AC condenser fan problem

I have a 20 y/o split system AC and recently the condenser fan stops running. Most of the time when I first start the unit the fans runs and the system cools but the fans does not seem to run at top RPM but it runs. Then after a couple minutes the fan stops. I've replaced the two capacitors and same thing, runs a couple minutes then fan stops. I ohm checked the fan motor per specs I got online which was this: Black & Brown, Black & purple, and brown & purple which is to be the sum of the first two and the results seemed OK. Total ohms for brown & purple was approx 120 ohms othe two were approx 60 oms each so it seems the fan motor is ok, I think.
Today I read that the fan could shut down if low on refridgerent through the low pressure switch.
Given what I've said anybody have an opinion on what the problem could be?
Thanks, Pete

 

At 20 years old Id say get a new AC unit or just the fan motor. 15 years is about the life of a AC .Yes you can Ohm out the wires there . But to tell just how good a motor is you have to use a amprobe on it to tell what it is doing.

I dont think you unit has a low cut out there. Also if you did it should turn off the fan and compressor both.

 

The problem with your motor is not electrical it is probably a bad bearing when it starts cold it will run for awhile but when the bearing heats up the resistance to rotation increases and the motor slows down and stops.
If this unit is 20 years old you could pay for the new one with your energy savings but if you have to fix this thing then replace the fan motor and capacitor and you should be fine.
Remember there is no better way dollar for dollar to green up your home than to properly upgrade your home Air Conditioning System