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MG MGB Technical - Tachometer calibration


I have removed the tachometer from my MGB and wish to calibrate it... does anyone know what peak to peak voltage of the signal input needs to be?

Thanks in advance,
R Sims

Rick, it can't be any higher than 12v. Depends on which instrument you have, the early current loop type or the later pulse type, and if the car has a ballast resistor in the system I'd guess the input would be around 8v, same as the voltage at the points.
Bill Young

I couldn't get any needle movement after connecting the tach to the power supply and signal generator. I set the generator to 100Hz and checked with an oscilloscope and the P2P voltage being produced by the generator was only 4V. The input voltage to the terminals was set to 12V although no current was appearing to be drawn from the supply when the tach was powered up.

My tack is an RVC type... am I correct in thinking the banana-plug style prong on the back of the tach is the signal input, with the one next to it being the positive terminal and the prong on the casing being ground?

R Sims

Rick - The input (through the two bullet connectors) is a current loop thrugh a transformer. The wires connecting to it are the wires from the ignition switch to the coil. It would take the full 12 volts at whatever current the coil pulls to trigger the tach. When using a signal generator, I always just connected to the point where the secondary of transformer connects to and then ran the level of the signal up until the tach started reading. I find it much easier to calibrate the tach by comparing the reading on it to that on a diagnostic tach connected to the coil. The tachs in our cars are not very linier, so using a signal generator or audio oscillator just isn't worth the trouble. I just set the car's tach to match the diagnostic tach at 3000 RPM and call it good. Cheers - Dave
David DuBois

With the later ballasted system the voltage signal to the tach is still 0-12v as it is connected to the negative terminal of the coil. It is the positive terminal of the coil which switches between 12v and 6v i.e only sees a 6v differential.

But the voltage triggered tach was used from 1973, when the 12v ignition system was still in use, the ballasted system didn't come in until rubber bumpers during 1974.

The earlier tach used current pulses through the coil to trigger the tach.

Both types can be calibrated from a typical battery charger. These usually don't put out a steady 12v but instead output voltage pulses at mains frequency, i.e. (in he UK) 100 pulses per second if a full-wave rectifier is being used or 50 pulses per second if a half-wave is being used (you won't know which until you try it, or connect it to an oscilliscope).

For a current pulse tach you will need a steady 12v supply to the tach electronics i.e. the car battery, and a coil or equivalent resistance (3 ohms) in series with the tach pickup and charger leads.

For a voltage pulse tach you will need the same steady 12v supply to the electronics, but should just be able to connect the charger +ve to the tach pickup terminal with the charger -ve connected to the tach case/battery negative.

A full-wave rectifier i.e. 100 pulses per second should indicate 3000 rpm, a half-wave rectifier i.e. 50 pulses per second should indicate 1500 rpm. I think, as I haven't tried it myself.
Paul Hunt 2

This thread was discussed between 11/04/2007 and 13/04/2007

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