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Author Topic: question for Nick or any other a/c guru  (Read 993 times)
thomasinnv
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« on: July 05, 2011, 08:31:16 PM »

my friend has a older coleman mach 15k unit on his bus. the fan motor will not start on its own. if you turn the unit on and give the fan a little spin it will take off and run fine, blows nice and cold. is there a replaceable capacitor for the fan motor? or is the motor toast? I'd hate for him to replace the whole unit for a easy fix.
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« Reply #1 on: July 05, 2011, 11:54:57 PM »

THERE IS A START RUN CAPASITOR. that starts the fan mounted under the top cover should be the smaller one.  cost about 10.00 . more than likely bad .
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Nick Badame Refrig/ACC
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« Reply #2 on: July 06, 2011, 03:15:05 AM »

Hi Thomas,

You could try the capacitor [because it's the cheaper part to replace] but, usually the capacitor that is

attached to the fan motor is a "Run" capacitor and only helps the fan motor run.  You could OHM out the

capacitor to check it's value. The value is marked on the side.  [5uf] [7.5uf] [10uf] and so on.

Good Luck
Nick-
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« Reply #3 on: July 06, 2011, 04:09:06 AM »

Nick, curious about "ohm out the capacitor to check it's value".  The only way I know how to use an ohm meter to check a capacitor is to look for a dead short, or fully open.  I think that if you connect an ohm meter to a good capacitor, depending on it's value you would see an initial short then rapidly increasing up to fully open as the capacitor charged up.  Is that what you look for?  I confess I have never actually tried it, never had the opportunity.  I'd make sure to discharge the cap first...  When i start out in the data communications business i worked in the factory on the test/repair floor, and we used to charge up tantalums and toss them at people - you'd get a little hit from them!

Brian

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« Reply #4 on: July 06, 2011, 09:59:02 AM »

yeah, u should be able to observe a charge, as well as the short test for a cap. (using ohmmeter0
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« Reply #5 on: July 06, 2011, 12:18:12 PM »

you will need a battery operated ohm meter when you make cotact be sure all wires are disconnected. The meter will go up and back down right away.
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Nick Badame Refrig/ACC
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« Reply #6 on: July 07, 2011, 03:10:31 AM »

Hi Brian,

For 0 to 10 MFD values, you can use this.
http://www.supco.com/images/pdfs/Manuals-Instructions/MFD-10%20INSTRgen.pdf

Nick-
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« Reply #7 on: July 07, 2011, 06:38:14 AM »

Haha. "Failure to discharge capacator" section in the last post's link... Reminds me when i accidently touched a large capacator in a OTR truckers illegal linear amp I was fixing...  Grin
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« Reply #8 on: July 07, 2011, 07:09:14 AM »

... You could OHM out the capacitor to check it's value. ...


Actually, no, you can't.  I am guessing what you meant to write is that, if you have a meter which can measure capacitance, such as some higher-end DMMs, you can use such a meter to check the value.  However, I would not call that "ohming it out."  (We can verb any noun here at BCM.)  I would reserve this particular bit of grammatical license strictly for measuring "ohms" which are only a measure of resistance.  As you know, capacitance is measured in farads.  Ohmmeters only measure resistance.

you will need a battery operated ohm meter


Umm, why?  Is there a difference in behavior between battery operated ones and ones with an AC power supply such as a bench meter?  I've never encountered this.

Note that all ohmmeters provide power to the circuit under test -- you can't measure resistance without it.

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The meter will go up and back down right away.


Actually, probably not.  Almost certainly not if it is a DMM and/or the value is below a microfarad or so.  Analog ohmmeters might show some deflection, and even a DMM might show a blip on the scale above a few microfarads.  For capacitors in the range of an AC start cap, you would probably see a small deflection on an analog meter.  Digital meters will be all over the map, depending on their design.  Bottom line: if you don't normally use your ohmmeter to check caps so you know what its expected behavior is, testing an unknown cap with an ohmmeter this way will not tell you much.  That said, here is some advice on using a multimeter (without a capacitance scale) to check a cap:

http://www.repairfaq.org/sam/captest.htm#cttcm



Note that these are the instructions for a very specific meter, one which has a capacitance scale.  Unless you have this meter, these instructions are not going to help.

You can't make a standard ohmmeter or even a multimeter into a capacitance meter without adding a bunch of additional circuitry -- there is no "simple trick."  Most capacitance meters work by using an internal power supply (usually a battery) to charge up the cap with a known current and measuring the time it takes to charge.  It is also possible to measure a cap by passing an AC current of known frequency across it and measuring the voltage drop, but most hand-held meters will not use this method.  Note that even meters with capacitance scales will not detect all problems.  That's because the voltages used for testing are very low, yet the capacitor may be designed for much higher voltages in-circuit, where the behavior might be different.

The best way to test a capacitor is in-circuit with a voltmeter.  However, you need to know something about the circuit and what the expected voltage should be on each side of the capacitor over time.  Also, you need a quality meter with very high impedance.

-Sean
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