General Assist Technical Reference
Testing Three-Phase Outdoor Units - PFC PCB
Diode-test procedure for Power Factor Correction boards
Applicable to AOTD**LATT, AOTD**LBTT and AOTG**LATT outdoor units
Three-phase outdoor units use a Power Factor Correction (PFC) PCB as part of the active-converter circuit. A common symptom of PFC PCB failure is the main circuit breaker tripping as soon as the system is energised.
Electrical safety: This procedure must only be completed by qualified service personnel. Isolate the unit from the mains supply, prove that power is disconnected and allow the DC capacitors to discharge before touching or testing the PCB.
1. Scope
| Item | Details |
|---|---|
| Applicable models | AOTD**LATT, AOTD**LBTT and AOTG**LATT |
| PCB under test | PFC - Power Factor Correction PCB |
| Required meter function | Diode-test mode |
2. Common symptoms
- Main circuit breaker trips when the outdoor unit is energised.
- The unit shuts down immediately after power is applied.
- There may be visible heat damage, carbon tracking or a burnt smell around high-current components.
A breaker that trips immediately on power-up, before compressor operation begins, is a strong indication that the PFC PCB or another component in the DC power circuit is shorted. Testing is required before parts are replaced.
3. Test points
| Terminal | Identification | Purpose |
|---|---|---|
| TM504 | Orange | Positive connection |
| TM505 | Purple | Negative connection |
| TM500 / TM501 / TM502 | Intermediate terminals | Used to verify the internal diode paths |
PFC PCB terminal locations. TM504 is identified in orange and TM505 in purple.
4. Testing procedure
| 1 | Isolate power. Disconnect the unit from the mains supply, prove the circuit is de-energised and wait for the DC bus capacitors to discharge. |
| 2 | Access the PFC PCB. Locate TM500, TM501, TM502, TM504 and TM505. |
| 3 | Select diode mode. Confirm the multimeter and test leads are functioning correctly before testing the PCB. |
| 4 | Complete every test below. Probe polarity is important. Record the result for each terminal pair. |
| Red lead (+) | Black lead (-) | Expected reading |
|---|---|---|
| TM504 | TM500 | OL |
| TM504 | TM501 | OL |
| TM504 | TM502 | OL |
| TM505 | TM500 | Approx. 0.4 V |
| TM505 | TM501 | Approx. 0.4 V |
| TM505 | TM502 | Approx. 0.4 V |
| TM501 | TM504 | Approx. 0.4 V |
| TM502 | TM504 | Approx. 0.4 V |
| TM500 | TM504 | Approx. 0.4 V |
| TM501 | TM505 | OL |
| TM502 | TM505 | OL |
| TM500 | TM505 | OL |
Example meter connection and original reference readings.
5. Interpreting the results
| Result | Interpretation | Action |
|---|---|---|
| Expected OL and approximately 0.4 V readings | The tested diode paths are operating normally. | Continue diagnosis if the breaker still trips. |
| 0.0 V on any test | The circuit is shorted and the PFC PCB has failed the test. | Replace the PFC PCB assembly. |
| OL on every test | The board may be disconnected, a fuse may be open or the probe contact may be poor. | Check harnesses, fuses, meter operation and terminal contact before condemning the PCB. |
Reading tolerance: The reference document shows approximately 0.4 V. Small variations are normal because the PFC circuit includes choke coils. A reading around 0.3-0.5 V is generally consistent with a healthy semiconductor junction, provided the reverse-polarity result is OL.
6. Final service checks
- Inspect the board for moisture ingress, carbon tracking, cracked components and heat damage.
- Check the three-phase supply for phase imbalance, under-voltage and over-voltage.
- Confirm the compressor and inverter/IPM circuits are not shorted before energising a replacement board.
- Confirm all earth connections and protective devices are secure before restoring power.