System Communication Troubleshooting - Trane ACONT900AC43UA Service Manual

Solving Err 91 Comm Errors
This DC voltage should be going up and down
between 0.152 volts DC and 0.174 volts DC. The
voltage meter will stop reading at times and then
start reading again.
Yes: Repair or replace the low voltage harness going
from the indoor blower to the gas furnace IFC.
No: Replace the indoor blower motor and reconnect
the wire removed from the gas furnace IFC or the air
handler.
Step 7
Is there 12 VDC at the gas furnace IFC or air
handler low voltage plug measured between
terminals 12V and GND? (See Figures 19a & 19b.)
Figure 19a
R - B 10
Figure 19b
Yes: Repair or replace the low voltage harness going
from the gas furnace IFC plug to the indoor blower
low voltage harness.
No: Replace the gas furnace IFC.

System communication troubleshooting

Step 1
Turn off the power to the furnace or air handler.
Remove the blower door to be able to see the furnace
IFC or air handler control board LEDs and their user
interface display. On an air handler installation, the
user interface display must be temporarily set aside
to be able to view the LEDs. Restore power to the
furnace or the air handler and watch the green BM
LED and the amber COM LED.
Does the green BM LED come on solid and
bright? (Note: BM LED must glow brightly, with no
flashing.)
Yes: Go to Step 3.
No: Go to Step 2.
Step 2
Is there 24 volts AC power at (1) the outdoor unit
Charge Assist™ Control, (2) at the furnace IFC
or the air handler control board and (3) the
Telephone Access Module?