R - B 15
Solving Err 91 Comm Errors
Voltage readings made at the furnace or air
handler LVTB
R to B = 0 volts AC - With 24 volts AC to control
board and fuse is OK, change control board.
D to B = 0 volts DC - 24 volts AC OK, change control
board
Voltage readings made at the Comfort Control
sub base. (24 volts AC and 12 volts DC OK at the
Furnace or Air Handler LVTB).
R to B = 0 volts AC - Open thermostat wire (R to B)
D to B = 0 volts DC - Open thermostat wire (D)
Voltage readings made at the Outdoor Unit
D to B = 16.5 volts DC - Open thermostat wire (D)
D to B = 13.6 volts DC - Open thermostat wire (B)
When outdoor unit is off (no 24 VAC) data line voltage
stays at 12 VDC. This 12 VDC is supplied by the
communicating indoor unit.
When outdoor unit is on (24 VAC present) and D
field thermostat wire is open the data line voltage
will read 16.5 VDC.
Voltage readings made at TAM
R to B = 0 volts AC - Open thermostat wire (R to B)
D to B = 0 volts DC - Open thermostat wire (D)
Communicating voltage to blower motor and
furnace inducer drive board
Furnace IFC
The following voltage readings are to be made at plugs
E8 or E9 with motor harness connected.
12V to GND = 12 volts DC
RX to GND = 12 volts DC
TX to GND = 0 volts DC
Note: The TX may have a voltage of 0.5 to 1.5 volts
DC when air handler control board is first powered
up. This may also occur if the blower motor and air
handler control board are not communicating with
each other.
Air handler
The following voltage readings are to be made at
plug VS-COMM with motor harness connected.
12 VCC (brown wire) to B/C = 12 volts DC
RX (purple wire) to B/C = 12 volts DC
TX (orange wire) to B/C = 0 volts DC
B/C (blue wire)
Figure 29
Note: When measuring voltage at pins 1, 2, 3, or 4,
the other meter lead should be connected to the
transformer 24V common.
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