by Chris Barker
originally reprinted with permission from THE ALPINE HORN, United Kingdom in 8/85

If things have worked out as Paul Norton and I intended this article should accompany one by him on the intimate details on electronic revcounters. He will tell you all about resistors and thermistors. I am going to show you how to check whether they are all telling the truth.

It’s human nature to believe implicitly what instruments say (except perhaps the oil pressure gauge), and I have met owners afraid to go above 70 mph because “the revs are on the limit”. Unless you’re still in 2nd or someone has fitted a 9: 1 axle this isn’t possible, and it is more likely that the speedometer is reading slow or the rev counter is fast. If you still have your license it’s probably the latter!

There are only two problems in checking a rev counter. The first is giving it some revs (pulses) to count; the second is to know how many. I had vaguely thought about using a thing called a signal generator, but these are not found in many peoples’ toolboxes and I am not sure whether one would work anyway.

No matter, a much easier way is to setup a real ignition system on the bench. Find an old coil and distributor, any will do but there is less math involved if it’s for a 4-cylinder engine. The only other things needed are an electric drill, an electric light and some way of cutting out sunlight. One easy way is to do it at night.

In case you have not already guessed, the drill is to spin the distributor. A variable speed drill is best, a two speed one will do and you can do something with just one speed. A piece of wood held against the side of chuck will give a fine speed adjustment. To join the drill to the distributor find a 4″ metal tube as big as will fit in the chuck. Cut a slot in the other end and spread this end enough to fit round the distributor drive. It should not be a tight fit. You now have to clamp everything together. I found a Black &Decker Workmate ideal. You should now have some revs to count.

The next step is to remove your rev counter. The earth connection is on one of the knurled fixing nuts and the live feed is on the only spade connector. The bulb holder pulls out completely and this only leaves the sensing coil wound through a nylon block on the back of the instrument. Make a careful note of how it’s fitted and remove the small knurled nut and steel bridge piece. The nylon block is part of the loom and stays in the car.

Wire up the system per your car wiring diagram. Arrange a turn of wire (without a nylon block) correctly under the steel bridge piece. All that is left now is to count the revs you are putting in and this is the clever bit.

You will need a patterned disc on the rotor arm. This is cut from a white card and is about 75mm (3 inches) diameter. You can mark a ring on it with four equally spaced black segments. It’s attached to the rotor arm with a dab of glue. When seen with fluorescent light powered by AC mains each ring will appear “stopped” at a particular speed. In this country, with 50 Hz mains the formula is: “stopped” speed = 12000/number of segments so rings with 3, 4, 6, and 12 segments will give you 4000, 3000, 2000, and 1000 rpm. For 60 Hz mains multiply these speeds by 1.2. (Editors note: For 60Hz, this will give you 4800, 3600, 2400 and 1200 rpm.)

If you have only a fixed speed drill choose the number of marks so that the “stopped” speed is just a little lower that twice your Drill’s speed e.g. for a 1300 rpm drill mark 5 segments and calibrate at 2400 rpm by applying a slight brake to the drill chuck.

I found that I only needed to alter the tachometer’s electrical adjuster to get an accurate instrument. It is best not to touch all those fine balance springs if possible. On the SV unit the adjuster is inside the back, between the bulb position and the nearest threaded stud. It’s black plastic, about 1/2″ in diameter. This may all sound a bit hairy but in fact it’s quite simple and very accurate so have a go.

The strobe method also has potential for checking cable driven speedometers and rev counters but you would need either a reversing drill or a means of reversing the direction of drive. One way would be to have a friction wheel on your drill driving a second wheel that carries the strobe disc and drives the cable the right way. For a speedometer the turns per mile figure on the face would also be the rpm for 60 mph and “stopped” rpm = 6000/# (7200/# in US) of segments.

Finally, to those curious enough to check my formulae I do not know why the disc only appears “stopped” at twice the speed one would expect but it does. I have allowed for the distributor only turning at half the engine speed so that is not the answer.

Even more finally, I haven’t tried it but I suspect that touching the H.T. coil terminal would not be pleasant so keep your fingers away when running!

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