Overview
Certain Fujitsu indoor unit models, such as ARTG/ and AIRSTAGE ducted series, utilize DC fan motors with two separate plugs — one for power supply and another for control/signal communication.
These motors differ from conventional single-plug DC fan motors and require a specific testing method to accurately identify whether a fault lies within the fan motor, control PCB, or signal communication path.
The following procedure outlines diagnostic steps for bench testing and in-circuit verification.
1. Identification of Motor Type
Two Plug Configuration
Plug 1 (Power Plug): Typically 3-core — provides DC voltage (V+, V–) and a ground reference.
Plug 2 (Signal Plug): Typically 5-core — carries control, feedback (tachometer), and communication lines to the indoor control PCB.
Refer to the schematic (page 1, lower right section of the provided diagram) which labels the FAN MOTOR (FM) connections at the control PCB, model ARTG65LHTA
DC fan motor _two plug test
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2. Safety and Preparation
Isolate power before disconnecting any plugs.
Wait at least 5 minutes after power-down to allow capacitors on the control PCB to discharge.
Use a digital multimeter with both DC voltage and resistance (Ω) functions.
3. Testing the Power Plug (Plug 1)
Procedure
Reconnect the system and power ON.
Measure between the V+ (Red) and V– (Black) wires at the fan motor plug.
| Test Point | Expected Reading | Interpretation |
|---|---|---|
| V+ to V– | 310–340 V DC (nominal) | Power supply from PCB is correct |
| 0 V DC | No power — likely PFC or inverter drive fault | |
| Fluctuating/unstable voltage | PCB regulation or IGBT switching issue |
Tip: If DC voltage is missing but the control LED indicators on the PCB are normal, the fan drive transistor section on the control PCB may be open-circuit.
4. Testing the Signal Plug (Plug 2)
Pin Functions (Typical)
| Pin | Function | Expected Reading |
|---|---|---|
| 1 | 5 V Reference (from PCB) | 4.8–5.2 V DC |
| 2 | Control / PWM Input | Variable 0–5 V depending on fan speed |
| 3 | FG (Feedback / Tachometer) | Pulsing 0–5 V signal during fan rotation |
| 4 | Ground (Signal) | 0 V |
| 5 | NC or Diagnostic | Open / floating |
When the system commands a fan speed change, the PWM input line should fluctuate in voltage, confirming the PCB is sending a drive signal.
5. Testing the Fan Motor (Resistance Check)
Procedure
With the system powered OFF and plugs disconnected:
Measure resistance across power plug terminals:
Between V+ and V–: should read several hundred kΩ (depending on model).
Between any terminal and frame ground: should read open circuit (OL).
If resistance is very low (<10 Ω), the fan motor’s internal driver IC or rectifier stage is shorted — replace the fan motor assembly.
6. Common Fault Patterns
| Symptom | Likely Fault | Corrective Action |
|---|---|---|
| Breaker trips when fan starts | Shorted fan motor or drive transistor | Disconnect motor and retest supply |
| Fan doesn’t start but 340 V DC present | Faulty internal fan driver IC | Replace fan motor |
| Fan speed erratic | PWM control or feedback (FG) line issue | Inspect signal harness and PCB |
| No 5 V signal to motor | Control PCB fault | Replace indoor main PCB |
7. Service Verification
After repair or motor replacement:
Confirm normal fan operation on all speed settings.
Ensure no fan-related fault codes are displayed (e.g., indoor PCB “E:EE” or “A6” depending on system).
Visually inspect both plugs for heat marks or corrosion — replace connectors if discolored.