Rev counters on cosworth conversions

[james280779]

hi guy,
I have been chatting with mike @ boosted today, the engine is spotless apparently which is always good news.
he has made me aware though that when running the cosworth grannie ecu the tvr rev counter will not work.
Is this due to the signal differing and is it fixable?
would a simple change to maybe a smiths counter work?

Any ideas guys?

[Barkychoc]

I think the reason for this is that the cosworth engine has a coil pack and no distributor - effectively one ignition coil per cylinder. (If I'm wrong this is tosh)

Anyway on the 'normal' V6 it has a distributor and single coil so there are 3 coil pulses per revolution of the engine (each cylinder fires every other revolution in the 4 stroke cycle). These 3 pulses per revolution are fed to the rev counter to indicate rpm.

On the cosworth you can't get 3 pulses per revolution as there's no single place to connect it due to the seperate coils.

It would be interesting (OK I should get out more) to study the cosworth wiring diagram to see how they do it on the granada. They may have a special rev counter for the cosworth engine but there's an outside chance that they use a standard one and supply pulses from the ECU or something - again most of this is guesswork.

Chris

[trickjohn]

Hi James - my rev counter works - Im not sure why - but I do have other problems, which I think where caused when the cosworth engine went in (prior to me buying it), maybe trapped / broken wires in the harness,ie one headlamp is wired externaly from one side to the other and the radiator fan is on a manual switch, I cant emagine that this has anyting to do with the conversion.
When my body goes back on, I am going to connect everything, then strip the outer casing off the wiring - i have a couple of auto electricians friends coming to have a look, I can ask them how the rev counter works - maybe they will know.

BR Sean

[Jed-S]

Barkychoc said:

On the cosworth you can't get 3 pulses per revolution as there's no single place to connect it due to the seperate coils.

If the tacho is driven off the coil and if there are 3 feed wires to the coils then it might be possible. I did a similar thing on my Westfield when converting from dizzy/CVH to coilpack/Zetec. It involves linking the 3 coilpack feed wires with 3 diodes plus a zener diode and take rev counter feed from that. The Megajolt site should have the diagram for that so I'll have a hunt around.

Edit: Can't find it on the new Megajolt site but fortunately I have an image of the fix for my Zetec/coilpack and it is now on my site.

http://deadpineapple.net/linked/modified-rev-count...

So you'd need to take a 3rd 1N4001 (or similar) diode to the zener. The reason for the resistors was that at a certain revs the tacho would stop reading. Putting in the resistors as a potential divider solved the problem. I actually used a small pot for one of the resistors so that I had some adjustment.

Good luck
Jed

Edited by Jed-S on Thursday 27th March 20:14

[Le TVR]

The standard tacho is current driven. It measures the current pulses drawn from the 12v supply to the coil.

In theory if you have multiple coil packs it would still work providing the coils have a common +ve that can be taken from the tacho.

[900T-R]

Le TVR said:

The standard tacho is current driven. It measures the current pulses drawn from the 12v supply to the coil.

In theory if you have multiple coil packs it would still work providing the coils have a common +ve that can be taken from the tacho.

That's how I remember things to work not only in theory, but in practice too when I converted the Saab from a distributor-based ignition system to a computer-controlled coil-on-plug system (Saab Direct Ignition).

Edited by 900T-R on Friday 28th March 08:14

[CTE]

I converted my S1 to cossie power, and fitted an Emerald ECU. This is obviously diferent to the standard Granada ECU, but I guess the ECU still supplies the ignition coil feed.
Anyhow, all I had to do, was to wire the 12v feed to the coil pack, through the Tacho, and hey presto, its still working after more than 2 years!