJLJ output?

[Josh_b]

i just installed the setup on a friends 2.5 litre 6 cylinder triumph gt6 race car. works great as i knew it would, problem is the dreaded tacho! no joy from the MJLJ tacho output, has anyone actually managed to get it to work from that output? there doesnt appear to be a tacho out on the EDIS 6 module so i had to resort to the diode method, which worked up to about 2700rpm! any help greatly appreciated
Josh
http://www.bowler.fslife.co.uk

[David Withers]

I had this problem with a Smith's tacho and pinto engine. What seems to happen is that as the revs rise, the voltage on the output of the diode circuit rises, which causes the tacho to cut out. I just measured it with a voltmeter as I don't have a scope.
In an attempt to solve this I connected a 1200 Ohm potentiometer between the diode circuit and ground. The wiper was connected to the tacho input. I managed to adjust this circuit so that the tacho worked over the entire rpm range. I measured the resistances and replaced the pot with fixed resistors, soldered into the loom. It seems to work at the moment but has only had a few miles testing.

[Josh_b]

interesting, bit confused, how many resistors,? what values? connected from where to where? cheers
Josh

[Josh_b]

the way you describe yout circuit it would earth out the coils wouldnt it?

[David Withers]

If you look at the diode circuit, the zener is marked "to tacho". I have disconnected the tacho from here. A 220 Ohm resistor (R1) is connected to this point. A 1100 Ohm (R2)(2 x 2200 in parallel as that is what I had available) connects between the other end of R1 and ground. This forms a potential divider. The junction of the two resistors connects to the tacho and provides an attenuated voltage to the tacho.

If it's still not clear I will try to post a drawing on a website.

[David Withers]

It doesn't seem to. Looking at the diode circuit it will only conduct through my resistors when the voltage across the zener exceeds 15 V. Since this is greater than battery votage, it can only occur when the primary circuit is oscillating during the generation of the spark when the electronic equivalent of the contact breaker has opened. It may damp the oscillations a bit, but I doubt that it is significant.

[Josh_b]

a diagram would be helpful if you could, as it may help me and others too

[David Withers]

I have added a diagram to the files area