Troubleshooting Err 91 Displayed At The Comfort Control - Trane ACONT900AC43UA Service Manual

Troubleshooting Err 91 displayed at the
Comfort Control
Step 1
At the User Interface Assembly, scroll down  to
the LAST 4 FAULTS and press the ENTER button.
Step 2
What is now displayed on the User Interface
Assembly?
If IND COMM ERR is displayed, see Troubleshooting
IND COMM ERR (Page 4.) If BLW COMM ERR is
displayed, is the green LED on the Inducer Motor
Drive Board on?
Yes: Perform 24 volts AC Go/No-Go Test for serial
port V.S. Blower Motor.
No:
(1) Unplug the wires going into plug E9 on the furnace
IFC; these wires go from the furnace IFC to the
Inducer Motor Drive Board (IMDB).
(2) At the Comfort Control, set the fan to the ON
position.
(3) Turn the power off to the furnace for 30 seconds
and then back on.
(4) The User Interface will display, wait until the
furnace IFC goes through the power up routine. Does
the Indoor Blower now come on?
Yes: ERR 91 is being caused by the Inducer Motor
Drive Board. Check for 120 volts AC at the Inducer
Motor Drive Board power plug, if 120 volts AC is
present at the IMDB power plug, replace the IMDB.
No: Check for the following DC voltage at the furnace
IFC plug E8. This is the plug going to the indoor
blower. Read the voltage with the blower motor plug
connected to the furnace IFC.
Plug E8 pin (labeled 12V) and then next pin (labeled
RX) should read 12 volts DC. Connect the volt meter
to the B/C terminal on the low voltage terminal strip
Solving Err 91 Comm Errors
and the pin in plug E8. Is the voltage correct?
Yes: If the 12 volts DC is present at both pins in
plug E8, the fault is with the harness or the indoor
blower motor. Perform the 24 volt AC Go/No-Go test
for serial port V.S. Blower Motor.
No: If the DC voltage is low or not present at the
furnace plug E8 pins 12V and RX, unplug the wire
from plug E8 and again check for the DC voltage at
these two pins, which should both read 12 volts DC.
Is the 12 volts DC present at both pins?
Yes: Fault is in the harness or the indoor blower
motor. See Troubleshooting BLW COMM
ERR (Page 7) for additional information.
No: Fault is the furnace IFC replace the furnace IFC.
Figure 3
R - B 3