/diy/ - Do It Yourself

Need help diagnosing AC motor, 120v 3hp
>starts and runs freely forever
>under load runs for a 60sec and trips thermal protection
>reduced load, (rebuilding air compressor piston, lubed, aligned etc)
>still trips after 1min
So the caps might be bad, 20µF 370v ac 60hz. Both the start and run caps show no signs of stress or bulging. Both caps charge and discharge within the limitations of my multimeter (resistance). I don't have a method of testing spec'ed µF capacity just resistance. How would one test them to determine if they are actually bad before troubleshooting other parts of the compressor?