Transpositions (Crossing of Lead Wires).

TM 55-4920-244-14

Figure 2-7. Continuity probe and sampling thermocouple.

cater must show a temperature rise as each en-

has cooled to ambient temperature, make another

gine thermocouple is checked.

test. If there is a transposition, the difference in

h. Place switch SW- 6in MECH. ZERO

the readings will probably be large because of

position.

the thermals that were present in the first test.

c. Disconnect and check each terminal

with a magnet. Chromel (+) is nonmagnetic;

2-13. Transpositions (Crossing of Lead Wires).

alumel (-) is magnetic.

An odd number of crossed lead wires (1,

d. Touch a hot soldering iron to each ter-

3, etc ) can be easily detected, since the aircraft

minal. A temperature rise or fall on the aircraft

EGT indicator will read backwards upon applica-

EGT indicator indicates a transposition when the

tion of heat to the engine thermocouples. An

crossed terminal is touched.

even number of crossed lead wires (2, 4, etc),

while much more difficult to detect, can be found

2-14. Securing the Tester (Figure 2-2).

by several methods:

a. Place switch SW-1 in OFF position.

a. The simplest method is with an ohm-

b. Place TEMP. REGULATOR knob in

meter. Chromel wire has approximately two and

OFF position.

one-half times the resistance of alumel wire. To

c. Remove heater probes from engine

have a transposition, the chromel wire would

thermocouples and disconnect from outlets of

have to be connected to the alumel wire. The

junction box.

resistance reading would be different than the

2-1/2 to 1 ratio.

NOTE

If continuity check probe

b. Where possible, make a normal engine

is connected, remove in

thermocouple test with tester connected to a hot

accordance with c above.

engine (shortly after being run). When engine

2-10