How to Test an Air Compressor Motor Capacitor

Is Your Air Compressor Motor Failing? It Might Just Be the Capacitor!

If your air compressor won’t start, runs sluggishly, or just hums instead of kicking on, the issue may not be the motor itself—it could be a faulty capacitor. Knowing how to diagnose capacitor failure can save you time and money by avoiding unnecessary motor replacements.

Signs of a Bad Capacitor

A faulty capacitor often mimics motor failure, but here’s how to tell the difference:

✅ Capacitor Failure Symptoms:

• Humming but Not Starting: The motor makes a low humming noise but doesn't turn on.
• Slow or Failed Starts: The motor struggles to start or needs a manual push to get going.
• Visual Damage: A bulging top, cracks, or leaking fluid on the capacitor indicate failure.

❌ Motor Failure Symptoms:

• Grinding or Rattling Noises: Bearings may be worn out.
• Overheating: The motor feels excessively hot or has darkened paint.
• Performance Issues: Inconsistent operation or complete failure to run, even with proper power.

How to Test Your Air Compressor Capacitor

Using a Multimeter to Test a Capacitor

Yes, you can test a motor capacitor with a multimeter! Here's how:

1️⃣ Safety First – Before testing, discharge the capacitor by placing an insulated screwdriver across the terminals.

2️⃣ Set Your Multimeter – Turn it to the capacitance setting (marked as "Cap" or "µF").

3️⃣ Measure the Capacitance – Touch the multimeter probes to the capacitor terminals. The reading should match the rated capacitance on the capacitor’s label.

4️⃣ Check for Shorts – Switch your multimeter to the resistance setting (Ohms). Touch the probes to the terminals. The reading should start low and increase to infinity (or OL for overload). If it stays low, the capacitor is shorted.

5️⃣ Interpret the Results – If the capacitance value matches and there are no shorts, the capacitor is likely fine. If not, replace it.

Need a multimeter? Check out our selection here.

When to Replace the Motor Instead

If you’ve tested the capacitor and your air compressor still won’t start—or you hear grinding, rattling, or excessive overheating—your motor may be failing. At Master Tool Repair, we carry a wide selection of replacement air compressor motors and capacitors to get your unit back up and running.

✅ Need a new motor? Browse our selection here.

✅ Want a visual guide? Watch our step-by-step video on testing a capacitor.