by Rick
(Terre Haute, IN, USA)

Problem with a Harbor Freight 21 gal. 2.5 HP 125 PSI Cast Iron Vertical Air Compressor.

The unit works great EXCEPT I cannot depend on it starting and keeping running. This condition is inconsistent.

When turning ON using the switch, it will sound bogged down and attempt to turn over slow and then blow thermal breaker on compressor. If I keep moving the switch from start to stop (to prevent popping the breaker) sometimes it will start and run fine.

Harbor Freight 21 gal. 2.5 HP air compressor

Harbor Freight 21 gal. 2.5 HP air compressor

My question – Could the problem be the capacitor? Are capacitor problems intermittent? I always thought that they either worked or did not but would not vary.

Hope someone can address my issue.



Harbor Freight won’t start dependably
by: Doug in

Capacitor CAN change value depending on temperature, but usually they are pretty stable.

From your description, it sounds more like an unloader problem.

Try this: next time it acts bad, try draining the tank some. Once you get to zero PSI, if it’s still not starting, it’s likely the cap or wiring to it/them. (the caps are worth checking, regardless)

But if it does start reliably at some lower pressure, then it’s the unloader or check valve – often you can disassemble and clean to fix.


Thank you to Doug
by: Rick

Thank you for your tips. I will be trying them when the weather warms up. My compressor is more of a summer tool. Use weekly to clean off mower.


Compressor won’t start dependably
by: Doug in

Sure Rick.

Let us know how you make out if the weather ever warms up…


Capacitor replacement
by: Ron

I had similar issue, & yes, it was the capacitor. Couldn’t find a 150 µF axial mount replacement for a decent price, so went with 100 µF (I think anything from 100 µF to 150 µF will work) can type CBB 60 or CBB65 start & run capacitor ($5 – $10 on eBay.)

Cut wires where they entered old capacitor, added female blade connectors and built new mount.

Used caulking gun ($4) tube as holder, by removing handle & cutting to length.

Removed 8 mm bolt from old capacitor & used to attach to old capacitor mount on compressor.

Wrapped black electrical tape around new capacitor to provide firm fit, and added a wire tie to secure it to new mounting.