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MG MGB Technical - Bench Test a Rev Counter

Is there a way to bench check a rev counter using a 12V battery (or lower voltage)? It's from a '68 and has 4 connections: One insulated and one uninsulated bullet connector, and one spade lug for earth and one other spade lug.



Rev counters "count" the number of pulses (from 0 to 12 volts in the case of a car)of electricity over a given time. Simply connecting 12V to the unit will only energise the the timing and control circuits. It needs to have a pulse input such as the wire from the dizzy to the coil which pulses according to the speed of the engine. The only way to test it is to either use a pulse generator or hook it up to the car. Make sure you know which wires are the 12 volt supply and earth and which wires are the pulse sensing wires.

Tony OIiver

I made an electronic tach a long time ago when transistors were just becoming coming avaliable.

The sensing circuit provided a standard output for each pulse, so the output current was proportional to the rate of the pulses, but not their duration or intensity.

Calibration was carried out using the output from an unsmoothed full wave rectifier - as found in your common or garden cheap battery charger.(But probably not the newer types with voltage sensing and charging control)

For a 50 cycle supply output is 100 Hertz ( 100 pulses a second)

On a 4 cylinder engine which has 2 firing strokes per revolution:

(100/2) * 60 = 3000 rpm.

For a 60 cycle supply, output is 120 Hertz ( 120 pulses per second)

For the same 4 cylinder engine:

(120/2) + 60 = 3600 rpm

Ian F
Ian Fraser

Tom's tachometer sounds like the earlier current-sensing type. If connecting this to the output of a conventional charger (constant half-wave or full-wave rectified output) you will need an additional load such as a 12v bulb in series with the charger and the two bullets. You will also need to experiment with connecting the charger to the tach in each of the two possible directions as it will only work in one of them. For the later voltage type tachs with only three connections the -ve output of the charger will need to be connected to the ground connection of the tach and the +ve to the third terminal. Both types will need a smoothed 12v supply, like from a battery, to the 12v and ground connections of the tach. Putting a diode in series with the charger output will convert a full-wave to a half-wave and so reduce the 100 pulses per second to 50 to give another point of reference.
Paul Hunt

I have a bench tach calibration system that I purchased on line several years ago from a fellow who worked at Intel. It connects to the serial port of a PC and has software that will generate pulses at any specified RPM. It can be set for 4, 6 or 8 cylinders. The interface box is handmade in a Radio Shack project box, but I've never looked inside to see what is there. Works great. He may still have or make some. Contact me offline if you want his info.
John Hubbard

FWIW I tried the battery charger method as described but for some reason beyond my IQ it gave a reading of about 2350rpm on a properly calibrated tacho instead of the perfectly logical 3000rpm.
Steve Postins

the tacho in my 1970 b gives a unsteady reading
at different needle positions
m r Sykes

I have found that the tachs in the MGBs, particularly the ones in the early cars are not linier. For that reason, I like to set it right on at a RPM that is close to what I normally drive at. I have found that the easiest way to do this is in the car with a good quality diagnostic tach that uses digital circuitry hooked up and inside the car so I can watch both tachs at the same time. I can then adjust the car's tach to match the diagnostic tach reading and I am good to go. Cheers - Dave
David DuBois

This guy has made a tach circuit with modern components

I use one and I am happy with it.

When setting it, I was surprised to see that my tach was perfectly linear between 1000 and 7000 rpm.
JM Thély

This thread was discussed between 30/12/2004 and 03/01/2005

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