MGB & GT Forum

Hi All,

I wanted to see if my Tach was bad so I bought an automotive multimeter that had the RPM
readout feature.
The instructions had me hook up the hot lead to the minus side of the coil
and it did work properly, and showed my tach is in fact not reading high enough.

However the Tach in the car is wired to the positive side of the coil as per the wiring diagram.
Can anyone shed some light on how this works?

Thanks

The tach in your car is current sensing. The newer aftermarket tach (and post 1972(?)) MGB tachs are voltage sensing.

Your MGB tach is wired in series with the coil. That is, power from the battery passes through the tach and on to the positive side of the coil. The tach can "view" the current pulses that occur whenever the points open/close. When the points close, there is a surge of current. When the points open, the current flow stops.

The newer tach simply monitors the voltage at the points (i.e., the negative side of the coil). When the points close, the voltage is zero. When the points open, the voltage is +12v. (I'm ignoring all the noise that is actually on the line for purposes of simplicity).

In terms of adjusting your tach, there should be a hole in the back of the tach that provides access to an adjustment potentiometer. You might try tweaking that to see if you can bring it into calibration.

In my experience, it is not unusual for the older RVI tachs to read a bit high at idle but be accurate above idle.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Thanks Terry mine is reading OK at idle but doesn't want to go higher than ~3500

The tacs have several electrolytic caps in them that age and will cause the tach to be off, sometimes buy quite a bit.

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Cogito ergo sum periculoso

Here is a picture of where to adjust on mine i needed to drill a hole so I could adjust with the tach assembled. Mine was way too high.

Thanks all

Is it possible that there is something physically binding at the 3500 RPM mark? I doubt you will be able to adjust around this problem. And definitely, capacitors do age and may need to be renewed.

Here is the schematic for the RVI tach. I *think* it should correspond to yours - it matches my 1971 tach.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

No it isn't binding cause sometimes it goes over 3500, but the engine sounds like 5000RPM,
so I don't push it any further. And the meter I bought is displaying the high rpm's.
Time for a shipment to Nisonger I believe. Thanks for all the info!