Trane ACONT900AC43UA Service Manual page 277

Error 91: Communication fault
Comfort Control = Err 91
User Interface = SYS COMM ERR
Air Handler Fault LED = 3 Flash
or
Furnace Fault LED = 10 Flash
Problem Description
• Loss of heat/cool demand
- Heat/Cool demand is generated by comfort control
every 1 minute and a fault is reported after 3 missed
messages
Possible Causes/Solutions
• Data line to comfort control broken or disconnected
- 0vDC measured between D & B at comfort control
with ~12vDC measured at indoor unit
• Data (D) line and common (B) line reversed at
outdoor unit
- The Bit Master LED on indoor control board will be
off, dim or flickering
- 0vDC will be measured with D & B wires connected
at outdoor unit
- 12vDC will be measured on D & B field wires when
disconnected from outdoor unit
• Data (D) line grounded going to outdoor unit
- The Bit Master LED on indoor control board will be
off, dim or flickering
- 0vDC will be measured with D & B wires connected
at outdoor unit
- 0vDC will also be measured on D & B field wires
when disconnected from outdoor unit
• Alert message and fault LED may take up to 5
minutes to display
Alert Code Response
Error 91: Communication fault
Comfort Control = Err 91
User Interface = BLW COMM ERR
Air Handler Fault LED = 3 Flash
or
Furnace Fault LED = 10 Flash
Problem Description
• Loss of communication between serial
communicating blower motor and control board
• (3-Stage/Modulating Furnaces only) Corrupted signal
between integrated furnace control board and inducer
drive control board can also cause this fault
Possible Causes/Solutions
• No line voltage to blower motor or inducer motor
control board
- Furnace 115vAC
• Faulty low voltage connection between control board
and blower motor
- 12vDC should be measured between 12v pin and
GND pin on control board
- E8/E15 connector and at terminals 1 & 4 at blower
motor low voltage plug
• Perform 24volt "Go/No-Go Test" by applying 24vAC to
pins 3 & 4 at blower motor low voltage harness
- If motor runs replace control board if motor does not
run and motor harness tests good, replace motor
assembly
• (3-Stage/Modulating Furnace only) Disconnect 4-pin
connector from E9 on furnace control board, cycle
power, then test blower operation
- If blower runs, the inducer drive control board is
causing the fault
- Air Handler 230vAC
R - A 11