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10
Ground
11
Evaporator power return
12
Evaporator power input
Table 4 LED List
INDICATOR
D25
Comp.
D26
Cond.
D27
Tie-in comp.
D28
Tie-in cond.
D29
Heat
D30
Com.
D31
Evap.
System Operation
Basic system operations are covered by the instruction manual (701465).
Thermostat Control Logic:
Temperature sensing
EC3.0 system has two thermistor type temperature sensors: cabin (internal) air temperature
sensor and ambient (external) air temperature sensor. The cabin air temperature sensor is
mounted in front of the main evaporator return air filter and the external temperature is
mounted externally but the location varies depending on vehicle models.
The electric resistance of the temperature sensor changes with the temperature. The sensing
circuit on the relay board can read the resistance and convert it to temperature reading. The
resistance of the temperature sensor is about 500 ohms at 70ºF. This can be uses as a
reference to determine if a temperature sensor is functional.
EC3.0 system only displays the temperature between 60 ºF and 90 ºF. If the actual
temperature is lower than 60 ºF, the display will show "Lo". If the temperature is above
90ºF, the display will show "Hi". The setpoint range is also set to be 60 ºF to 90 ºF.
Thermostat control logic
The compressor clutch, condenser and heat output are controlled by the thermostat based on
the temperature differential between cabin air temperature, Tin, and set point temperature,
Tset. The control logics are listed in Table 5.
Table 5 Thermostat control logic
MODE
CONTORLLED
COMPONENT
COOL
Clutch,
condenser
HEAT
Heat relay
DEFOG Heat relay
External temperature protection
FUNCTION
Illuminates when compressor relay is engaged.
Illuminates when condenser is engaged
Illuminates when tie-in compressor relay is engaged
Illuminates when tie-in condenser is engaged
Illuminates when heat relay is engaged
Illuminates and flickers when board is powered up
Illuminates when evaporator is on
LOGIC
Tin>Tset+2 clutch & condenser turn on
Tin≤Tset+2 clutch & condenser turn off
Tin<Tset-3 heat relay engage
Tin≥Tset+1 heat relay disengage
Clutch & condenser stay on
Heat relay regulate same as in heat mode
EC3 Service Manual Jan. 8, 2008
3
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