Lotus Elan and Lotus Elan Plus 2 Forums

I have a stock Elan S3 - positive earth. I accidentally wired the battery for negative earth. Car runs fine, however, no rev counter. Have I damaged the rev counter? If I simply reverse the battery poles and re-polarize the dynamo, should all be well? I have read conflicting opinions on this. Any changes required on the coil? Thank you.

The standard Smiths rev counter contains among other electronic components a zener diode used to stabalize the internal voltages. It is likely to have burned out with the reversed voltages.

A good opportunity to replace the innards with a Spiyda circuit board.
https://www.spiyda.com/tachometer-electronics.html

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If you intended to change your car to negative earth and it is running on negative earth, the instrument might possibly be undamaged and if you changeover the polarity inside the rev counter, it might still work. The details of that changeover were on this website somewhere.

However iff it was just a mistake and you reverted to positive earth and the rev counter doesn't indicate, that pretty much means the electronics are busted.

There's a good chance it is not damaged, however it won't work with the wrong polarity supply. The rev counters fitted to s1and 2 cars was an easy conversion - replace the single transistor with a modern npn type, replace any electrolytic capacitors and reverse any diodes as I recall. I don't recall a zener diode specifically but that might be used in the later types. You will need to recalibrate the rev counter though as I recall most behaved exactly the same. I must have done several dozen of these back in 1970s! The same basic circuit was found in mg midgets and b's and similar cars and was pretty robust. An advantage of using a modern transistor is it is less heat sensitive.

You really don't need to do all those changes. Inside the meter the positive and negative supply leads are easily swapped over, One goes to the power feed connector on the back and the other to the internal metalwork.

The changes you suggest would enhance the long term reliability, but if you were intending to do all them you might as well fit a modern circuit such as that from Spiyda. Which will then work better if you subsequently fit electronic ignition.

The earlier tachometers had two transistors not one.
Yes there is a zener, it just looks like any other diode.



https://www.hazelden.ca/austinhealey/RebuildingBJ8Tach/


The later tachometer had only a single transistor and no zener (personally I would think this made it less accurate).

http://www.mgb-stuff.org.uk/harvit.htm

I replaced the majority of my tacho internals, wasn't terribly hard and it runs very well now. I put a thread up on how I did it all

Chris, where would I find your thread on replacing tachometer internals? thanks

Yes, anyone with electronic skills can easily make up a more modern circuit using say a 555 chip, but alas many of the members on this forum don't seem to even understand basic electrics let alone electronics.

I bet several wish they had paid more attention during physics lessons in school.

Hi

With and bit of luck the tacho may have survived and in both circuits the current that will flow through the transistor
terminals is limited so you might be OK. However the tacho will need to have its internals +ve and -ve swapped over to work at all.

The dynamo will work fine and should re polarise OK when the engine if run.

Final check should be that you do not have a solid state voltage regulator for the fuel and temperature gauges. On cars this old the PO may have replaced these with a modern solid state unit. I make this comment based on my own
+2S I am not sure whether this is used on the earlier S1 and S2 Elans but worth a check

Hope this helps best of luck

Bob

The tacho is not fed from the 10v regulator only the slow-respond gauges are fed from the regulator.

The regulator is merely mounted on the tacho screws as a convenient earthing point.

CG901 wrote:Chris, where would I find your thread on replacing tachometer internals? thanks

Here you go:

lotus-electrical-f38/tacho-repair-t41110.html

I didn't touch the transistors or the actual needle driver.

Thanks to all. I now have a plan to evaluate the rev counter condition and return to positive earth.

To post a follow up - I reversed poles at the battery to the correct positive ground, and my rev counter now registers reasonably well. I do suspect that it is not accurate above 3k rpm however. It seems a bit slow to get above 4k. It is smooth, with no jumping at all.
I am wondering whether it can be calibrated to correct this, or if I did do damage. Possibly one of the other remedies suggested above should be effective.

There is an adjustable resistor (potentiometer type) inside the tachometer on the circuit board.
You will need to have the tacho connected up without it`s case and hanging out of the dashboard on it`s wiring,
You will also need an independent means of measuring revs, some strobe testers do this.
Then adjust for least worse situation. Mine responded well and was surprisingly near enough.
If you have not been in there already, then that is the time to replace the ageing capacitors. They can affect the calibration.
Best of luck
Eric in Burnley
S3SE DHC

Eric - many thanks. Scott

Remove the case and drill a hole in the case where the potentiomenter would be accessible. Put the case
back on. Now, it's easier to hold, or simply mount, the gauge and makes adjusting easier with a
screwdriver through the back of the case.