Electronic Control Unit Circuit Board (ECU)
The Electronic Control Unit
operates the outdoor fan
motor, crankcase heater,
EEV stepper motors and the
4-way valve. This board also
controls the general operation
of the system and makes all
of the diagnostic decisions.
The ECU is connected via
communication cables to the
Module Circuit Board, Power
Circuit Board and the Service
Voltage to operate the ECU is provided by the PCB on
terminals ACN and ACL.
When this power is present, the GREEN LED on the ECU
should be lit.
The communication cables to the PCB and MCB boards
connect via Plugs CN6 and CN-9.
The SMB connects to plugs CN-23 and CN-8. When
these cables are connected to the SMB, the SMB digital
display should be illuminated.
Plug CN-21 connects the data path between each indoor
unit and the outdoor unit ECU board. The connections
from this plug terminate at the Number 3 terminal at the
voltage connection terminal strips for the indoor units.
The Outdoor Fan Motor is a DC voltage variable speed
type that connects to the ECU at terminal Plug CN-11.
The 4-Way Valve is energized by line voltage from a
connection via Plug CN-5. This valve is energized in HEAT
The Crankcase Heater is energized via a connection at
terminals CON-9 and CON-8 on the ECU.
The EEV Stepper Motors are controlled via connections
at terminals CN-15 through CN-20. These EEV Stepper
Motor connections include the connection for the HEAT
MODE EEV located at the outdoor coil.
OUTDOOR TECHNICAL OVERVIEW
Each EEV has a set of temperature sensors that monitor
the temperature of the exiting liquid and entering
vapor from each evaporator circuit. These sensors are
mounted in a group near the center of the circuit board.
There are 6 system temperature sensors that
monitor refrigerant line temperature and outdoor air
temperatures. These sensors plug into the ECU via 2
Plugs CN-14 and CN-7.
The system has two refrigerant pressure switches, a
Low Pressure Switch and a High Pressure Switch. These
switches are connected to the ECU via Plugs CN-12 and
There are 3 sets of DIP Switches located on the circuit
board. They are SW-7. (Factory Settings Only), SW-5
(Defrost Adjustments) and SW-6 (Not Currently Used).
There are 4 surface mounted buttons located next to
SW-5 and SW-6. These buttons are for factory use only.
The ECU board has two LED Indicators, a GREEN power
indicator and a RED Diagnostic Indicator LED. When
power is present, both the GREEN and RED LED lights
A 15A 250V rated ceramic fuse is located on the ECU.
This fuse will open if excessive current occurs or if a
power surge is present. This fuse is field replaceable.