I have a fan motor from a mobile a/c unit here which stopped working. It is for the fan that blows the warm/hot air trough the a/c condenser (exhaust air).

240V AC motor with a 4uF (+/-10%) 450V starting capacitor that has two speeds (high and low). 5 connections/wires:

blue, red (low speed), yellow (high speed), brown and white (which connect to the capacitor terminals)

Not running on either speed. No humming, motor does not run even if given a manual kick start turning. 240V AC present and good.

I tested the capacitor with a capacitance meter and it read at less than 2.8uF which is well below specification.

Replaced it with a new capacitor 4uF 500V testing at correct capacitance.

Motor still not running (minimal humming sound), even with manual kick start.

I removed the capacitor from the wiring and bridged the two capacitor terminals.

This way, eliminating the capacitor from the wiring completely, the motor runs now in both speeds perfectly!

Well, kind of: in low speed connection it requires a slight tip to get it running (as is to be expected without a start capacitor). Without kick start there is just the typical and well noticable humming sound of the motor.

In high speed the motor just about manages to start on its own, gradually starting up and then comes to full speed.

Any ideas what the explanation for this is?

My understanding is that in these types of single-phase AC motors the capacitor is wired in parallel to the auxiliary winding.

Would it be possible to bridge the two capacitor terminals in parallel to the capacitor? Can that work or are there other problems with that?

The motor has a thermal fuse in the main winding, I already check that and it is not blown. The blown thermal fuse is a known problem on these types of motors, I had that before. But even in that case the motor would not run with the capacitor terminals bridged.


I checked the wiring again and it is

  • red - blue OL
  • yellow - blue OL
  • red yellow 67 ohms
  • white - yellow 30 ohms
  • white - red 98 ohms
  • white - blue OL
  • brown - blue 0.5-1 ohm
  • brown - red OL
  • brown - yellow OL
  • brown - white OL

-> Motor doesn't run, main windings open

If brown and white are bridged (capacitor connections) it is

  • red - blue 95 ohms
  • yellow - blue 30 ohms

With this bridge the motor runs in both speeds correctly, but does not start without a little manual push in low speed (capacitor not connected in that szenario)

  • 1
    \$\begingroup\$ Could the main or aux winding be open? The cap is only there to provide a phase-shift for starting torque (that's why the motor will spin without it, but not start itself reliably.) \$\endgroup\$
    – rdtsc
    Commented Apr 27, 2022 at 17:20
  • \$\begingroup\$ @rdtsc: Yes, one of the windings is open, or both? See the ohm measurements \$\endgroup\$
    – Rick2000
    Commented May 7, 2022 at 18:37

2 Answers 2


The capacitor is connected in series with the aux winding, not parallel. It seems most likely that the aux winding circuit is open. That could mean an open winding or an open centrifugal switch or other mechanism for disconnecting the aux winding once the motor is at full speed. The means of disconnecting the aux winding could be external to the motor. Determining that would require a circuit diagram for the a/c unit or tracing the wires coming out of the motor.

If the motor is really the capacitor-run type rather than the capacitor-start type, the problem would most likely be an open aux winding. I suspect it is a capacitor-run motor because the capacitor value seems low for a start capacitor and capacitor-start motors are not often used (likely never) for fans.

In either case, there might be some reason that the aux winding is connected through the switch, but that seems extremely unlikely.

  • \$\begingroup\$ There is no centrifugal switch or mechanism for disconnecting the the aux winding. The only external connections are the red (line for low speed), yellow (line for high speed) and blue (neutral). The line connections come directly from the main bord (which controls the compressor and regulates according to temperature settings and sensors etc), blue comes from a main distribution point for neutral (there is another separate fan and the compressor). The brown and white wires go directly to the connectors of the capacitor which is mounted by the side of the motor. \$\endgroup\$
    – Rick2000
    Commented May 7, 2022 at 17:53

The cable colour code, for connecting the motor, is as follows:

Red - connection to line, through selector switch, for low speed

Yellow - connection to line, through selector switch, for high speed

Blue - connection to neutral

Brown - connection to capacitor

White - connection to capacitor

The final schematic, based on your resistance measurements, shows that a break in the winding, marked 'X' in red, is the root cause of the motor refusing to start.

However, with the capacitor bypassed (brown and white cables interconnected) and the shaft manually rotated, the motor will start and run at both the speeds.

enter image description here

  • 1
    \$\begingroup\$ Thank you for the schematic. I have added all ohm measurements above. \$\endgroup\$
    – Rick2000
    Commented May 7, 2022 at 17:41
  • \$\begingroup\$ Hi Rick, My answer has been edited. I have re-assigned the cable colours based on your winding resistance measurements. \$\endgroup\$
    – vu2nan
    Commented May 7, 2022 at 18:17
  • 1
    \$\begingroup\$ Hi vu2nan, thank you, but I do not think that swap is correct. The blue wire goes definitely to the main neutral connection block of the whole ac system which feeds other components with neutal. The yellow wire goes to the high speed line connection point on the main board, not neutral. I think your original schematic was corect. \$\endgroup\$
    – Rick2000
    Commented May 7, 2022 at 18:21
  • \$\begingroup\$ Thanks, Rick, that error has been rectified by swapping the line and neutral. \$\endgroup\$
    – vu2nan
    Commented May 7, 2022 at 18:49
  • 1
    \$\begingroup\$ Blue is neutral, yellow high speed, red low speed. The switch for hi-low is between red and yello. Not with neutral as in the new schematic. \$\endgroup\$
    – Rick2000
    Commented May 7, 2022 at 22:19

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