Changing capacitor, possible fan motor burn out

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Old 06-12-09, 05:05 PM
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Changing capacitor, possible fan motor burn out

I had an HVAC repairman come out and take a look at my central AC after the outside fan refused to turn on. He determined that the capacitor for the fan needed to be replaced and that the compressor capacitor was about to go too (it's bulged out and does look like it's seen better days). The unit is a 1984 Trane (model BTD742A100B0) and the capacitors were GE Dielektrol (12.5 uf and 40 uf respectively).

I purchased a couple of generic Packard capacitors of the same uf and voltage as those I wanted to replace (all other specs appeared the same). I decided to install the fan capacitor first. Doing so resulted in nothing new happening. The same sounds persisted (a buzzing sound once or twice, then a hissing sound a few minutes later. I could hear the compressor turn on like normal). I decided to soldier on and replaced the compressor's capacitor as well. This resulted in absolutely nothing happening, not even the compressor turned on. I thought about posting here to find out what happened but determined that it was worth another try.

Today I installed the fan capacitor again. To my surprise the fan turned on and appeared to work normally. This was so up until I detected a faint odor (possiblly electric). A spark then emitted from near the fan and the odor became more prononced. There was also a faint "puff" sound. I shut down the unit and opened it up. The motor and its supporting struts were too hot to touch. I replaced the new capacitor for the old one and left the unit disconnected from it's power source.

My question is this: Did I kill my fan motor? Should I reconnect the unit and try again? I don't know for sure if the capacitor I installed for the fan was a run capacitor or a start capacitor, how do I tell? If it's a run capacitor did the fan spin up to the point where it burned itself out? Why didn't the fan start up the first time I installed the capacitor but did the 2nd? Am I doomed to spend the rest of my summer sweltering in the New Jersey sun listening to my wife gripe about how I killed the AC? What do you recommend I do now? Any help would be greatly appreciated.
 
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Old 06-12-09, 09:30 PM
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You probably did not kill the motor. It was probably on it's last legs. If you bought a 12.5uf and a 40 uf cap, then you got run caps. They had a metal body, right? Are you 100% sure that you wired them exactly the same as the old ones had been wired? 12.5uf is a large for a fan motor cap in a unit your size. Was the old one the original? Trane used to use a scheme where they used 2 caps for the compressor and used off cycle heat from one of the compressor motor windings as a crankcase heater. Are you sure that both caps are single caps. Any chance one of them is a dual cap?
 
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Old 06-12-09, 10:53 PM
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Yes the old and new ones have a metal body and I definetly wired them the same. I believe that the old ones were original (they certainly look 20+ years old). I'm sure that they are single caps. They look identifcal to the originals only the 12.5 uf is about 2/3 the size of the original.
 
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Old 06-13-09, 05:37 AM
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if the motor body was super hot to the touch new cap or not the motor is on its way out.....get the motor info and head over to your local Grainger to match it up it is an easy changeout.
 
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Old 06-15-09, 05:19 AM
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Why didn't you just let the HVAC service tech make the repairs he had determined were needed?
 
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Old 06-15-09, 09:00 PM
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He was going to charge about $300 for the repairs. It seemed like a steep price considering the caps cost about $15-$20 apiece.
 
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Old 06-16-09, 05:06 AM
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It may have sounded like alot of $ then, however your system would be running and your motor would not be suspect and you wouldn't be risking further damage, sorry.
 
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Old 06-16-09, 07:21 PM
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Saving money was definetly a part of why I decided to take on this project but a big part of it too was my father-in-law. He went out and earned an AC tech certificate not because he worked in the field but because he wanted to know how things worked and do his own work himself. He passed away about a month ago and everytime I look at this thing I think that he could've diagnosed and fixed it in less than a day. I only knew him for a few years but in that time he influenced how I view my house and my responsibilties towards its upkeep. So I'm going to go out and buy a new motor. I give myself a 1 in 10 chance of being able to install it and make everything run again. But you know what, even if I only break even on this little endeavor I'll feel good about it because that's what he would've wanted me to do. If it doesn't work then at the very least I'll have that much more knowledge under my belt for the next time some similar problem crops up. I'll also have a better idea of what I can and can't do. Ultimately if I totally mess up this thing and have to buy a whole new unit I'm not going to shed any tears about replacing a system that's almost as old as I am. So instead of rubbing my nose in what I should've done maybe you could just say "good effort, try doing this now..." To everyone else I say thanks for your help I'll be looking for more shortly.
 
