The use of a temperature stick is another way to test
a thermostat quickly. The temperature stick is a pencil-
like device that contains a wax material containing
certain chemicals that melt at a given temperature.
Using two sticks (one for opening temperature and the
other for full-open temperature), rub the sticks on the
thermostat housing. As the coolant warms to operating
temperature, the wax-like marks will melt. If the marks
do not melt, the thermostat is defective and needs to be
radiator and pressure cap. Proper service of the
components ensures an efficient cooling system and
extends the life of the vehicle.
Engine Fan Test
A bad water pump may leak coolant, fail to circulate
coolant, or it may produce a grinding sound. Rust in the
cooling system or lack of antifreeze is the most common
causes for pump failure. These conditions can
accelerate seal, shaft, and bearing wear. An
over-tightened fan belt will also cause water pump
A faulty engine fan can cause overheating,
overcooling, vibration, and water pump wear, or
damage. Testing the fan ensures that it is operating
To test a thermostatic fan clutch, start the engine.
The fan should slip when cold; as the engine warms up,
the clutch should engage. Air should begin to flow
through the radiator and over the engine. You will be
able to hear and feel the air when the fan clutch locks up.
If the fan clutch is locked all the time (cold or hot), it
is defective and must be replaced. Excessive play or oil
leakage also indicates fan clutch failure.
When testing an electric cooling fan, observe
whether the fan turns ON when the engine is warm.
Make sure the fan motor is spinning at normal speed and
forcing enough air through the radiator.
If the fan does not function, check the fuse,
electrical connections, and supply voltage to the motor.
If the fan motor fails to operate with voltage applied,
If the engine is warm and no voltage is supplied to
the fan motor, check the action of the fan switch. Use
either a voltmeter or test light. The switch should have
almost zero resistance (pass current and voltage) when
the engine is warm. Resistance should be infinite (stop
current and voltage) when the engine is cold.
If these tests do not locate the trouble with the
electric cooling fan, refer to the manufacturers service
manual for instructions. There may be a defective relay,
connection, or other problem.
SERVICE AND REPAIR OF COOLING
The individual components of the cooling system
which require servicing and repair include the water
pump, thermostat, hoses, fan and fan belt, and the
To check for a worn water pump seal, pressure test
the system and watch for coolant leakage. Coolant will
leak out of the small drain hole at the bottom of the
pump or at the end of the pump shaft.
Worn water pump bearings are checked by
wiggling the fan or pump pulley up and down. If the
pump shaft is loose in its housing, the pump bearings are
badly worn. A stethoscope can also be used to listen for
worn, noisy water pump bearings.
Water pump action can be checked with a warm
engine. Squeeze the top radiator hose while someone
starts the engine. You should feel a pressure surge (hose
swelling) if the pump is working. If not, pump shaft or
impeller problems are indicated. You can also watch for
coolant circulation in the radiator with the engine at
Whether a defective pump is replaced or rebuilt
depends on parts supply and cost. A water pump rebuild
involves disassembly, cleaning, part inspection, worn
part replacement, and reassembly. Few mechanics
rebuild water pumps because rebuilding takes too much
time and is not cost effective.
The removal and installation of the water pump
varies with different vehicles. Therefore, the applicable
shop manual must be consulted for the step-by-step
When you replace a pump, install a new gasket.
Make sure the mating surfaces are clean and smooth.
The application of a gasket sealer to both sides of the
gasket is recommended. Then after refilling the cooling
system, the pump should be checked for leaks, noise,
and proper operation.
There are no repairs or adjustments to be made on
the thermostat. The unit must be replaced when it fails