Figure 2-26. Air trapped in cooling system by thermostat being closed while system is filled.

2-30.

Preventive Maintenance

During scheduled maintenance services, or during climatic

change service, applicable testing of the system coolant

and inspection of the cooling system, a through c below

will be performed and recorded.

a. Testing for freeze protection, when applicable, by

use of a combination antifreeze-and-battery tester.

b. Before reading tester, hydrometer barrel should

be filled and emptied several times in order to equalize

temperature of all parts. The first number or letter on the

float above surface of the liquid is first read and solution

temperature is then noted from the first division or number,

Figure 2-26. Air trapped in cooling system by thermostat

above top of the thermometer column.

These two

being closed while system is filled.

readings should be made at the same time and as soon as

possible after drawing solution into hydrometer. Freezing

(8) With engine stopped, test freezing protection of

protection of solution is determined from float and

solution, and add water if necessary to bring coolant up to

thermometer readings by referring them to the protection

specified level Complete details on antifreeze installation

chart on the hydrometer. Tests will be inaccurate if made

procedure will be found in TB 750-651. The capacity of

immediately after adding either water or antifreeze. Most

the cooling system can be obtained from the vehicle

antifreeze hydrometers give best reading accuracy at

maintenance manual.

solution temperatures around 110F.

Even with

e. Slush Freeze-up (fig 2-25). At temperatures

hydrometers designed to read at solution temperatures

below its freezing point, antifreeze solution does not freeze

below 0F, tests should always be made with temperature

solid, but a mass of small ice crystals forms in the solution.

of coolant above 60F if possible, because the solution is

The slush ice stops circulation through the radiator core.

Even after the engine is started, the slush ice may not melt

more viscous when cold. This condition prevents the float

in the radiator, due to the cold draft of air from the fan. If

from finding its true level quickly and may result in a false

operation is attempted before the radiator is thawed out,

float reading.

the engine will overheat, and steam pressure in the water

jacket will force large quantities of solution out the overflow

CAUTION

pipe. Serious heat-cracking damage may follow.

A freeze protection indication below -55F will

CAUTION

require partial antifreeze drain and replacement

Always maintain adequate freezing protection

with water and inhibitor solution (para 2-29b(3Xb)).

against the coldest weather expected.

Freeze protection must not exceed -55F.

c. Refer to TB 750-651 for testing procedures for

Testing Antifreeze. To be sure that antifreeze will

reserve alkalinity.

give protection at the coldest temperature to which the

engine is likely to be exposed, and to avoid overheating

2-31.

Properties of Antifreeze

difficulties from slush freeze-up, the freezing protection of

Compound Solution

solution should be tested at least weekly and more often if

the need in indicated because of water additions or

a. Freezing Protection (fig.

2-27).

Missing

weather conditions. Only hydrometers designed for testing

antifreeze compound with water lowers the freezing point

ethylene-glycol type antifreeze compound will give an

of the water in proportion to the amount of antifreeze

accurate reading. Check the accuracy of the antifreeze

compound used. The addition of 1 gallon of antifreeze

tester occasionally by taking readings on prepared solution

compound to 3 gallons of water (25 percent concentration)

of known freezing point. A mixture of one part of

lowers the freezing point of the water from +32F to +10F.

antifreeze compound and two parts of water should test to

A solution containing one-third antifreeze compound and

zero degree F, and one part antifreeze compound and one

two-thirds water will freeze at 0F, while a solution which is

part water should test to -34F. Tester manufacturers'

half antifreeze compound and half water will protect

instructions should be followed for proper use and care of

against freezing down to -34 F.

the hydrometer. Accurate readings are not possible if the

float and Inside of the glass barrel are dirty.

2-24