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Old 07-13-2008, 11:59 AM   #1
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Trouble shooting a Blower Motor


I have a Trane XE 80 Furnace, Yeah Yeah I know probably Junk. However, It was in the house before I purchased the house 6 years ago. It is only around 7 years old and has worked fine with no problems other than a dirty filter. Yesterday the A/C was working fine then all of a sudden no air was coming from the vents. I checked the furnace and the blower was running, however, it seems as if its running in slow motion. I removed the blower motor and cleaned it out and there is nothing blocking the flow of air or the motor itself. I came across a test for a motor run capacitor. Which confused me. Using a 100watt light bulb and a DVOM. Would someone tell me. Is it the motor run capacitor or is it simply the blower motor took a dump? Also the entire A/C system froze over. Is that caused by the lack of air not being moved out of the furnace?


Last edited by cougar; 07-13-2008 at 02:10 PM. Reason: adding more information
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Old 07-13-2008, 09:42 PM   #2
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Trouble shooting a Blower Motor


froze because of lack of air probably, need to check the cap with a multi meter set to Mf.

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Old 07-14-2008, 08:57 AM   #3
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Trouble shooting a Blower Motor


If you don't have a C meter, put the cap in parallel with a 9v battery, a one megohm resistor and a high impedance (20 megohm input impedance) DVM.

Disconnect the 9v battery and time how long it takes the voltage to drop to 37% of its initial value. This number of seconds is the value of the cap in microfarads.
E.g., initial voltage = 9.6v, and it takes 11 seconds to drop to 0.37 x 9.6 = 3.6v, so the cap is 11 uF.



Or, wear face protection because sometimes caps behave unpredictably, and be mindful of a shock hazard so that you don't qualify for a Darwin Award,
put the cap in series with 120v and a 25w to 100w incandescent bulb and measure the voltage across and the current through the cap.

If the cap is shorted the bulb will probably light full brightness.

If the cap has failed to an open state. the current, I, will be ~ 0.

Otherwise,
Z = V/I, with V in volts and I in amps, and at 60 Hz, C in farads = 1/(377 x Z).

E.g., let's say V/I comes out to 240 ohms. 1/(377 x 240) = 0.000011 farads. This is 11 uF.


You use a bulb since everyone already has bulbs and so you don't have to buy a hi wattage resistor.
This second method more nearly approximates real world conditions.

Last edited by Yoyizit; 07-14-2008 at 09:14 AM.
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Old 07-14-2008, 03:20 PM   #4
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Trouble shooting a Blower Motor


For safety's sake and to protect measurement instruments it's good practice to discharge capacitors before working with them.

For 11 uF and 1 megohm, it takes 1 minute to discharge the cap by putting the resistor across the capacitor. If the cap is 100 uF it might take 10 minutes.

You can confirm that the cap is discharged by measuring the voltage across it. Theoretically the voltage never goes to exactly zero, it just gets less and less.

For capacitor values much larger than ~11 uF, the Grainger catalog recommends a 15K 2w resistor [which you can buy at Radio Shack].
With a resistor this low across the cap, 1 minute will discharge any value cap that you'd reasonably find in this type of application.

Don't use a lamp to discharge the cap. It might work, but it might blow out the lamp.
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Old 07-14-2008, 04:52 PM   #5
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Trouble shooting a Blower Motor


While your at Graingers, pick up a new capacitor for about $8.00.
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Old 07-14-2008, 05:23 PM   #6
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Trouble shooting a Blower Motor


Not an HVAC guy, but been down this road, and similar roads, many times.

The cap most likely failed for its own reasons or not so likely because it was overstressed by some external, root cause.

Some possible outcomes:

You buy a new cap for $8 and up [mark it so you know which cap is which], and the symptoms are gone, and the motor works for several years more.

You buy a new cap for $8 and up, and the symptoms are unchanged but the new cap hasn't been fried: problem elsewhere and old cap was good.

You buy a new cap for $8 and up, and it promptly blows out because the problem is elsewhere, and the symptoms are unchanged, and you may or may not know that the new cap is now fried: bad but unlikely outcome.

You run one of the tests and find the cap is bad and confidently buy a new cap. If the symptoms are still unchanged, check the new cap. In fact, check it before you install it using the 9v method.

You run the test and the cap is good: look elsewhere for the problem, hold off on the trip to Grainger because you might be buying whatever other part is bad.



Out of these five outcomes, how many are good? Other things being equal, those are your odds of a good outcome.



I guess the test buys you confidence.
If you pay yourself ten tax-free dollars per hour to do this kind of thing, you paid $10 for the boost in your confidence.
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Old 07-14-2008, 08:20 PM   #7
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Why oh why is it that every time I try to be helpful, Im...Im...just...just...misunderstood!!
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Old 07-14-2008, 08:30 PM   #8
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Quote:
Originally Posted by 8 Ball View Post
Why oh why is it
Why Oh Why Is It? Yoyizit?
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Old 07-14-2008, 10:49 PM   #9
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Trouble shooting a Blower Motor


cougar,
Does your blower motor have an attached motor control module on it? If it does, that's probably what's blown.
I have a 1998 Carrier 4-ton heatpump, and I average replacing that module about every other year (about $200 for that part). I think that it was a design defect.
I learned how to change it out by watching the serviceman a couple of times, and I keep one on hand and DIY now it whenever it blows.
It invariably fails on a holiday or weekend. LOL
Mike
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Old 07-14-2008, 10:53 PM   #10
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I would just like to say Thank you to Everyone who replied. The problem has been fixed and dint cost me anything other than 20 minutes of my time being nosy. I removed the access panel for the evaporator area. And found that over a course of 7 years there was an over abundance of Pet Hair built up. Cleaning this out fixed my problem. So it was lack of air flow. Now the A/C is blowing out of the vent stronger than I remember. So, Again. THANK YOU!!
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Old 07-15-2008, 08:14 AM   #11
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Quote:
Originally Posted by Yoyizit View Post
Out of these five outcomes


Make that six; title it "other".

Troubleshooting 101; if it can be done quickly and at no cost, look for simple stuff first.


Last edited by Yoyizit; 07-15-2008 at 08:51 AM.
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