Lesson 5/Learning Event 1
The vacuum valve in the cap operates after the engine has been stopped and begins to cool off. As the water
cools, it contracts, forming a vacuum or low pressure in the cooling system. When the vacuum gets great
enough, normal air pressure entering the overflow pipe forces the vacuum valve off its seat and flows into the
cooling system. If a vacuum valve was not used, the hoses and perhaps the radiator tanks would be drawn
together by the vacuum.
You will not work on many cooling systems before coming in contact with the expansion plugs. These plugs
are commonly known by such names as core hole plugs or freeze plugs. Actually, they are metal plugs that are
driven into, and seal round core openings in, the outside wall of the engine coolant jacket.
The core openings in the coolant jacket of the cylinder block and head are necessary for the casting process
when the block and head are being manufactured. But once they have been built, the openings have no further
use, so they are plugged with the expansion plugs.
Expansion plugs may be forced out of the openings if the water freezes in the coolant jacket. However, they
cannot be depended upon to keep the engine from cracking in case of a freeze-up.
Temperature Sending Unit
A temperature gage sending unit is installed in the cylinder head or some other part at the top of the engine
that contains water passages. On combat vehicles an electrical-type unit is used. Electrical units always have a
wire attached and are screwed into a hole tapped into a coolant passage. The sending unit has a heat-sensitive
element that extends into the coolant. The element senses the temperature and sends corresponding electrical
signals to the temperature gage.
It is not a good practice to operate a vehicle with water alone in the cooling system. If just plain water is used,
parts in the cooling system will rust and corrode, causing leaks and plugging small passages. Also, in cold
weather water will freeze, which may crack the engine block and radiator.