2-4. Troubleshooting Temperature System.

TM 55-4920-401-13&P 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 (para a). 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 limits. 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 engine thermocouples. (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 disconnect. (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 thermocouples. ( 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. D’Arsonval 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 2-4