Communication Loop Mapping
Test the serial voltage wave across interconnect terminals (e.g., S1/S2/S3) to isolate bad wiring from control board failures.
System Diagnostics
Variable-Speed & Inverter Diagnostics Without the Guesswork
From communication lockouts to compressor sync loss, ACLogics guides your technicians through serial line analysis, DC bus voltage measurements, and EEV coil tests.
FAULT SYMPTOM
Inverter compressor will not start, LED shows a compressor sync loss or drive fault code
CHECK 01
Disconnect compressor plugs. Measure winding resistance across all 3 phases (U, V, W). Are windings balanced and isolated?
Yes, balanced under 2Ω, no ground path
CHECK 02
Power unit on. Measure the DC Bus voltage across the capacitor bank. Is voltage around 300-340VDC?
Yes, voltage is stableNo, voltage is extremely low or 0V
No, open windings or short to ground
DIAGNOSIS
Failed compressor motor windings (shorted or grounded). Replace compressor.
Complete diagnostic tree — every branch, every fault path
Try ACLogics Free on App StoreThe Three-Step Workflow
01
Select the Fault Code or Symptom
Enter the specific LED blink code, thermostat error, or operation symptom (such as failing to ramp up speed).
02
Follow Step-by-Step Measurements
ACLogics orders diagnostics from low-voltage communication checks to high-voltage inverter bus measurements to pinpoint the exact failure point.
03
Verify Before Part Replacement
Validate coil resistance, voltages, and sensor curves to document the definitive cause of failure before ordering high-ticket inverter parts.
Built for Precision
Everything you need to diagnose with confidence and accuracy.
EEV & Sensor Diagnostics
Verify EEV stepper coil resistance, thermistor resistance curves, and valve feedback loops to isolate metering issues.
Inverter Power Path Testing
Measure incoming AC power, DC bus capacitor voltage, and compressor terminal voltage to safely isolate board failures from locked rotors.
Frequently Asked Questions
How do I distinguish between a faulty inverter board and a bad compressor?
Measure the resistance across all compressor terminals (should be identical and balanced, typically under 2 ohms) and verify no continuity to ground. If the compressor is fine, check if the inverter module outputs balanced AC voltage to the compressor pins briefly during startup.
What is the correct way to test variable-speed communication (e.g., S1, S2, S3) issues?
Disconnect S3 and measure DC voltage between S2 and S3 on the outdoor board. It should pulse dynamically between 0V and 24VDC. A flat or static voltage indicates a communication circuit failure on one of the boards.
How do I verify if an Electronic Expansion Valve (EEV) is physically stuck?
Measure the resistance of the EEV actuator coil windings (usually 40-50 ohms). If the coil is good, use a strong magnet or manual EEV driver to listen for mechanical movement or monitor the change in refrigerant pressures.
What does a DC Bus Undervoltage fault code typically point to?
It indicates that the DC voltage on the main capacitors has dropped below the threshold (typically around 200VDC). Check for low supply voltage, a faulty rectifying bridge, a failed reactor, or heavy compressor load pulling down the bus.
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