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paulcats02

dead Friedrich AC unit, Model KM18J30B-3

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paulcats02

Unit is a Friedrich Quietmaster Model KM18J30B-3.

It doesn't turn on (doesn't work in "cool" or "fan only" modes). I have measured voltage at the outlet I am plugging into to make sure that is not the problem and there is 110V there. I opened up the unit and tested the switch, it appears to be fine (switched "off, I measure no continuity where in any of the "on" position, I have continuity). There is a big metal dual AmRad Ultramet capacitor that looks to be for the compressor and fan, I could measure the capacitance on that but am not sure what #'s to look for. On the cap it says 30.0/7.5V, I'm guessing that the former is for the compressor and the latter for the fan. Manual and or schematics sure could help!

Thanks. :?

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jkirksey1889

30 between common and herm or whatever and 7.5 between common and fan. And that's not volts it's microfarads.

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paulcats02

oh, i was thinking to measure capacitance (with my multi-meter set to capacitance) with the unit powered off... but that's makes sense to check with the unit turned on (either to "fan only" to look for lower value or any of the "cool" settings for the higher) albeit carefully with multimeter.

attached is the image of the cap...

post-51113-129045118173_thumb.jpg

post-51113-129045118612_thumb.jpg

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paulcats02

front label of cap

post-51113-12904511818_thumb.jpg

post-51113-129045118615_thumb.jpg

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jkirksey1889

To check the capacitor you need to unplug the unit (for safety) and remove any wire(s) from the common on the capacitor.  Check between common and fan (should be around 7.5) then check between common and herm (around 30).  If this doesn't read within +10% or -6% of those values you have a bad capacitor.

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paulcats02

would that still be the case if the unit has not been working for awhile and the cap has no charge left?

even moreso, prior to testing it the other day i shorted it out to discharge it (unit powered off and unplugged, placed screwdriver blade between leads common and green for fan then common and black for compressor)

in any event, i'll try what you suggest and post reply.

 

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paulcats02

got about 6-12 nF between the test point(s) and common; basically nothing. it's about a $15 part but i'm not a big fan of shotgunning it so maybe i'll double check continuity on the switch to make sure that's not the prob...

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jkirksey1889

6-12? The fan should be about 7.5, according to the unit.  The compressor (Herm) should be about 30.  If you do not have that you have a bad cap.  It does not matter how long the unit has been unplugged or has not worked.  It has nothing do do with the "charge".

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paulcats02

you're saying that with the unit off and the wires off of the common lead of the cap i should still get a read?

you know i used to be handier with components, meters and electronics but my work has become more and more computerized so as time goes on i become rustier and rustier except when i work on my home appliances, infrequently as it is...

 

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RegUS_PatOff

[user=51113]paulcats02[/user] wrote:

got about 6-12 nF between the test point(s) and common; basically nothing.

yes, capacitors are tested without power ...

12 nF .. that is practically nothing ...

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paulcats02

actually i bought a replacement capacitor which i measured the capacitance of (right out of the box); it measured fine as expected:bananadance:.

prior to replacing the "suspect", for grins and giggles i measured it... lo and behold, it measured fine; when i measured it previously i had only removed the wires from the common terminal and left the wires on the fan and herm terminals so maybe that threw off the measurement? :?

so, at this stage of the game, i guess i need to look at the schematics and recheck the switch (maybe it's bad after all though it seems to test fine) as well as measure voltage at various points to see where it's dropping off.

 :nerd:

what's throwing me off is that neither the fan nor the AC work, that's why I was suspcious  that there was a commonality that was/is the culprit but i just don't know which way to go...

:yikes:

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appl.tech.29501

your cold control ( thermostat) could be bad. bypass it by tying the 2 wires together.

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paulcats02

>your cold control ( thermostat) could be bad. bypass it by tying the 2 wires together.

thanks for the reply by the way. was away for a while and now we're back it's hotter than heck here in NYC. what 2 wires am i to tye together, i have an alligator clip to alligator clip cable ready to go...?

by the way i have a schematic but it's for a KM18L30-C,-D which is not the same as the unit i have...

regards

p

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