8 Service Manual SM-MDS_(ii)
Piping and
Components
Layout
General layout of MDS system piping and components
PWM (Pulse Width Modulation) valve is an ON/OFF valve. When signal is send from
outdoor PCB, the pulse voltage will react on the coil and generate a rotating magnetic field.
This will allow the rotor to drives the valve to move up or down and hence opening or
closing the valve.
EXV4 governs the sub-cool circuit of MDS system. It is controlled by the readings of
temperature sensor TH9 and TH10. The difference between TH9 and TH10 is normally
about 8 to 12ºC (cooling mode). For example, when the difference between TH9 and TH10
is very low, EXV4 will increase its opening step to maintain the sub-cool circuit.
Defrost starts when:
1. After initial 1st heating cycle for 25 minutes and the intersection of TH8 & TH12 is
fall in the “I” area for 3 minutes.
2. After initial continuous heating cycle for 45 minutes (45 minutes from previous
defrost) and intersection of TH8 & TH12 is fall in the “I” area for 3 minutes.
Defrost will terminate if:
1. High pressure reading >19 bar (R22) OR >30 bar (436 psi) (R410A)
2. Unit entered to defrost mode for minimum 10 minutes
3. Unit entered to defrost mode for minimum 2 minutes & TH9 (temperature before
SCC) is more than 15°C
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