Kobalt VT6389 Air Compressor Just Hums & Doesn’t Build Pressure

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Going nuts here, says James. I  bought a Kobalt VT6389 30gal compressor in August. It suited my needs perfectly and I hit a good price and sale. Honestly, it sat in the shop for a while until I got around to plumbing air lines. I did fire it up a couple of times, but never drained the tank until the motor kicked back in.

Here is my issue. The tank fills to 150psi with no problem. Once it hits 100 or so, the motor goes to start, but just hums. It will not fill start up correctly until it hits 25psi. At that point, the motor turns over just fine and begins to fill.

Kobalt air compressor

I have been through your site top to bottom and have tried everything suggested. Changed the unloader valve and the check valve, checked the caps, and confirmed that I was on a 20amp breaker. Only things that are plugged in is a wireless router, some surge protectors with nothing connected and a 2 LED overhead lights. I have turned them off and still am having problems.

Any suggestions of where to start?

James, please, don’t go nuts! Your air compressor isn’t worth it! We suspect your humming compressor has a power related issue.

We assume it’s 14/2 wire that feeds the socket into which you plug your compressor.

No extension cords, right? How far is the socket into which you plug your compressor from the power panel?

If it’s a long way, amperage drop over the long run may just put your motor out of the sufficient-power category. That’s one issue, though since you say it used to work fine, not the most likely one.

That you compressor will start OK with 25 PSI in the tank, but not the normal cut in pressure of what… 105 or so, says to us that the additional load against the pump when trying to pump air into the tank is enough to stop the motor from starting.

In the hope that it’s not the motor itself, we would still wonder about whether the start cap is weak and can’t fire up the motor against the normal back pressure from the tank. You say you checked the caps… what does that mean? Can you please explain how you checked them?

One more thing.  Is the Kobalt still under warranty? If so, the next response should be to take it back. Good luck.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)

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