MK1 tachometer wiring

[Jacko1998]

Hi, my engine is currently out of the car and put in a test frame so I can run it up to test it out so obviously not using any of the car loom only what I wired up for the basic running function.
i have the Lumenition set up with a 12v 3ohm unballasted coil.
I want to connect my tacho from the car and put it in the test frame to check the revs and see if it works.
Im assuming it will work with Lumenition and 12v coil.
so terminal 1 12v ignition supply (via the voltage regulator)
terminal 2 earth
terminal 3 - should this be 12v supply direct?
terminal 4 - coil what side?
Many thanks

[richardthestag]

Im assuming it will work with Lumenition and 12v coil.

did it work before everything was removed and with luminition and 12v coil? if so then it may have already been converted. If not then you can connect it up and get a very jerky needle movement, if anything. Similar issues with 1970s Rover p6 that I was working with. changed the gauge to a 1973 and the dial was the same. then have to modify the wiring behind the dash

Others will be along shortly to advise whether you need mk2 tacho internals and dress them with mk1 dial etc etc etc

[richardthestag]

Dug out some more detail on the Rover p6 I fixed. The original Tacho, wired in a similar fashion to the mk1 stag was labelled up on that face as RV1.

It is well known that this type (RV1) is less keen on working with distributors retrofitted with electronic ignition of any kind.

The trick with the p6 is to source a tachometer with RVC stamped on the face and the adapt the wiring, however this is less easy with stag as the face of the instrument is completely different between RV1 and RVC instruments.

A quick google found this https://youtu.be/VzToKEeCoWc?feature=shared may be useful to modify an original RV1 tacho to RVC spec?

There are a fair few mk1 stags around and probably most have electronic ignition of some kind, what have the owners done?

[Philip Wardle]

There's a SOC Magazine article which may be of interest (pdf below), the article gives a reference to this website www.clocks4classics.com which is still alive and kicking. No connection or experience myself.

[marshman]

Hi, my engine is currently out of the car and put in a test frame so I can run it up to test it out so obviously not using any of the car loom only what I wired up for the basic running function.
i have the Lumenition set up with a 12v 3ohm unballasted coil.
I want to connect my tacho from the car and put it in the test frame to check the revs and see if it works.
Im assuming it will work with Lumenition and 12v coil.
so terminal 1 12v ignition supply (via the voltage regulator)
terminal 2 earth
terminal 3 - should this be 12v supply direct?
terminal 4 - coil what side?
Many thanks

Terminal 4 goes to the positive/supply side of the coil - opposite terminal to the wire that goes to the distributor/lumenition module.

Essentially the RVI tacho sits in series with the supply to the coil. The coil current passes through a transformer in the tacho which has electronics to detect the pulse of current that occurs every time the points close (or the electronic ignition module switches).

The late RVC type has a single wire which attaches to the coil and detects the voltage pulse which occurs everytime the points/ignition switches.

As RTS says the earlier RVI type struggle with electronic ignition as the switching is usually much faster and giving a different current pulse characteristic that the electronics were never designed to work with. Couple that with 50 year old components that have deteriorated (mainly the filter capacitors) and you get a rev counter that may not work correctly. You can "renovate" the electronics - fit new capacitors and tweak the design to get it to work - but it is a lot of work and quite hit and miss OR you can get a conversion PCB like the one linked to by Philip Wardle or as produced by a company called Spyder : (https://spiyda.com/tachometer-electronics.html). OR get a later MK2 RVC type rev counter and swap the innards.

I eventually fitted the Spyder conversion to my MK1 Stag RVI rev counter - easy to do and it worked well. You end up with just a single wire to the coil as it effectively converts an RVI type to an RVC. Easy to calibrate as well.

[Jacko1998]

did it work before everything was removed and with luminition and 12v coil? if so then it may have already been converted. If not then you can connect it up and get a very jerky needle movement, if anything. Similar issues with 1970s Rover p6 that I was working with. changed the gauge to a 1973 and the dial was the same. then have to modify the wiring behind the dash

Others will be along shortly to advise whether you need mk2 tacho internals and dress them with mk1 dial etc etc etc

thanks for the reply, the engine came separate to the car so couldnt test it. but the reply below with a company that sells modern internals might be my answer.

[Jacko1998]

There's a SOC Magazine article which may be of interest (pdf below), the article gives a reference to this website www.clocks4classics.com which is still alive and kicking. No connection or experience myself.

wow this looks like a very good product, i guess i need the Unitacho kit for lumenition.

[marshman]

Try what you have first, you may be lucky and it works. If it is erratic then get either the clocks4classic or the spyder kit. I fitted the Spyder kit and tested/calibrated it "on the bench" using the instructions supplied. No experience of the other kit.