Emerson Rosemount Oxymitter 4000 Instruction Manual page 100

Oxymitter 4000
Figure 8-4. Fault 2, Shorted Thermocouple
8-8
Fault 2, Shorted Thermocouple
Figure 8-4 shows the electronic assembly for an
Oxymitter 4000 with a membrane keypad (upper
view) and an Oxymitter 4000 with an LOI (lower
view). The upper view also shows J1 and test
points TP1 through TP6, located on the
microprocessor board, below the membrane
keypad or the LOI module.
Membrane Keypad
When Fault 2 is detected, the HEATER T/C LED
flashes twice, pauses for three seconds, and
repeats.
1. Using a multimeter, measure the voltage
from TP3+ to TP4-. If the reading is 0 ±0.5
mV, then a shorted thermocouple is likely.
2. Remove power and disconnect J1.
3. Measure the resistance from TP3+ to TP4-.
The reading should be approximately 20K
ohms.
4. If so, the short is not on the PC board. The
thermocouple wiring or the thermocouple is
shorted. See "Heater Strut Replacement" in
Section 9: Maintenance and Service.
LOI
When Fault 2 is detected, the LOI displays
the"O2 T/C Shorted" message.
1.Remove power. Unscrew and remove
the LOI module from the electronic
assembly.
2. Reconnect power to the Oxymitter 4000.
3. Perform the diagnostic steps 1
through 4 shown for the membrane
keypad.
Instruction Manual
IM-106-340, Rev. 4.3
July 2017