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9
MICROPROCESSOR CONTROL
1. COOLING/HEATING OPERATION
Outdoor unit operation is activated by delivery of a 12VDC signal from the indoor unit. Cooling only units have 2 low
voltage control wires from the indoor unit to the outdoor unit. Heat pump units have 3 low voltage control wires from
the indoor to the outdoor unit. The function of these wires are:
Wire #1: Common
Wire #2: Initiates operation of the compressor and outdoor fan when 12VDC is supplied from the indoor unit.
Wire #3: Initiates operation of the reversing valve for heating operation. 12VDC is present constantly when
the indoor unit mode is set to heat.
2. OUTDOOR ABNORMALITY DETECTION BY INDOOR UNIT (P8 FAULT CODE)
A. The indoor unit constantly monitors the difference between return air temperature and indoor coil temperature.
During a call for cooling, if after several minutes of operation (approx 10 minutes) the indoor coil does not
become at least 9 degrees colder than the return air temperature, the indoor circuit board will cause the system
to shut down. At this point the problem code "P8" will be displayed at the indoor unit remote controller and the
12VDC signal to the outdoor unit will be stopped.
B. The "P8" code does not necessarily mean that the failure is caused by the outdoor unit. It simply means that the
indoor coil did not become colder than the return air during a call for cooling.
C. During the heat mode the indoor coil temperature must get at least 9 degrees warmer than the return air tem-
perature. If this temperature difference is not achieved the unit will shut down as indicated above.
D. If a "P8" code is displayed:
i. Turn the indoor remote controller off and back on to reset it.
ii. Set the remote controller to a "call" for cooling.
iii. At the indoor unit 12VDC control terminals check to see if 12VDC is present on the #1 and #2 wires.
1. You may encounter a 3 minute delay in cooling or a 6 minute delay in heating before the 12VDC signal
is generated.
iv. If 12VDC is present and the outdoor unit is not running the problem is not caused by the indoor unit and the
following should be checked:
1. Check the 12VDC terminals at the outdoor unit for 12VDC. The polarity must be the same as that of the
indoor meaning wire one at the indoor must be on the same terminal at the outdoor. If they are
reversed no harm will be done but the unit will not operate.
a. If no 12VDC signal is present when 12VDC is available at the indoor unit check for a break/open in the
control wire.
b. If 12VDC is present insure polarity is correct. If polarity is correct and a 12VDC signal is present check
the LED indicators on the outdoor board.
i. Is the "Status" LED lite constantly? If so the outdoor board is in time delay, wait for it to expire
(make sure the 12VDC signal is still available from the indoor unit).
ii. Is the "#2" LED lit? This indicates that the board is receiving 12VDC from the indoor unit.
NO LED: Recheck for 12VDC on the control wires, check for correct polarity and check the wires
from the terminal block to the board.
LIT: The board should be outputting 24 VAC to the compressor contactor and line voltage to the
fan (unless in low ambient control). Is the compressor/fan operating?
YES: System is OK
NO: Is the "Status LED" flashing?
NO: Bad outdoor board
YES: 2 Flash sequence indicates that the High Pressure Switch is open. 4 Flash sequence indicates
that the Low Pressure Safety Switch is open
SP007-28
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