My Harbor Freight Model 90234 also trips the thermal circuit breaker after running up to where the tank air pressure exceeds 70psi.

This model has a cap-start cap-run split phase motor. The start capacitor feeds the second winding to provide starting torque, the run capacitor feeds the same winding to provide improved breakdown torque at speed.

When the run capacitor fails, the compressor motor has poor breakdown torque, falls below the normal speed range, draws excess current and trips the breaker. (You may even hear it falling back into the start capacitor through the centrifugal switch.)

Harbor Freight 90234 air compressor

Harbor Freight 90234 air compressor

Under the shroud, there is a centrifugal switch inboard of the fan blades. Under the motor are two capacitors stud mounted to the frame. The one with no terminals and only two wire leads is the run capacitor. Mine had exploded like a science fair volcano with simulated puffy lava, and electrolyte and oil dripping down the compressor casing.

You can get a replacement cap from Harbor Freight, but since the original part clearly doesn’t meet USA / UL safety standards, you’re better off getting something from a supply house with proper safety vents. I’d suggest changing the start cap at the same time. I found the .250″ quick connect terminals had been soldered in place. Don’t desolder them, cut them off and crimp new ones on. Don’t solder them on the new capacitor, you’re more likely to overheat the capacitor than add significant reliability to the mechanical connection. (If the mechanical connection is loose the solder will fail quickly under vibration anyway.)

Dear anonymous… an excellent, informative post. Our thanks.



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