All About Tachometers

A modern electronic tachometer uses a signal from the engine to generate a readout of engine RPM (revolutions per minute).  The readout may be a moving pointer on a calibrated scale or some digital electronic display.

Historical development

In the beginning, tachometers were strictly mechanical. The first photo shows a Jones Portable (centrifugal) Tachometer
I have that probably dates from WWII. When the drive shaft rotates, a set of weights is rotated through a gear train.  As RPM increases, the weights fly further apart by centrifugal force and move a collar on their common shaft.  The collar is linked to a readout dial.  The gear train affords three different ranges: 50-500, 500-5000 and 5000 to 50,000 RPM.  The rotating weight mechanism recalls the flying ball governors on steam engines.  This is a fairly delicate, expensive device - something that would have been limited to calibrating the mechanical tachometers on the engines themselves or setting mechanical governers.  The oiled cloth cover of the Jones tachometer case still smells like war-surplus from the 1940's to me.  I have checked this tachometer against modern electronic reference tachometers and it is still accurate within 5%.

A tachometer is more generally useful at the engine control panel than at the engine itself. A flexible spring-steel spirally wound cable was used to mechanically transfer the rotation of the engine to the tachometer.  If you are old enough you might remember automotive speedometer cables.  Same thing.  In the old mechanical speedometer, a flexible rotating cable with it's ends ground square started in the hub of a front wheel and ended in the speedometer body.  Tachometers tended to run off the end of an engine camshaft.  This was physically convenient place to attach a cable and, as the camshaft of a four-stroke engine rotates at half the crankshaft speed, less prone to wear.  Even a two-cycle diesel, which lacks a cylinder valve camshaft, has other coupled half-speed shafts to drive the injection pump, water pumps or the blower. The rotating cable was enclosed in a larger, flexible spirally-wound shell.  Lubricating the cable was a challenge. Grease packing at the factory had to be periodically renewed through preventive maintenance.  There were aso practical lengths beyond which a rotating cable could transmit a small amount of power against friction between the cable and the shell. 

The mechanical tachometer readout itself might consist of a magnet rotated by the cable which transmitted force to a closely positioned aluminum disc  The disc was connected directly to the readout pointer. A spring biased the pointer back to zero. Eddy currents were induced into the disc by the rotating magnet.  These eddy currents opposed the magnet polarity with a force proportional to the rotation rate. The faster the magnet rotated on the end of the cable, the more force transmitted to the disc and the further the pointer moved up the scale against the return spring.   There was very little to wear in this mechanism compared to the flying ball designs.

Electric Tachometers

Problems with mechanical cable wear, and probably cost, led to the development of the electric tachometer.  A very short shaft coupled the camshaft to a small electric permanent magnet alternator.  The shaft was shaped like the end of a tachometer cable where it entered the engine.  The sender screwed on the same threads as the mechanical cable housing.  Everything was therefore backward and forward compatible. The sender generated an alternating current whose power (and frequency) was proportional to engine speed.  The sender was connected by copper wires to a sensitive meter.  The faster the sender turned, the more power transferred to the meter and the higher the pointer moved against it's dial.  Note that, although the frequency of the alternating current signal is also proportional to rotation, the electric tachometer only used the alternating current's power.

Some mechanics are still biased against electrical or electronic instruments.  Mechanical thermometers and pressure gauges as well as tachometers are "calibrated" by their design tolerances in manufacturing.  They always tell the truth, or so we believe.  Poor connections and calibration differences between senders and gauges can make electric gauges lie.  A mechanical gauge is physically coupled to and an integral part of it's sensing apparatus.  Mechanical pressure gauges have tiny capillary tubes which transmit pressure at the engine directly to the gauge.  The difference between "electric" and "electronic" is the presence of electronic components such as vacuum tubes or transistors.

