Air conditioner fan motor capacitor question

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Boonieman

Boonieman

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Joined
Feb 19, 2017
Posts
382
Location
Meade County, Ky
This is for the electrical gurus here. I?m working on an old Rheem air condition on an old house we bought. It was working, but now the condenser fan motor won?t run. I suspect the dual mode capacitor, but it doesn?t show any physical signs of being bad like they usually do. I took the capacitor out and used my old Simpson analog volt-ohm meter (I have digital as well, but I?m old school, ok ?) and put the meter to +DC ohms and set it on 100. One side to common, and one side to fan motor....no reading. One side to common and one side to Herm?s...no reading. Usually, the needle will bump up on the fan side and come back down on a good capacitor. On the Herm?s side it usually builds ohms slowly. On the one, nothing. Don?t get me wrong, I?m a jacket mechanic and just learned by experience. With the hotter weather now, air conditioner guys are at a premium with estimated two weeks before anybody can come, so trying myself. I didn?t want to test voltage from the capacitor to the fan with the unit attempting to run, because I wasn?t sure what voltage to expect from the capacitor and don?t want to fry my beloved old Simpson meter.
 
I admit it does sound bad

As you noted (I am assuming this is an analog VOM) when you first connect to a fully disharged Cap the needle should show near short slowly building up .. in fact some meters have a diamond on the scale. you start a stopwatch when you touch the lead to the cap and when the needle crosses the diamond you stop the watch. the time is determined by the value of the cap.

Showing open circuit is not good nor is a continous short.
 
It does sound like the capacitor is open, you should see the meter "kick" upwards.  The capacitor charges from the meter, so try shorting the capacitor's terminal to ground to bleed off any charge before you test it.  Of course, make sure the power is off before you do this.

Does the compressor motor run?  If so, you can kickstart the fan motor by giving the fan blade a spin with your hand like starting a propeller.  It should then run until you turn it off.
 
That is what is confusing me, I tried the ?hand spin start? method and the fan wouldn?t run. I did check voltage at the start relay and voltage is coming thru both sets of contacts, but I realize that doesn?t necessarily mean the contacts are good. This is an old unit with only mechanical contactors and coils, no circuit board. The compressor does come on, just not the fan. I did not check ohms across the motor windings on the fan motor.  I have a new cap in hand and will swap out tomorrow and let you know if that work. I just wasn?t sure if I was testing the capacitor properly. Sounds like according to you guys I did it correctly. Hard to diagnose this stuff over the internet because you never know what the original poster (me) actually did and how much working knowledge the person has. I appreciate your responses.
 
Thank fellas for your advice and encouragement. Replaced the motor run capacitor. The fan and rest of system fired right up. So giving the fan a spin to find out if it is a bad capacitor might work some of the time, but not always. Even at age 63 still learning. Thanks again!!
 
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