Retrofit electronic analogue rev counter

[Paul Bennett]

B 143 WG
Positive Earth

I have often wondered what my engine ticks over at and have been looking around without success for an electronically driven rev counter / tachograph to position in my cubby hole. Also, at speed, my only idea of revs is that a standard 3.73:1 rear axle supposedly delivers circa 24 mph per 1,000 revs.

I wonder what the red line would have been for a new car, (not that I ever push it past 70 mph, which must be nearly 3,000 rpm?)

Size or make not critical, as it could be fitted at small or large end of aperture to suit. As long as it is black and white with a needle!

There are dozens available on line but I can't find one to suit a 6 cylinder positive earth car and I wouldn't think it possible to reverse its polarity?

I have only found 4 cylinder positive earth examples or same 2nd hand which may be from MGB's?

Any suggestions would be appreciated,

regards
Paul

[John Murch]

When I fitted one years ago on a special, I seem to recall that it had a black and a red wire to attach suitably to the power and a green(.?) wire which went to the distributor, so polarity wasn’t an issue. The one I had could be switched to deal with 4,6 or 8 cyl. motors. It may be that such units are no longer available.
Good luck!

Edited. I never got one to work with a magneto!

[John Murch]

Had a look on ebay, would this sort of thing work?
https://www.ebay.co.uk/itm/12V-2-52mm-LED-Digital-Car-Rev-Counter-Tacho-Tachometer-Pointer-Gauge-Meter-UK/153015432917?epid=12019145322&hash=item23a06e36d5:g:MXcAAOSwNxda8pKc

[Paul Bennett]

Thanks to you John, I have now found a slightly superior cheapo oriental version further down that page, which had instructions on display confirming suitable for either - or + earth just by transposing the red and black wires.

Had not thought it would be so easy.

regards
Paul

[Mark Taxis]

I have a Smiths gauge with the loop inductor this will work on + and - ve earth, you just have to re thread the pick up wire in a different direction through the pick up
_________________
Mark Taxis
Sunshine Coast
Queensland 4560

[Paul Bennett]

Looks like a Mk VI special, Mark; with a good instrument layout.

I had been looking at the Smith's rev counters, as I would have preferred an English-made one for a Bentley!

However, it was specifically advised on the dealer's website that they could not be used on positive earth.

Excuse my electronic ignorance, but what is a "loop inductor" and is there a Smith's model including it, (that I may have missed,) or is it something you have bought separately and specially installed? Is the "pick up" the wire that would go to the coil side of the distributor?

I mentioned what I had ordered to an auto electrician who thought that if I just transposed the red and black wires the instrument would try to go backwards!
(Or maybe he was joking?)

I just realised today that the distance from the cubby hole on the extreme right of dashboard is a long and difficult way to the distributor, and the coil is as far away as possible near the left of the radiator.

This is what happens when you get carried away buying something on the internet too late at night!

Thanks for your help.

[Mark Taxis]

Hi Paul
Yes, it is a Mk 6 special with a Harry Rose body, built by my father in the seventies.
This car was originally +ve earth, I converted it to – ve earth.
The Rev counter was a + ve unit, I adapted it to -ve earth this is the instructions that I followed. https://mgaguru.com/mgtech/electric/et202a.htm
Maybe that it is not possible to go the other way.
I have just had this gauge refurbished and it came back with a single pickup pulse wire, I think they hide the induction loop inside the casing.
There are Smith +ve earth gauges available here https://www.ebay.co.uk/sch/i.html?_nkw=positive+earth+tachometer&ul_noapp=true

_________________
Mark Taxis
Sunshine Coast
Queensland 4560

[Paul Bennett]

Thanks Mark.

I had looked at the + ve ones on e-bay first of all, but I think they are all for 4 cylinder MGB's, Lotus, etc. (unless anyone knows if they ever put a BMC 6 cylinder engine in a + ve earth version?)

I only remember that there was a later beefed-up version with the Rover V8 engine, (as also went in the Range Rover.)

[Paul Bennett]

Epilogue:

Oriental tachometer came yesterday; was jury-rigged but could not be made to give realistic readings.
Although lit up and read zero ignition on, read 2,000 rpm at tickover, (is maybe 500 in reality?)
Tested the 4/6/8 cylinder switch, (read 3,000 when experimentally switched to 8 cylinder.)
All neat multiples of pulses from the distributor!

Conclusion: not worth messing about with so gave it away.

Re. MGB positive earth tacho's, (my memory was prompted yesterday by a car auction entry,) they did make an MGC version in the late 60's with a 3 litre BMC 6 cylinder engine. If they were still positive earth cars maybe there is a source for their rev counters?

[Christopher Carnley]

Why not change the polarity of your auto R Type?

[Mark Taxis]

Changing to -ve earth is very simple and opens up a new world, but the jag tacho might be worth looking at
https://www.ebay.co.uk/itm/Jaguar-MK-2-E-type-reconditioned-Smiths-Rev-counter-with-electric-clock/382567259489?epid=20006847029&hash=item5912c91d61:g:c8AAAOSw8H1boAsl
_________________
Mark Taxis
Sunshine Coast
Queensland 4560