my water pressure pump just 404d
I still haven't figure it out if it was the capacitor, or if the motor coil just burned
how can I diagnose a bad capacitor using only a cheap multimeter?
What kind of pump is it? If it is an in-well submersible pump, check the electrical wire that runs down to the motor for wear and tear. They tend to vibrate on anything they touch and short out eventually when the insulation is worn off. "Torque Arrestor" can help stop the twisting motion. I have like 3-4 on mine. However, the pump motor uses so much electric that the wires can actually move a fraction of an inch, especially if they are in a conduit pipe anywhere. This too can short them out by rubbing the insulation away.
Don't do this.
One other issue I have with my 1980s vintage one is insects getting in the pressure switch. If you leave it powered up they tend to get btfo before they get between the contacts but if its powered off at the wall they invariably get stuck between the contacts and next time you switch it on at the mains it doesn't go.
If your multimeter has a capacitance function you could use that, but its not an absolute test. If the capacitance is way off then it failed but if its not it could have failed in some other way that the meter can't detect (high ESR or high leakage)
Set the meter to resistance, test across the cap terminals. A healthy cap will start with a low resistance and in under a second rise to infinite resistance. A fucked cap wont reach 100% resistance or will rise very slowly. A totally fucked cap will judt be straight short circuit.
Not 100% reliable but its the best you get without a real cap tester.
This is how to test capacitors.
As for the motor, disconnect power and put multi meter on both legs while set to ohm, if it goes up into the millions the motor is shot. Their shouldn't be much resistance at all through the motor.