tolerance. If system is not within tolerance, note the
number of degrees out-of-tolerance and proceed to
troubleshooting, paragraph 2-4.
N O TE
If an engine thermocouple is suspected
of opening up at the test temperature,
each thermocouple may be tested sepa-
r a t e l y b y m o m e n t a r i l y r e m o v i n g t he
heater probe and observing the tester
or aircraft indicator for a drop in av-
erage thermocouple temperature. If a
Thermocouple is open, there will be no
d r o p i n t e m p e r a t u r e w h e n i t s h e a t er
probe is removed .
(9) Turn FUNCTION SELECT and MASTER
POWER switches to OFF position before removing
heater probes from aircraft engine and tester.
2-4. Troubleshooting Temperature System. I f
the temperature system error exceeds applicable
technical manual tolerance, one or more of the
causes listed below will probably be the source of
trouble. The complete temperature system and all
of its components must be correct before the tem-
perature system can be used for reliable tem-
perature readings. The possible sources of trouble
a r e:
1. Thermocouples and thermocouple harness
2. Resistance of circuit out of tolerance (para d).
3. Temperature indicator error (paras b and c).
4. Shorts to ground (para e).
When troubleshooting, keep in mind the system
error detected in paragraph 2-3, subparagraph b. As
each malfunction is corrected, the amount of error
reduction can be compared to system error to deter-
mine if system has been returned within allowable
a. T h e r m o c o u p l e a n d H a r n e s s C h e c k . T h e
thermocouple and harness check tests the aircraft
thermocouple harness and the aircraft thermo-
couples by heating all the thermocouples and com-
paring the average output of the engine thermo-
couples with the average output of the heater probe
thermocouples as read on the TEMP indicator.
Refer to figure 2-3 and proceed as follows:
(1) Turn FUNCTION SELECT, STD DAY,
and master power switches to OFF position and
turn PROBE CONTROL fully counterclockwise.
(2) Connect BH405 heater cable to HEATER
CABLE receptacle on tester and connect junction
box to heater cable.
C A U T I ON
Never connect a cold heater probe to
the tester when it is at an elevated
t e m p e r a t u r e . T o d o s o c o u l d c a u s e
overheating of the hot probes.
(3) Connect as many heater probes having the
correct part number to the junction box as there are
(4) Place a heater probe over each engine
thermocouple. The heater probes must fit firmly
and be positioned properly on the thermocouples.
(5) Connect BH450 check cable to CHECK
CABLE receptacle on tester and to aircraft
thermocouple harness at engine disconnect. If
required, use check cable adapter at engine
(6) Turn FUNCTION SELECT switch to
HEATER PROBE position, T/C INPUT switch to
HEATER CABLE postion, TEMPERATURE switch
to HEATER PROBE TEMP SET position, and
MASTER POWER switch to ON position.
(7) Adjust PROBE CONTROL until TEMP
indicator reads test temperature.
(8) T u r n T E M P E R A T U R E s w i t c h t o
OPERATE position to read probe temperature.
(9) Place T/C INPUT switch in CHECK
CABLE position to read temperature of engine
thermocouples and in HEATER CABLE position to
read probe temperature on tester TEMP indicator.
(10) The difference in engine thermocouple
temperature and probe temperature read in step 9 is
the error of the engine thermocouples and should
not exceed technical manual tolerance.
(11) If temperatures are within tolerance,
perform temperature indicator check in paragraph
b or c. If temperatures are not within limits,
t r o u b l e s h o o t a n d r e p a i r h a r n e s s a n d / o r
( 1 2 ) T u r n P R O B E C O N T R O L f u l l y
counterclockwise and turn FUNCTION SELECT
and MASTER POWER switches to OFF position
before removing heater probes and cables from
aircraft and tester.
b. DArsonval Temperature Indicator Check.
In the aircraft temperature indicator check, a
simulated thermocouple signal from the tester is
applied to both the aircraft indicator and the tester
TEMP indicator and the two indicator readings are