Electronic Tachometers

The development of the transistor in the mid-1950's led to the electronic tachometer.  An electronic tachometer overcomes several issues with electric tachometers.  The electric tacho "meter" must be fairly sensitive thus somewhat delecate and expensive.  The calibration of an electric tachometer is variable.  The tight mechanical lock provided by a rotating cable is not available.  Hence an electic tach can be either calibrated more accurately or drift out of calibration just as well.  The electronic tachometer measures frequency.  An alternating current comes from the engine at some frequency proportional to RPM.  An easy source is the familiar tachometer sender, this time counting cycles instead of measuring power. Tachometers also operated from magnetic pickups that put out a pulse each time a gear tooth rotated past.  The problem here is that different engines have different gears with 30, 113, 126, 132, 136 or 159 teeth.  So a mag pickup tach tends to operate on a significantly higher frequency signal than tachometer sender. 400 Hz might correspond to 3000 RPM on a signaflex mag sender and 182 RPM on just one of the several possible mag pickup ratios.

Electronic tachometers based on vacuum tubes probably existed, I just never saw any.  They would have been far too expensive and unreliable for mass-produced automobiles.  These are two themes of the tachometer story: reduce manufactured cost and increase reliability. I do have a tube-based electronic stroboscope, probably from the late 1950s, which measures RPM very accurately if a bit ambigously. Stroboscopes are very bright, fast calibrated flashing lights.  Strobes are used to "freeze" repetitive actions by illuminating them at the same point in every cycle.  A rotating fan appears to stand still.  The ambiguity in the use of a stroboscope for measuring RPM is that it is very hard to tell whether something is being made visible every revolution or every other revolution.  It's pretty easy to lock on to some multiple or sub-multiple of the rotation rate.

The magnetic sender or magnetic pickup both have a cost penalty.  The sender or pickup is specifically there to measure RPM, nothing else.  With the switch from generators to keep the starting battery charged in the 1940's to alternators by the 1960's, another source of tachometer signal became available.  An alternator produces alternating current at a frequency proportional to engine speed.  You normally don't think about this as the AC is immediately rectified to DC within a modern alternator.  The term "alternator" came into vogue when the diodes to rectify the power were still physically seperated from the alternator. The name stuck even though power has come out of the "alternator" as direct current for 50 years.  Alternators are almost universally belt driven.  The speed of an alternator is porportional to pully ratio and number of alternator poles as well as engine speed.  Still, the electronic tachometer can be set to count pulses of AC any way it needs to.  So, with a tachometer signal available for the insignificant cost of adding a small tab terminal to the alternator, tachometer senders and mag pickups started to disappear.  As I write this in 2011, if you want an electronic tachometer to operate from a mag pickup or a mag sender, it's a special order.  Alternator driven tachometers have taken over.

Tachometer Interface Issues

If you have replaced a tachometer in the last ten years you will have learned that the process is not necessarily simple.  Getting the tachometer to read at all may be a chore if you have a magnetic sender or a magnetic pickup.  Usually you can pick a signal off the alternator and bypass that whole issue.  But getting the tachometer to read the right number given dozens of possible relatiuonships between RPM and alternator frequency is a challenge.  Virtually every electronic tachometer has some complicated, mechanically flimsy way to set the ratio.  First, remember that there are no "marine" tachometers.  The market only mass-produces one tachometer for bulldozers, passenger busses or powerboats.  The tachometer, like so many other things, has been "value engineered" to be inexpensive and as good as it needs to be but no better.  The switches and potentiometers to set up the tachometer only do the job because they get adjusted once, at installation.  Some tachometer suppliers use tiny "dip switches" to set the ratio.  Dip switches have been abandoned in most applications because they are notoriously noisy and unreliable.  If you can correctly read the manufacturer's chart that links pully ratio and alternator poles to dip switch settings you are a more patient person than me.  But I digress.  Lets attack this systematically.  First, lets get the dip switch reliability issue out of the way.  It's true, dip switches are bad, but there is a trick the manufacturer employs to get past this. 

in process of being completed, 8/17/11 DS