Monitoring the heat

Sensors have an important function in any engine.

On modern fuel-injected engines, where everything is controlled by an on-board computer, a host of sensors continuously feeds information to the control unit, much like our own senses of sight, touch, smell, taste and hearing continuously supply information to our brains.

Today, we focus on a sensor that has been fitted to cars for more than 60 years, namely the coolant temperature sensor, or temperature-sender unit as it used to be called.

Formerly, its purpose was to send information about the temperature of the engine coolant to the temperature gauge on the dashboard where it could be monitored by the driver. Nowadays, it still does that, but also sends information to the engine's electronic brain to be processed together with all the other inputs.

Newer engines have several temperature sensors. The humble temperature-sender unit is usually screwed into the cylinder head, close to the thermostat housing. It protrudes into a coolant passage and is thus in contact with the circulating coolant at the point where the latter is at its hottest, just as it leaves the engine to go to the radiator to be cooled. How does this sensor communicate with the temperature gauge on the dashboard and what makes the pointer on the gauge move across the dial to show changes in temperature?

Any conductor of electrical current changes its resistance to the flow of current when its temperature changes. An electromagnet, which in its most basic form is an iron bar with a coil of conducting wire around it, increases its magnetic force when the current flowing through the coil of wire increases.

It's not difficult to design a temperature sensor and the gauge to go with it. Inside the sensor you put a small pellet of a special material whose electrical resistance is very sensitive to temperature changes.

Some thermistors have a further useful property in that their resistance decreases with increasing temperature, exactly the opposite of what happens in ordinary copper wire. If the pellet inside the probe is this kind of thermistor and you send an electrical current through the pellet, the current will become stronger as the temperature of the pellet rises because higher temperature means lower electrical resistance in the pellet.

If the same current flows through the windings of a small electromagnet inside the temperature gauge, the magnetic pull exerted by the electromagnet will become stronger at the same time, because of the increasing current.

By placing the electromagnet in such a position that it can exert a pull on one end of a pivoting pointer, the other end of the pointer will move in an arc when the pull increases and the needle will move across the dial of the temperature gauge.

The well-known property of a bi-metal strip is that it will curl more as the temperature rises. It can be used in both the sensor and the dash gauge. When a bi-metal strip is used, you need an instrument-voltage stabiliser or the normal voltage fluctuations, occurring when loads are switched on and off, will give erratic readings.

When the temperature gauge stops working, the problem often lies in a faulty sensor or a loose wire to the sensor. Have an auto-electrician check the sensor and wiring before replacing the gauge.