Pour ton CA18DET si j'ai bien lu tu peux le faire soi avec le harnais de diodes ou avec une résistance.
Circuit avec la résistance:

Harnais avec les diodes (diodes No. 1N4004, 1 amp 400 volt )
*je vais traduire plus tard mais voici l'explication en anglais*
The tacho works by counting the rate at which ignition pulses are being fired. With the CA20 (and distributor-based ignition systems in general), all four pulses are on a single wire, so all four pulses are counted by the tacho. With the CA18/SR20 (and distributor-less ignition systems in general), each pulse is on its own seperate wire so any given wire carries only 1/4 the pulses of a distributor system, like so:
CA20 _|-|___|-|___|-|___|-|___
CA18#1 _|-|_____________________
CA18#2 _______|-|_______________
CA18#3 _____________|-|_________
CA18#4 ___________________|-|___
So, you should get 1/4 the tacho reading from a CA18 that you do from a CA20
To complicate things, some ignition systems (like my Microtech) fire "waste sparks", i.e. they fire on the exhaust stroke as well as the ignition stroke. Supposed to make it burn cleaner and produce more power or something. That looks like this:
WS#1 _|-|_________|-|_________
WS#2 _______|-|_________|-|___
WS#3 _|-|_________|-|_________
WS#4 _______|-|_________|-|___
With a waste spark system, you should get 1/2 the tacho reading from a CA18 that you do from a CA20.
What the diodes that Gerry has suggested do is to allow you to mix the signals from the CA18 wires onto a single wire, without affecting the original signals. The output from the junction of the [edit]cathodes[/edit] of the diodes then looks like the CA20 ignition signal:
_|-|___|-|___|-|___|-|___
so the tacho is happy and displays what you want.
As someone else noted, you can also use the output of an aftermarket (or even factory CA18) ECU, with some additional circuitry to increase the signal to the right voltage level. While it would appear that a pullup resistor works, personally I wouldn't trust it. I would put a proper level shifter circuit in - but that's a bit of work to do right so its easiest just to use the diodes imho.