Mitsubishi Electric MXZ-SM72TAM-U1 Service Manual page 48

1508 (EF): 4-way valve trouble in heating mode
Abnormal points and detection methods
4-way valve does not operate during heating operation.
Any of the following temperature conditions is satisfied
for 3 minutes or more during heating operation when the
outdoor temperature is −4°F (−20°C) or more:
TH22j−TH21j ≤ −18°F (−10°C)
TH23j−TH21j ≤ −18°F (−10°C)
TH22j ≤ 37.4°F (3°C)
TH23j ≤ 37.4°F (3°C)
Note:
• For indoor unit, the abnormality is detected if an operating unit satisfies
the condition.
TH21:
Indoor intake temperature thermistor (RT11 or TH1)
TH22:
Indoor liquid pipe temperature thermistor (RT13 or TH2)
TH23:
Indoor gas pipe temperature thermistor (TH-A to E)
Diagnosis of failure
Is the connector for outdoor multi
controller circuit board or 4-way valve
coil disconnected or loose?
Disconnect the connector for outdoor multi
controller circuit board or 4-way valve coil to
check the resistance.
Is the resistance detected?
Check the voltage and appearance of the
outdoor multi controller circuit board.
Is the detected voltage normal?
Is it free from any trace of over-heat-
ing or burning?
Refer to ''How to check the parts'' for ohm values.
OCH800A_48
Yes
No
No
Yes
No
Yes
Causes and checkpoints
• 4-way valve failure
• Disconnection or failure of 4-way valve coil
• Clogged drain pipe
• Disconnection or loose connection of connectors
• Malfunction of input circuit on outdoor multi controller
circuit board
• Defective outdoor power circuit board
Reconnect the connector or connect it tightly.
Replace the 4-way valve coil.
Replace the outdoor multi controller circuit
board.
Replace the 4-way valve.