Pulse Signal (Twin Coil) 'Addition'

[f0xy]

Hi there,

Im trying to find a solution to get the rev-counter/tacho working on a car. The original engine had a single coil, and displayed the revs on the dials correctly....the new engine (custom) has a twin coil, (both engines 4 cylinder), I connected one of the coil wires into the clocks, and it just displays half revs, so at 6k, the clocks are reading 3k, not ideal...

There is also a second wire coming through from the same location (as its twin coil), so I was wondering if anyone has any ideas of how these two wires/signals can be 'added' together, to give one reading....I didnt want to put them together incase I damaged something (The two coil wires/readings are coming directly from two pins on the ECU), without first knowing how it can be done...

Thanks if anyone has any ideas!

 

[tcmtech]

Put a V8 in it!

Does each coil only fire two cylinders?

What type of car and what year and model?
Actualy you will need to post a schematic so we can get some idea of how your ignition system is set up.
Or talk to the people that work withcustom modes like yours. they will already have an adapter box that will correct the problem.
If you can afford a custom engine you can also aford a custom tach adapter.

 

[crutschow]

If each coil fires two cylinders, that would explain the half rev display.

If the custom people don't have a box, you should be able to tie the two signals together with resistors. Try a 10k ohm resistor in series with each line. Connect the other end of the resistors together at the tach input.

 

[f0xy]

Sorry, not a custom engine so to speak...

Its a Renault Clio Mk1 Williams, 2.0 16v. Originally they run a single coil, and a distributor that is spun by the end of the exhaust camshaft.

We have put a Renault Clio 172, 2.0 16v (10 years newer engine) into the car, and adapted the engine loom from the newer engine to mate upto the original car loom (usually single coil)...

Yeah, each coil fires two cylinders, both wires come out from the ECU.....

There are about 4/5 people in the whole of the UK that have done this conversion, and not one has managed to get a fully working original rev counter as of yet....always reads half...

What other info do you need?

 

[f0xy]

We found this diagram on the web a short while back....but dont understand the need for diodes?

**broken link removed**

If we need the diodes, ive no idea what rating etc they would need to be....

Well basically, im a 100% electronics newbie...common = earth?

 

[tcmtech]

Tachs that run off the coil need a higher level pulse to work. And if it is connected to one coil directly then it sould work with both feeding it. But I would go with a fairly high voltage rated diode set. Perhaps 400v to 1000 volts 1 amp rated. Each doide will let the individual coils signal go to the tach but will block the signal from going back to the other coil. The resistor helps pull down the signal between pulses and is connected to the common ground of the electrical system.
Try it see what happens!

 

[Diver300]

Some rev counters have a resistor - capacitor network inside them that controls the range. It may be possible to alter the resistor (probably halving or doubling it) to get the range right.

I fitted a rev counter to a car with twin coils like yours. The rev counter is an after - market one and it has a switch on the back for 4, 6 or 8 cylinders where I want 2, the number of cylinders fed by each coil.

The switch selects different resistors. I forget the values now but the resistor for 4 cylinders was half the value of the one for 8 cylinders. I just changed one to a value of half the resistor for 4 cylinders and it works fine.

The diode circuit should work fine.