Revcounter Mod For Aftermarket Ecu's

[brumster 135 1 Cars]

If it helps anyone, I did this trick on an old coil-driven tacho when I moved to an Emerald ECU....

 

http://www.brumster.com/articles.php?article_id=10

 

Not really 205 specific, but might be an alternative for some people with aftermarket tachos not driven off the ECU. Just for reference, really.

[welshpug 1,661]

ooohh, worth trying for sure, so that works off the tacho signal from the ecu with an older tacho head?

[brumster 135 1 Cars]

ooohh, worth trying for sure, so that works off the tacho signal from the ecu with an older tacho head?

 

I guess it should work with any tacho that expects a high (12v) spike to drive it, yes.... it's basically a little mini coil setup

[CaptainK 11]

The way it's been done works well now, so I will leave it be. The problem must have been a plug connection when I fitted it. Out of interest, who did it and when? The fact it's lasted so long without 'frying' the chip gives me no reason to change it. Unless people have seen them done this way fail often?

Its was either that complete wuckfit retard Jonnie Wigmore that originally put the engine in it, or Dan Taylor (TaylorsPug) who fixed it all up properly afterwards.

 

Reading the following posts (from yours) above:

 

I have got a Spikey Rev Counter fix loom /cable thing somewhere too as I originally got one, but didn't need it. I can't remember exactly with Sandy, but he noticed that my rev counter had been "fixed" and thus didn't add the above suggestion to my wiring loom (which was custom made for my specific 205).

Edited September 20, 2012 by CaptainK

[[L'e$kro] 2]

I'm using the diode bridge trick on mine, like Welshpug

Advantage is that it saves the tacho output for something else on my Megasquirt, which has almost no spare pins left on the CPU

The downside is that if you cut or retard ignition for your rev limiter and launch control/flat shift, the engine speed seems to decrease more than it does in reality.

Nico

Edited September 25, 2012 by [L'e$kro]

[Wod 0]

Nice one! Works a treat

[j_turnell 137 3 Cars]

Digging up an old thred, I'm installing Omex and will be using bx16v dials. I read somewhere that they don't need modding at all, can any body confirm this??

[Anthony 1,003]

BX 16v ones will need modding the same as 205 GTi ones - I suspect its late BX GTi ones (that had MP3.1) that won't need modification

 

Basically, if the clocks are driven from the coil then it'll need modifying for late model engine swaps like GTi6, whereas if they're driven from the ECU (most things that have a wasted-spark coil or coil on plug) then it won't.

[j_turnell 137 3 Cars]

Thanks for clearing that up Anthony.

[floodys 4 1 Cars]

Sorry to bring this old thread back to life but I've read through it and can't see where anyone has mentioned how to wire to a coil pack if one can?

I've got omex 500 with one aux out that is being used for the fuel pumps, so I can't use that, it's also got a pug coil pack with 4 pins, from memory one is 12v ignition, and 2 other 2 are from the omex (one I think is 2 of the HT Leads and other is the other 2) I did wonder if it was as easy to put the existing tacho/rev counter wire to that?! If anyone can let me know step by step what and where I need to connec to that would be great. I did see something re soldering on the back of the Rev counter?! Cheers

[welshpug 1,661]

coil will have two wires that are pulled to earth by the ECU, one pin will be power and the 4th is not used. is there a coil plug on your loom?

 

tacho either needs an adapter to pickup from the two trigger wires or a mod to the circuit board to use the signal from the ecu.

[floodys 4 1 Cars]

This is the wiring diagram, the loom now has a coil plug wired In, it's got 4 pins, I've used the 3 as per diagram.

[floodys 4 1 Cars]

Sorry, poxy thing won't attach a picture. Basically 4 pin coil pack, has a 12v ignition live to it and one yellow and one violet from ECU.

[floodys 4 1 Cars]

coil will have two wires that are pulled to earth by the ECU, one pin will be power and the 4th is not used. is there a coil plug on your loom?

 

tacho either needs an adapter to pickup from the two trigger wires or a mod to the circuit board to use the signal from the ecu.

When you say trigger wires do you mean the 2 wires I mention from the omex? I can't use anything out of the omex as its being used for fuel pump?

[floodys 4 1 Cars]

Quick reply here if anyone can help? So I've figured on the omex 500 the aux wire will only do one thing so it's powering the fuel pump.

I have a yellow and a violet that go to the coil, I've found if i put the tacho wire 112 to either one the tacho will work but not properly, is this because it's only getting a signal for 2 sparks/pistons etc as I'm not connecting to the other feed?

[petert 588]

You'd only be seeing half the RPM. You need to add the two coils together with some diodes, the output of which goes to 112. I'll try and find the circuit diagram. It's very simple.

[petert 588]

Here you go.

Tacho Adaptor 4 cyl.pdf

[floodys 4 1 Cars]

Many Thanks .

[floodys 4 1 Cars]

I've tried that and nothing shows up on the gauge? I found the same layout on another forum, the only difference was that the diode in your picture is a 4744 15v and the one I brought was a 4746 18v. Will that make it not work?

[Anthony 1,003]

It should work as I've built similar for people before.

 

Double check you've the diodes around the right way and that you've got the earth as per Peter's link - some other diagrams don't have that and it is needed.

 

I've used variances of the Zener diode depending on what I've been able to source without issue so I'd be surprised if that's your problem, although I'm no electronics engineer.

[welshpug 1,661]

I doubt it will make much difference, its the frequency of the pulse that alters the RPM reading not the strength (voltage) of it.

[floodys 4 1 Cars]

Think it's the earth one i need, is it as simple as looking for it with those number? R1 1K?

I guess you can't go straight to earth without that R1 thing?

[Anthony 1,003]

It's just a 1k ohm resistor, ten a penny.

[DrSarty 90 1 Cars]

On an Emerald for example there's a tacho output, but even that needs a little gadget (which they supply) to make certain tachos work properly e.g work with more than one coil (wasted spark etc).

 

Hasn't the Omex got similar?

 

I'm guessing not, so you will need help with a solution that handles your coil pack trigger(s) as proposed.

 

Not sure if that helps.

Edited October 24, 2017 by DrSarty

[Anthony 1,003]

On an Emerald for example there's a tacho output, but even that needs a little gadget (which they supply) to make certain tachos work properly e.g work with more than one coil (wasted spark etc).

Modifying the tacho to remove some of the filtering works fine on an Emerald tacho output, I've done it several times now. Don't see why that wouldn't work with anything else with a negative-pulse tacho output too, certainly does with later OE ECU's (eg 2.0 Turbo, GTi-6) for example.

 

You only need the "gadget" or the diode trick (see Petert's post above) if you want to use an unmodified tacho that needs to see a spike back from a coil to work. The filtering circuit on the tacho to clean up that "spike" is what stops them working with a clean low voltage signal from an ECU.