036-21544-001 Rev. B (1103)
Mitsubishi Electronics 2003 29
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. The functions 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: Receives 12VDC from the indoor unit when set to the "Heat Mode".
NOTE: The Reversing Valve is energized during the cool mode (whenever 12VDC is NOT PRESENT) on wire #3, the Revers-
ing Valve is de-energized during the Heat Mode.
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 the indoor coil does not become at least nine degrees colder than the return air temperature after
approximately ten minutes of operation, the indoor circuit board will shut down the system. The indoor unit will display
the problem code “P8” at the indoor unit remote controller and terminate the 12VDC signal to the outdoor unit.
B. The “P8” code does not necessarily mean that the outdoor unit has a problem. It simply means that the indoor coil did
not become colder or warmer than the return air during a call for cooling or heating.
C. 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 before the 12VDC signal is generated or after the 12VDC signal
is delivered to the outdoor unit before the compressor contactor is energized.
iv.If 12VDC is present and the 3-minute delay has expired, the outdoor unit should be operating. If it is not, the indoor
unit is not the source of the problem 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.
That is, the control wires must be connected to matching numbered control terminations on the indoor and out-
door units (1 to 1, 2 to 2). If the wires are not connected properly, the unit will not be damaged, but it will not oper-
ate.
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 lit 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.
LED 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
3. OUTDOOR BOARD FUNCTIONALITY
A. High Pressure Safety Switch (HPSS). This switch opens at approximately 400 PSIG and closes at approximately
300 PSIG. There is no bypass on this switch. When the switch opens, the control immediately stops the compressor
and outdoor fan. When the switch closes, the system can restart if the indoor unit has not entered a P8 fault code. If
the switch opens three times within a one-hour period, the outdoor unit operation is “locked out” and must be serviced.
Turning line voltage off and back on to the outdoor unit will reset the lockout. The indoor unit must also be reset to clear
the fault code. Turning the remote controller off and back on will reset the indoor unit.
B. Low Pressure Safety Switch (LPS). This switch opens at approximately 7 PSI and closes at approximately 22 PSI.
The switch is bypassed during the first 3 minutes of cooling operation. If the pressure rises to the “cut in” point, the
compressor will restart if the delay has expired and the indoor unit is not in a P8 fault. If the switch opens 3 times within
a one-hour period, the outdoor unit will be “locked out” and must be serviced. Turning line voltage off and back on to
the outdoor unit will reset the lockout.
MICROPROCESSOR CONTROL11