Watch these fans when the heat's on
Page 50
If you've noticed an error in this article please click here to report it so we can fix it.
THERMOSTATS not only ,speed the warming up process, but also control the water temperature during normal operation. Another use for them is the control of some fan drives and radiator shutters. These days, nearly all thermostats have a wax element.
Bellows-type thermostats were used for many years, and the valve was controlled by a partially evacuated bellow containing a small quantity of volatile fluid. As the temprature rose, so the fluid caused the bellows to expand, opening the valve. But the bellows took up a lot of space, while the continual flexing led to wear and tear.
In the wax element thermostats, there is a wax pellet that is very sensitive to changes in temperature, and as it expands, so it pushes an elastorneric moulding against a piston, thus opening the valve.. The wax element thermostat is compact, and is not affected by changes in pressure. With either type of thermostat, opening is progressive, the valve not opening fully until the temperature has risen some 14-20°C above the initial opening temperature.
If the bellows type fails, it remains in the open position, so the inefficiency of the heater and the slowness of the warming up are the symptons. The wax element unit fails in the closed position normally, leading to instant overheating.
If the engine .does overheat suddenly, and the water hasn't leaked away, and the fan belt is still intact, the chances are that the thermostat has failed in the
closed position.
The thermostat cannot allow for changes in the weather, so engines tend to be overcooled in winter, as they do when they are running at light load. This is why radiator shutters are used. The aim is to control the operating temperature between close limits, although the fact that the fan runs only when the engine is hot also saves some power with the thermostatically controlled fan drive.
The shutters are closed when the engine is cold, and are opened as necessary to maintain the correct water tempera ture. The fan drive either incorporates a clutch or a viscous drive, and in either case, a thermostat is used to bring the fan into operation when the coolant temperature exceeds the optimum level — and that isn't often.
The use of these devices is increasing, although they make life a bit more complicated for the mechanic. Generally the fan drives are designed to fail leaving the fan driven, so diagnosis of the fault is usually straight-forward, if the fan is driven when the engine is cold, then something is wrong.