Rev Counter Connections

[111Robin]

I have searched this subject but as yet can't find anything that answers my query. My Plus 2 is negative earth, fitted with the Aldon Ignitor type electronic ignition. The rev counter was working when I got the car initially however the wiring was a bit of a mess so I removed any spurious looking bodged wires that I could not identify so I could start with a clean slate. Since then the rev counter has not worked so obviously I removed/disconnected something relative to its function. The sketch below shows the arrangement. There's a 12V feed from the ignition switch to the voltage regulator mounted on the instrument this is then connected across to the instrument. There is a red/green wire from the regulator that is to the fuel gauge. The other yellow wire connected to the instrument is just hanging in the passenger footwell. I tried connecting this to each side of the coil but nothing worked. I'm assuming this instrument has an internal loop as all I can see in other posts are ones with external loops. Hopefully this makes sense.

[mbell]

For it to work you need:
- 12v supply - This should be the green connection that is common with the voltage regulator
- Ground connection - This should be a spade connector on the rear of the tacho case. (There is also a onward ground connection via one of the mounting posts on the rear)
- Signal connection - In the standard setup you should have a separate white wire that runs from the ignition switch to the tacho and then to the coil + to power the coil. The tacho works by "counting" current pulses down this wire to the coil.

This white wire should either wrap around a post on the rear of the tacho on early cars or have two bullet connections (in & out).

If the tacho has been converted to RVC then the sense wiring will need to be different, normally these have a dedicated sense wire (white/black is standard colour for it) to the coil ground connection. It maybe the tacho has been converted and you have removed this sense wire by mistake.

I would make sure you appear to have the right connections, if it still doesn't work I'd remove it and try asses if it is standard tacho or modified.

[111Robin]

I assumed the sense wire is the yellow, currently not connected. I tried this on both coil posts but it still didn't function. I didn't previously disconnect anything from the rev counter, only the yellow that possibly went to the coil but I can't remember. There were so many odd wires in the engine bay I preferred to remove them and start from scratch

[mbell]

You need to check all the connection and assess what you have. If your not sure what your setup is there is very little chance anyone here can can tell you what is wrong.

You need to check you have a working 12v supply correctly connected, a working ground connection and some form of signal connection. If you don't have those you need to fix that, if you do and it doesn't work then you need to pull the tacho for further testing/investigation.

If there is an added yellow wire between the coil area and the tacho that would suggest it might have been converted to RVC and that added as sense wire.

How is the coil getting power? Are there any signs of the white wires behind the tacho? (Maybe joined together.)

[111Robin]

It does actually work when connected to coil "-", I just didn't notice as it's reading very low and barely registers at idle so it's about 1000rpm out. Not much use but at least I now know how to connect it.
It has been converted, there are no white wires and the coil is powered direct from the ignition switch, not via the tacho.

[mbell]

Cool, it may have a calibration adjustment somewhere.

If it is Spydia conversion it is a screw setup in one of the holes on the rear of the tacho case. So you can probably improve the situation via adjustment of that screw, but will need some kind of reference tacho to adjust it against. My experience is that you can only adjust then to be correct at one range of revs, so need to calibrate it at the rev range you care most about.

[111Robin]

Thanks, I'll take a look. One thing I thought may be an issue is that I used a very long wire to connect between the signal wire and coil, probably about 4 metres long (just used what was handy at the time, no other reason) . I wonder if that would have an effect on the function ?. I'll try a much shorter length to see if there's any change.

[mbell]

I doubt the wire length would make a difference, as very low current.

I happened to have a (poor) photo of the adjuster on my spydia conversion which may give you an idea where to look. It's the brass screw on the blue piece.

[111Robin]

Thanks I'll have a look.