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Old 06-17-09, 02:23 AM
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so where are we now? The caps are new but the condensor fan motor is kaput? If the $300 was for capacitors only, that's way out of line. Do you have a multimeter you can test the fan otor with? Maker asketch of where each wire from the motor goes, remove the wires, scratch a clean place on the motor end bell or on the body, the idea is to get to clean metal. One motor wire at a time, test from the wire to the clean spot [ohms scale on the meter]. if you get a reading, the motor is grounded. let us know your progress. thanks.
 
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Old 06-17-09, 05:22 AM
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I'am not trying to "rub your nose in it" just being honest. If I insulted you then I apoligize. If you read my posts I help many on this forum.

OK, your best shot at this point is to remove the mtr, make sure you mark on the shaft where the blade is positioned. Take the mtr to a local supply house and match it up, get a cap for the mtr and get a new comp cap.

I'am still a little suspect of a 12.5uf for a CFM on a 3 1/2 ton unit.

Try to get an exact match, if they give you a multi-speed or multi-H/P rescue mtr it will cause more problems.

Replace mtr and both caps (there should be no soldering involved) chk and tighten all electrical connections, make sure you have rotation set correctly.

Cross your fingers and fire it up.
 
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Old 06-21-09, 09:44 AM
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I removed the motor from the unit and was able to check the wires today. I'm not getting any reading from the wires to the motor body (excluding a green wire which appears to ground the unit and is clearly connected to the outside body instead of going into the motor).
 
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Old 06-21-09, 10:50 AM
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Sorry, I should have clarified about the ground wire. Put your meter on its lowest ohm scale and read wire to wire [exclude the ground wire] and post that [each set of readings- for example- the motor may have yellow, purple and brown leads. read yellow to purple, yellow to brown, and purple to brown]. Make sure you're not holding the meter leads to the wire connectors with you fingers because you'll get a false reading thru your body. Holding the motor and meter leads to each other with a plastic or wooden clothespin works well. Also, if the motor has any ventilation openings, smell it up close and personal.
 
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Old 06-21-09, 12:17 PM
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Got it, there are three wires excluding the green: Black, Brown, Purple. Black to Brown gives a reading of 104.1 at a 200 ohm setting on the meter. The other connections are reading 1.
 
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Old 06-21-09, 04:57 PM
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I believe you have a bad motor. The resistance readings should be much closer to each other.
 
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Old 07-14-09, 05:25 PM
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I got a replacement motor and fan blade (old fan blade was rusted onto the old motor's shaft). The new motor however came with four wires (two browns, a black and white). The old just had three (purple, brown, black). I connected the brown wires to the capacitor (a 7.5 uf now) and the black wire to where the old black wire was originally. I decided to attach the white wire on the terminal oppostie the black. When I reconnected the breaker the fan started up immediately. This happened despite the fact that I didn't have a chance to turn the unit on at all. I disconnected the white wire and put a wire nut on it. With this detached the unit seems to work fine (that is for the ten minutes I tested it). Both the compressor and fan come on and I get cold air through the vents. Even the direction of spin for the blades is right. Should I be concerned about this loose white wire?
 
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Old 07-14-09, 05:35 PM
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I forgot to mention that there are two wires leading from the new capacitor (both black with a purple stripe). They are in the same position as they were on the old capacitor. I don't want to give the impression that the only wires attached to the new capacitor are those that come from the motor. I don't know what these wires do but I suspect they serve the same purpose as the white wire.
 
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Old 07-14-09, 06:34 PM
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the new motor should have a label that tells you how to hook it up. Why not just follow that?
 
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