EDIS-8 to drive the tachometer

[cinquecento]

Has anyone used the EDIS-8 IDM signal from pin 2 of the EDIS-8 unit to drive the tacho. Alternatively, any tricks'n'tips to drive the tacho when the coil is removed in a Megasquirt/EDIS-8 setup. Has anyone tried the 4 diodes off the CP lines?

Thanks..

[adyw]

I'm after this answer myself,
What if you don't connect it at all.

[eliot]

Yes you need a matt-tach. Matt dreamt it up when he fitted edis on his griff and the rev counter didn't work. Essentially you use a transistor do drive a coil from an old relay which makes a nice collapsing field that drives the tvr tach.



He put the circuit inside the actual relay:

[Uncle Fester]

When fitting an Emerald it needs a tacho driver. These are available from Emerald. IIRC it was about £30 Might be a quick fix?

[neal1980]

When I fitted my megasquirt there was a wire to drive the tacho from the megasquirt loom. Had to put this in to a relay then in the tacho and this works fine. www.extraefi.co.uk was the diagram I used.

Neal

[adyw]

I have the same problem, question how can you find the wire to the tacho without taking the dash out, or is that the only solution. And if so which one is it, with out having currently got that far.

[Pupp]

adyw said:

I have the same problem, question how can you find the wire to the tacho without taking the dash out, or is that the only solution. And if so which one is it, with out having currently got that far.

Off the back of the original coil... I ended up going to the back of the tacho just to be sure as the wire colour changed but IIRC it was a blue/white stripe at the footwell/engine end (changing to something I can't remember on the dash side)... there are threads on here that describe so serach but if you get stuck, I'll pop the dash top off and confirm as the original is still in situ but not connected.

[adyw]

Is the tacho feed from the -ve side of the coil or +ve side. as there is a split in the -ve wire with several connectors going off somewhere.

[GreenV8S]

adyw said:

Is the tacho feed from the -ve side of the coil or +ve side. as there is a split in the -ve wire with several connectors going off somewhere.

-ve

[cinquecento]

I did try the "Matt tach".. however wasn't sucessful it getting it to drive the tacho. It seems to work OK when cranking... however when the engine fires the tacho drops from 200 rpm to 0.

I did wire the Matt tach to a battery.. and pulsed the EDIS-IDM signal between 12v and ground.. and saw the Tach signal change.. so I assume the transistor and wiring are OK...but don't have a scope to see things at higher RPM.

Any tips..??

[GreenV8S]

cinquecento said:

I did try the "Matt tach".. however wasn't sucessful it getting it to drive the tacho. It seems to work OK when cranking... however when the engine fires the tacho drops from 200 rpm to 0.

I did wire the Matt tach to a battery.. and pulsed the EDIS-IDM signal between 12v and ground.. and saw the Tach signal change.. so I assume the transistor and wiring are OK...but don't have a scope to see things at higher RPM.

Any tips..??

I guess the coil will need to have a suitable charge time (the equivalent of 'dwell angle' in ignition terms). If the supply current was too low relative to the inductance it might not be able to charge far enough to trigger the tacho at higher revs. Can you reduce the series resistance? Also if you have left the solenoid in place this might affect the charge time, try removing it?

[cinquecento]

Thanks Peter.. I did try jumpering out the resistor all together.. now I am wondering if there is a happy medium between 0 and abt 8400ohms which is what the inline resistor is. I will play with a 10k pot. I also have some diodes and will try wiring these up..

Many thanks..

[cinquecento]

Here's any update


I have also tried using the diodes/zener suggested in http://www.megamanual.com/ms2/EDIS.htm (at the end) ... but still no luck.

I have tried both connecting the relay and the zener via the 6.8k ohm resister and also bypassing this.. same result which is tacho erratically attempts to raise the needle of 0 ever second is short bursts.

Peter mentioned a solenoid.. I am not familar with this.. is this in the tacho??

I think the best way is to scope both disti and IDM signal.. just don't have a scope handy.

Any suggestions

[eliot]

Maybe Matt got lucky with his one then, as he made one for me and it wouldn't drive my tacho either. I scoped it and it was producing a nice spikey signal. There is a circuit diagram for something very similar over at msextra.com

[GreenV8S]

cinquecento said:

Peter mentioned a solenoid.. I am not familar with this.. is this in the tacho??

An electrical relay has a coil with a moving metal core connected to the switching mechanism. The core moves when the relay operates. The back EMF from this movement could affect the electrical characteristics of the coil. I've never tried to use a relay in the way being suggested, but if it doesn't work with the core in place it might be worth trying without it.

[cinquecento]

Thanks..

Back to basics... Could someone please clarify how the tacho actual works. I got into a debate over this at the races today..and this has open a new line of thought..

Does the tacho sense the 12v drop across the coil input (leading or trailing edge)...or the emf spike when 12v to the coil is removed?

If the tacho reacts to a square wave that's basically controlled by dwell, then there would be a nice square wave. If the EMF spike is used, then a short pulse would be seen. It's a real pity I don't have access to a scope..this would make life so much easier.

[rev-erend]

I though that they just counted every 8th pulse.. but with wasted spark you get double the sparks ..

[Pupp]

cinquecento said:

Thanks..

Back to basics... Could someone please clarify how the tacho actual works. I got into a debate over this at the races today..and this has open a new line of thought..

Does the tacho sense the 12v drop across the coil input (leading or trailing edge)...or the emf spike when 12v to the coil is removed?

If the tacho reacts to a square wave that's basically controlled by dwell, then there would be a nice square wave. If the EMF spike is used, then a short pulse would be seen. It's a real pity I don't have access to a scope..this would make life so much easier.

If it helps, the Emerald interface instructions state: 'some early tachos will only trigger from high voltage spikes seen on an ign coil primary...', goes on to state the module steps up the voltage from 12 to the 150-300 needed