Dodgy old rev counter punt

yes it does.

 

That's a pic of a + earth one, but maybe they all look the same?

 

Correct, one end goes nowhere effectively the other to the sensor on the tachometer.
The spark creates a pulse in the wire and the tachometer senses it. Hence two HT leads.

 

I'll give it a bash. What do I do with the other terminal?

 

That's the one thing that isn't on the sticker... I didn't worry because I'd read on the net you can just switch a couple of wires inside to make it -ve earth if needed.

Here's what happened when I experimented with the wiring yesterday.
Connect either terminal to the coil -ve, then the other to either 12v or gnd and it works just the same measuring half revs. Earthing the case makes no difference.
Points or electronic ignition makes no difference.
Wire lengths make no difference.

I don't think it's a current sensing one as these have somewhere to loop the wire on the outside of the tach, or more terminals so the loop is inbuilt. I took it apart and there is no obvious loop inside as I've seen on pics after googling.

I have read just one article that stated two terminal tachs are generally coil -ve and earth and doesn't matter which way round hence the terminals aren't marked which seems to describe mine.

So I'm with the view that it wants 2x as many signals and must have come from a 2-stroke or summut. I know nowt about 2CV's but the tach is French so that adds up. Didn't think a 2CV would merit a tach at all as they seem to be very basic cars but what do I know!

 

I had the same problem with my 205. I put a 306 gti-6 engine in it and the rev counter only showed half the revs. I brought a little converter box that double the signal and bobs your uncle.

 

That looks like it's out of a Citroën GS.

 

Tell me more...

 

Eh? English please...

 

No

Nothing digital going on here but I take your point. Halve the resistance at the appropriate point and the double power will move the needle twice as far. I like.