TEMPERATURE GAUGE EVERYTHING YOU NEED TO KNOW

Why it’s such a heated topic for classic owners

The temperature gauge perhaps ranks second only in importance to the oil pressure gauge in the classic car driver’s arsenal of disaster avoidance weapons. Glances at the temperature gauge are an essential ingredient in the rich gateau that is a spin in an ancient car.

Early gauges often took the form of a Bourdon tube dial sitting proudly atop the radiator cap, viewed along the bonnet from the driver’s seat. This solution was fine if the radiator was kept well topped-up, as a lack of coolant could lead to a false reading.

Not everyone was enamoured by this solution and pressurised cooling systems led to radiators and their caps being hidden away, often behind a false grille and surround. Dashmounted temperature gauges thus became the norm.

Earlier types were operated by a liquid-filled bulb and capillary tube to the gauge’s internal Bourdon tube-operated needle. The bulb was typically screwed into the engine’s cylinder head, or sometimes into the radiator header tank.

The liquid within the bulb, often alcohol, was characterised by a high coefficient of thermal expansion, thus its capacity to operate the gauge. As it heats, it expands directly up the tube, operating the gauge.

Electrically-operated devices worked by means of a bi-metallic strip-operated gauge, acting in much the same fashion as a voltmeter.

The temperature of the coolant was detected by the sender unit, a heat-operated variable resistor, again mounted in the cylinder head.

When the vehicle’s ignition was switched on, battery power was sent through the gauge’s bi-metallic device. Meanwhile, in the engine bay, the cold sender’s high electrical resistance meant little current flow through the gauge, keeping the bimetallic device cool and the attached needle toward the ‘cold’ position.

As the engine warmed up, so the sender’s resistance reduced, allowing a greater current to flow through the bi-metallic device and so warm it up. This moved the mechanically-linked needle towards a warmer reading.

Less well-appointed vehicles had to make do with a simple temperature warning light. This was in a circuit supplied by the car’s battery, within which was a thermally-operated switch, located in much the same position as previous senders.

If the engine coolant reached too high a temperature, at a set point the thermal switch would complete the circuit, illuminating the warning light in the process. This was an effective means of notifying the driver of an occurred event, but was something of a blunt tool, as the use of a gauge offered greater insight into the cooling system’s situation.

For as long as cars continue to require coolant, the temperature gauge will remain an important monitoring device for drivers.

’Less well-appointed cars had to make do with a simple warning light’