11 replies to this topic

[zebidee]

Hi all.

Currently fitting a nice old smiths rev counter to the mini. Its a 1987, so just three wires, switched live, coil and earth. Fitted it but it didnt move at all. Its an RVC, so should work, and has a good earth. When i checked the wires with a voltmeter, the light green/orange which should be the switched live had no voltage (this was with the ignition on !) , and the white/black from the coil had 12v. Im guessing the switched live has a dodgy connection somewhere, but should the wire from the coil have 12v with the engine not running??

[MiniMick]

You did wire it to the 12v - terminal and not the 12v + terminal didn't you?

[Bungle]

yes the coil will have 12v with the engine not running

thats why you should not leave your ignition on II as you can burn out the coil

where did you take the power for the switched live from ?

(is it a rev counter from a 4 cylinder engine ?)

[zebidee]

Cheers. I took it from the original wire meant for the rev counter which was hiding behind the old dash (this is the light green/orange which is showed in the haynes). Yep, its for a 4 cylinder, came from an MG. I'll trace the wire back and see where its gone wrong

[morley]

[GreaseMonkey]

Are you running electronic ignition? As RVC is used with electronic ignition and RVI with the points system

(Just checking as this may not be the actual factor causing your problem but if you are running a points system it could cause problems later on unless RVC does work with electronic ignition as well as points?)

I dont think RVC can work with points as well as electronic igition, can anyone clear this up?

Thanks

[dklawson]

RVC will work with either points or electronic ignitions. RVI will not work with electronic ignitions (most of the time... though there are rare exceptions).

Yes... the white/black wire will have +12V on it, but ONLY when the points are open if you have wired it correctly. The white/black wire should be on the coil (-) terminal (the terminal with the wire going off to the distributor). When the points close (or electronic module is conducting) the coil (-) terminal will be at earth potential (zero volts). The biggest concern you have should be finding why you do not have power on the green wire. If you can't figure out why, run a new power wire to supply the tachometer. Also make sure you have a good earth from the MG tach's case to chassis ground.

[zebidee]

Hehe, yep, and its why i have the same rev counter in an RVI and RVC (if anyone would like a Smiths RVI tacho from a 70's MG?? )! Can i splice into the feed for the no-charge warning light for the switched live or would this cause problems??

[dklawson]

Can i splice into the feed for the no-charge warning light for the switched live or would this cause problems??

Well... you should be able to splice into the white wire for the warning light since that is on the "high" side of the warning lamp. On early cars like mine, that's an unfused circuit, I don't know about a car as late as yours. If it's not fused, or if you can't determine if it's fused, put an inline fuse in the supply line to the tach.

Regarding the RVI tach... the first rule of owning an old British car (over here at least) is that you throw nothing (absolutely nothing) away. Someone will want your RVI tach either complete or for parts.

[Goopster]

I was wondering if I could hijack this thread a little and ask a (connected) question?

I have a period Smiths 'Pod' type rev counter from the 1970's that I eventually want to fit to my car - my Mini already has an aftermarket tacho fitted, and it has ONE wire going to the coil. My period Smiths tacho has TWO white wires, just like the diagram that was kindly attached to this thread. I don't know if I'm being dense, but I don't understand what you do with the second white wire - I can see one of them is attached to the coil, but what is the other one attached to? On the picture it just seems to be stuck on the side of the distributor - and also, what does that dotted line signify?

Thanks in advance for any help you could give me!

Goops

[dklawson]

The Smiths tachs with two white wires are the RVI type. My first comment is that if your car has electronic ignition (factory or aftermarket) don't bother trying to fit the period tachometer as it isn't likely to work.

If your car's distributor still has points refer back to the magenta (Smiths) section of the JPG posted above. Note that as you say there is a white wire connected to the coil, there is also the white wire connected to the distributor. That black dashed line is probably what's confusing you. Normally, that's the white/black wire running between the coil and distributor. It's shown dashed above... because you remove it. With the white/black wire removed the coil current flows through the Smiths tach, then through the points to earth.

The wiring was a bit different for cars that had RVI tachs fitted as standard. One tach white wire would come from the ignition switch and go to the tach. The second white wire would go from the tach to high-side of the coil. Thus, only one wire had to pass through the firewall. The white/black wire between the dizzy and coil was retained in those installations. The wiring shown in the JPG above is the simplest aftermarket installation method for most people.

Regardless, if you fit the period tach and it doesn't work as expected there are two things you can try. First, swap the connection points for the two white wires to reverse the direction of current flow through the tach. Second (if the first doesn't help) take the tach out of its cup and see if there is an external loop of wire as shown in the JPG above. IF there is, see if you can make a second loop under the retaining clamp. Sometimes that helps. If you still have no luck, Google for some of the many articles on the net that talk about how to replace components inside old RVI tachs. Remember that this is a piece of almost 40 year old electronics. Some components do fail just due to age.

[Goopster]

That's absolutely fantastic Doug, thanks so much for your detailed reply. Now it makes perfect sense to me

Your information and help is much appreciated.

Best Wishes, Mark