Split AC compressor not working but fan is running. Why?

Today, in our AC compressor troubleshooting series, I will be guiding you on how you can troubleshoot and fix a window or split ac compressor not working but fan is running problem.

Well, here’s what might be causing this problem and how you can fix it:

Window split ac compressor not working but fan is running – what you can do to fix it

Start by…

Testing the capacitor – it could be to blame

Some split AC models come with what we call a dual/split capacitor – one capacitor that houses two capacitors.

On such models, it could be that the herm part of the capacitor has failed but the fan capacitor is still fine hence the fan running.

In turn, this would result in the condenser fan running but not the compressor.

Similarly, on models with two capacitors, it could be that the herm part of the run capacitor has failed but not the fan part.

With that in mind, the best way you can troubleshoot if the capacitor is to blame is by testing it…

Now, to test the capacitor, use a multimeter to measure the capacitance (in microfarads) of the capacitor terminals.

Next, compare this reading with that written on the capacitor.

If the reading you get is different from the capacitor’s rated reading, it means that the capacitor is faulty.

The good news is that the capacitor is fairly cheap – check current split ac capacitor prices on Amazon.

Test the compressor’s thermistor (thermal switch)

This thermistor is normally attached to the side/top of the compressor.

Its purpose is to shut down the compressor when the compressor overheats.

Now, if this thermistor fails, then your compressor might not start.

To test it, you can measure its resistance and compare it to the manufacturer’s readings.

Alternatively, you can jumper the two leads it’s connected to (and be sure to wrap it with electric tape to isolate).

After this, proceed to run your AC – if the problem goes away, replace the switch as it is bad.

Test the air conditioner’s evaporator thermistor

This is a temperature sensor located on/near the evaporator coils.

Its work is to monitor the temp on the evaporator coils by detecting when there is frost on the coils.

If there is frost, it will relay this info to the control board which will in turn toggle off the compressor thereby allowing the coils to defrost.

Now, if this thermistor is faulty, it will end up relaying the wrong signal to the control board. And, this will result in your split ac compressor not turning on though the fan might still be running.

But, before we crucify this thermistor, it’s good that we test it to see if it is to blame.

How to test the thermistor

  1. While measuring the temperature on the thermistor, test the resistance between its wire connections/terminals.

Pro tip: The readings might vary from manufacturer to manufacturer. So, be sure to have your AC’s service manual nearby. Nonetheless, in most models, the evaporator thermistor’s resistance will be around 6.1 kΩ at 20⁰C and around 4.9kΩ at 25⁰C.

  1. Warm the thermistor by covering it with your hand. As the temperature rises, the resistance reading should start going down since this is an NTC (Negative Temperature Coefficient) type of thermistor.

If your reading does not lower (or does not match your manufacturer’s recommended reading, it means that the thermistor is gone and will require to be replaced.

The start relay might be bad – so, test it

As you might know, the start relay is responsible for starting the compressor.

As a result, if the start relay is faulty, it won’t start your compressor (won’t affect the functioning of the fan).

Now, the best way to test the start relay is by measuring its resistance.

However, this brings in a new problem – start relays vary from manufacturer to manufacturer with each having a slightly different testing procedure.

That being so, follow the steps in the video below – it details the different ways you can test different start relays:

Split AC compressor not working but fan is running / Window ac compressor not working but fan is running – more troubleshooting steps

Moving on…

Test to see if the compressor is faulty

As you might know, a bad compressor will definitely not start.

Fortunately, you can easily test the compressor to know if it is faulty.

To do so, follow the below steps:

  1. Unplug the AC for at least 30 seconds – if you unplug it for less than 30 seconds, the compressor will not start for 3 minutes.
  2. Reconnect the AC to the outlet and power it on.
  3. Set the thermostat settings to: cool mode, lowest temp, and highest fan speed.
  4. Carefully listen for the compressor starting (might take up to 30 seconds). Now, if the compressor is healthy, you will hear it engage followed by a steady hum.

However, if you don’t hear it start or hum, it could be that it’s gone.

Subsequently, you’ll need to have an EPA-licensed HVAC technician test/change the compressor.

This is because the freon-based refrigerant that runs through the compressor is toxic.

It could be that the control board is gone

Finally, newer ACs have a control board that controls the other components (including the compressor).

As such, if the control board is faulty, it might result in your split AC compressor not working but fan is running issue.

Now, to test the PCB, first confirm that all the wire harness connections to the board are good.

And, if any are frayed/burnt, be sure to replace them as necessary.

Also, if the board appears to have a burnt relay or any other visual sign of damage, replace it.

Pro tip: Control boards cannot be tested using a multimeter. As such, it could appear fine but it is not. As such, sometimes replacing the entire board does the trick as the board could be bad.

Can AC capacitor run but not compressor?

Yes and no. Allow me to explain…

Normally, it depends on the capacitor in question.

If the AC has two capacitors, it is possible for the start capacitor to run but not the compressor.

This is because the compressor depends on the herm’s part of the run capacitor which is independent from the start capacitor or the fan part of the run capacitor.

The same thing applies to split capacitors – the capacitor can run (and so will the fan) but the compressor might not start.

Again, this is because the compressor relies on a different part of the capacitor and this does not affect the entire capacitor.

Split AC compressor not working but fan is running – recap

In conclusion, to troubleshoot and fix your split AC compressor not turning on problems do the following:

  • Test the capacitor(s)
  • Test the compressor’s thermal switch
  • Check if the start relay works
  • Test the evaporator’s thermistor
  • Check if the compressor is starting
  • Check if the control board is faulty

Till next time!


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