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FORM 150.73-NM2
ISSUE DATE: 09/07/2020
PROBLEM
"LOW SUCTION PRESSURE
FAULT"
"HIGH DISCHARGE PRESSURE"
FAULT
"MP / HPCO" FAULT
JOHNSON CONTROLS
TROUBLESHOOTING (CONT'D)
CAUSE
1. Improper suction pressure cut-
outs adjustments.
2. Low refrigerant charge.
3. Fouled Filter Dryer.
4. TXV defective.
5. Reduced air flow.
6. Defective Suction Pressure
Transducer/Low Pressure
Switch or wiring.
7. LLSV defective.
1. Condenser fans not operating
or operating backwards.
2. Too much refrigerant.
3. Air in refrigerant system.
4. Defective Discharge Pressure
Transducer.
1. Compressor Internal Motor
Protector (MP) open.
2. External overload tripped.
3. HPCO Switch open.
4. Defective HPCO Switch.
5. Defective CR Relay.
SECTION 9 – SERVICE AND TROUBLESHOOTING
SOLUTION
1. Adjust per recommended set-
tings.
2. Repair leak if necessary and
add refrigerant.
3. Change Dryer/Core.
4. Replace TXV.
5. Check air flow.
6. Replace Transducer/Low Pres-
sure Switch or faulty wiring.
See Section 9 – SERVICE AND
TROUBLESHOOTING for pres-
sure/voltage formula.
7. Replace LLSV.
1. Check Fan Motor, and Con-
tactors. Assure Fan blows air
upward.
2. Remove refrigerant.
3. Evacuate and recharge system.
4. Replace discharge pressure.
See Section 9 – SERVICE AND
TROUBLESHOOTING for pres-
sure/voltage formula.
1. Verify refrigerant charge is not
low. Verify superheat setting of
10° F - 15° F (5.6° C - 8.3° C).
Verify compressor is not over
loaded.
2. Determine cause and reset.
3. See "High Press. Disch." Fault.
4. Replace HPCO Switch.
5. Replace Relay.
9
CONT'D
145
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Table of Contents
Troubleshooting
