Classic Car Instruments

A guide to tachometer, ammeter, speedometer & more

classic car instruments

Classic Car Electrical Instruments

Most classic cars have far fewer instruments than modern cars and in particular fewer designed for driver convenience. What instrumentation they have being far more functional in nature.

At the time most classic cards were built the only legal requirement for instrumentation on a car was the speedometer. The remainder being solely for the convenience of the driver. Most panels have a fuel gauge, a water-temperature gauge and various coloured warning lights. Larger, more expensive cars, and those driven by enthusiasts, have a number of additional instruments to tell the driver more about the efficiency of the engine.


The usual speedometer has a round dial, clearly marked with numbers in tens. Road speed is indicated by a pointer needle. Another type has a coloured band operated by a revolving drum and moving along a horizontal scale. Some designs of this type have different colours in the band so that the driver, without reading his exact speed, can more quickly tell whether he is below 30 mph, between 30 and 60 mph, or above 60 mph.

Mileage recorder

The speedometer usually incorporates an odometer, a mileage recorder which shows the distance covered. It is geared to the speedometer drive and its reading is shown in an inset in the speedometer dial. Some speedometers give two odometer readings, one showing the total mileage and the other the current journey mileage.

Tachometer or Rev Counter

A tachometer, also referred to as a revolution counter, is fitted to many cars, especially sports, fast touring and racing cars. It is operated either electronically or mechanically and. shows the speed at which the engine crankshaft is rotating in revolutions (revs) per minute. The dial is usually marked in units of ten and the reading must be multiplied by 100 to give the rpm.

Water-temperature gauge

The temperature gauge and the fuel gauge commonly found in the modern car work on similar principles. They have different dials, but both usually incorporate a bi-metal strip and a needle. When current flows through a coil wound around it, the strip is heated and bends because its metals expand at differ-ent rates. As the strip bends, the needle linked to it moves over the scale.

The amount of current, and so the temperature of the strip, is governed by a sensor unit. In the temperature gauge the sensor unit
is a thermosistor, an electrical resistance that is sensitive to heat, set into the water jacket. The hotter the water gets, the more current it allows to pass through.

Fuel gauge

The sensor unit for the fuel gauge is a rheo-stat, or variable resistance, mounted on the petrol tank. A float which rises or falls with the fuel level moves a lever across the variable resistance, changing the amount of current passing through the gauge. The higher the fuel level, the lower is the resistance, the greater the current and the higher the reading. The time the heat takes to affect the bi-metal strip is the factor governing the comparatively slow swing of the needle to its reading when the igni-tion is first switched on.

Oil-pressure gauge

The oil-pressure gauge shows the pressure of oil in the engine's lubrication system. A reading noticeably lower than normal may indicate wear in the main bearings or big-end bearings ; wear implies wider clear-ances, whith result in lower pressure. Erratic variations in the reading, or a fall from normal when cornering, indicate that the level in the sump is so low that oil is surging away from the point where it is picked up by the oil pump.


On some cars the ignition warning light is supplemented by an ammeter, to show the rate at which the battery is being charged or discharged. When the electrical system is working perfectly and the battery is in a reasonable state of charge, the ammeter should show a fairly high reading for the first few minutes while the current used by the starter motor is being replaced in the battery. The reading will then drop back to indicate a low trickle charge.

Battery-condition indicator

Whereas the ammeter measures the rate of flow of electrical current, the battery-condition indicator registers the voltage or electrical charge of the battery. The indicator is wired so that when the ignition is switched on, it records the voltage between the two battery terminals, and hence gives an indication whether there is enough stored energy to allow prolonged parking with lights and a subsequent restart. If after a few miles' motoring the battery indicator still shows a low state of charge, have the battery checked.

Troubleshooting Classic Car Electrical Problems

The single most useful thing when troubleshooting electrical problems and instruments is to have a comprehensive wiring diagram. We have a range of classic car wiring diagrams on the site available for free download.

You might also find our post useful on how to read classic car wiring diagrams

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