MGB Register Forum

Hi,
I have a 1967 MGB GT which has been converted to Negative earth however the rev counter has never worked very well since I have had the car and is now not working at all.

I wanted to check if it is possible to simply replace the positive earth unit which has the loop on the outside for a negative earth unit which has the loop on the inside?

Thinking this will be easier then getting another positive earth unit and taking it apart to convert it.

Many thanks
David

David,
not a specific answer to your question, but a general point - rev counters are extremely simple devices, they simply work on the number of pulses from a spark plug (plus of course a source of power for lighting). So I wouldn't think that fitting any rev counter would be much of an issue. It may require a little rewiring, but that would be all.
No doubt someone will respond with a wiring diagram for you!
Ian F.

Hi David
If you have converted your car to negative earth, It will also be necessary to convert the Tacho to negative earth. This is easily done. Just twist off the bezel and release the screws on the rear securing the mechanism. Then just unsolder and swap over the two wires securing the earth and power feeds. There is also a small potentiometer that is used to calibrate the Tacho. This may need to be adjusted to set the calibration correctly. However If the Tacho has been connect the “wrong way round “ for any length of time , this could have damaged some of the polarity sensitive component such as the electrolytic caps. Easily and cheaply available. Once back together you may need to reverse the connections to the loop to ensure the correct direction of the magnetic field (basic Right hand rule)
If you want to fit later type tacho you will need to disconnect the bullet connectors to the loop ( Loop should be Brown /Black wire) and connect the remaining two white wires together . Then run a new wire (White/Black if you want to be correct) from the negative (CB) connection on your coil back up the head connection on the new tacho. Connect the existing power and earth to the new tacho and it should work.
I have a spare small diameter Tacho from an early rubber bumper car (V8 style tin dash).It would be very easy to transfer the guts of this to you housing and retain the original dial. You are welcome to this if it is of interest. Could bring to the MGB spring run if it helps.
Best regards
Mike Barclay

Hi David.
Some more thoughts for you to consider if you go down the road of changing the tacho completely.
As the positive earth tachos were fitted for 3 years (1964 to 67 - before that the tacho was mechanically driven off the camshaft), there are more Negative earth tachos around as they were fitted for 4 years (68 - 72) and a greater number of cars were produced per year than positive earth ones. They sell in good condition for around £50 at a show. Look for 'Negative Earth' and RVI on the dial. There is another type that is available for negative earth cars and that is labelled 'Negative Earth' and RVC. These were fitted to the chrome bumper MGBs (so they are the 4in diameter dial) in 1973/4 so they are rarer and people want a bit more (£60 - £65) for these. If you are thinking about fitting any electronic ignition system - even the ignitor type, then you need the RVC type. The RVI type will not give a steady reading with electronic ignitions - it jumps all over the place! It is a bit more involved to fit the RVC type but why not take the opportunity to future proof the car a little, after all it has already been changed for standard spec by fitting the alternator. The RVC type works with coil and points or electronic ignition systems.

For all about MGB tachos see here: http://www.mgb-stuff.org.uk/tach.htm
This also gives another, external method for reversing the polarity.