16 replies to this topic

[Qball]

Like the title says, I have a Trane XR80 furnace and the blower motor makes a loud buzzing when it's trying to spin. It never achieves any RPM, even if I manually spin it. I removed the blower motor and it seems rotate smoothly. Do you think the run capacitor is the culprit? This unit is @ 10 years old. Thanks for any advice you can give.

[RegUS_PatOff]

... Do you think the run capacitor is the culprit?

could be ..
you can test the Capacitor (somewhat) with an OHM meter
After discharging the Capacitor, use your OHM meter on the 20 Meg scale...
should start with a low reading and slowly rise.
Reverse the leads and try again ..
(may start with a "negative" OHMs and rise to 0 and then infinity)

[Qball]

Thanks! I'll try that tonight.

[Dan Webster]

Look for swelling and leakage. Carefully disconnect the cap and try to discharge it with a insulated screwdriver. It should make a small pop if it is holding a charge. No pop no good.

Edited by applianceman18007260692, 01 August 2011 - 03:19 PM.

[RegUS_PatOff]

... It should make a small pop if it is holding a charge. No pop no good.

... not necessarily...
It's an AC Cap, if the unit was turned OFF at or near the "0 crossing" of the AC Sine Wave, it won't "pop"

[ACtechGUY]

In all my years of dealing with "run" capactors in HVAC equipment , I have only had one hold a charge.(and zap the crap out of me) That was a capacitor on a failed (open winding motor).

Run Capacitors discharge via the motor. When a "good" capacitor is in the circuit with a "good" motor the capacitor will discharge any voltage Very rapidly (as in seconds or less).

If you can get a capacitor to "pop" . you likley have a failed motor or broken wire in the motor circuit.

YOU SHOULD NEVER SHORT THE TERMINALS OF A CAPACITOR . You may DAMAGE a capacitor by dead shorting the terminals or it may decide to explode in your face.

If you are unsure if your capacitor is holding a charge, Use Your voltmeter set to the AC volts setting . Put your leads on the terminals. the capacitor will discharge thru your meter . and because you have a meter that is designed to not be a dead short , you do not damage the capacitor.

Edited by ACtechGUY, 01 August 2011 - 05:50 PM.

[ACtechGUY]

In response to the motor problem..... It is not likely the capacitor that is the problem. If you have a bad capacitor you can almost always "push start " a fan motor when it is starting and the motor will run , but not a full RPM and it will likely overheat and shut down.

At 10 years old I would think a failed motor is very possible. 10 yrs is the standard lifespan of a motor these days( sometimes only 5 years!!)

A failed (electrically ) motor will spin just fine by hand .

You can Check for a failed motor by taking an OHM reading with your digital volt meter.

On a 1/2 hp 115volt blower motor here are the readings you should generally see:


Between White(common wire) and Black (high)- 6 ohms
Between White and blue (medium) - 9 ohms
Between White and red (Low) - 15 ohms
Between white and brown with white stripe(capacitor) - 0 ohms or very little
Between White and brown ( capacitor) - 45 ohms

Resistances will vary from motor to motor.
Also your motor may be a different horsepower along with the very distinct possibility that it may also have more speeds than I have shown .
If your motor differs from the one i show the resistances would be not be the same ,but as you can see , there is an increase in resistance as you go from the highest speed to the lowest speed. One of the Brown wires will always have little to no resistance and the other brown wire will have the highest resistance of all.
This will apply to all motors.

If your motor does not conform to this , it is BAD.

Edited by ACtechGUY, 01 August 2011 - 05:42 PM.

[Qball]

On a 1/2 hp 115volt blower motor here are the readings you should generally see:


Between White(common wire) and Black (high)- 6 ohms
Between White and blue (medium) - 9 ohms
Between White and red (Low) - 15 ohms
Between white and brown with white stripe(capacitor) - 0 ohms or very little
Between White and brown ( capacitor) - 45 ohms

Resistances will vary from motor to motor.
Also your motor may be a different horsepower along with the very distinct possibility that it may also have more speeds than I have shown .
If your motor differs from the one i show the resistances would be not be the same ,but as you can see , there is an increase in resistance as you go from the highest speed to the lowest speed. One of the Brown wires will always have little to no resistance and the other brown wire will have the highest resistance of all.
This will apply to all motors.

If your motor does not conform to this , it is BAD.

Ok, I did what you suggested and thanks so much for that. My readings are:

It's a 1/2 horse 115v 1075rpm 4 speed GE motor Part number (I think) is P918AS

Black = 2.8 ohms (High)
Blue = 3.4 ohms (M-High)
Yellow = 4.1 ohms (M-Low)
Red = 4.9 ohms (Low)


Brown /white stripe = 49.2
Brown = 5 ohms

Bad motor, eh?

Edited by Qball, 01 August 2011 - 08:09 PM.

[Qball]

could be ..
you can test the Capacitor (somewhat) with an OHM meter
After discharging the Capacitor, use your OHM meter on the 20 Meg scale...
should start with a low reading and slowly rise.
Reverse the leads and try again ..
(may start with a "negative" OHMs and rise to 0 and then infinity)

My cap behaved exactly like this. So much for a cheap fix...

[Qball]

Anyone have this motor for sale? GE motor 5KCP39LGP918AS

[ACtechGUY]

All you need is a generic 3 speed(because you are only using 2 or maybe 3 of the speeds) . 1075 rpm 1/2 hp 115volt blower motor.

You are near big city . take a short drive and you should be able to get one at a good price locally. you should pay no more than $100 for that motor.



******It does not have to be a GE motor. Blower motors are almost universal.

This motor is VERY COMMON and not too expensive. I would include a link to ebay for a motor but Mr samurai seems to forbid ebay links. maybe Mr samurai should think about selling some HVAC parts if he is gonna stop links to ebay............................

Link to price compare site for your motor

Edited by ACtechGUY, 01 August 2011 - 10:13 PM.

[Qball]

ACtechGuy, you're my savior! Thanks SO MUCH!

[Qball]

One more question. If I do go with a 3 speed motor (which I am leaning heavily) which speed should I not hook up? I'm going to assume to omit the M-Low as my current yellow wire, which a three speed will not have, goes to M-Low.

[ACtechGUY]

Here is how it works.......

HIGH speed is used for cooling
Medium high or medium low can be used for Heat
Low speed is for constant fan.


More modern boards tend to have three outputs .This is where 4 speeds may be handy , but not really needed.
Modern boards will have a " cool" speed and "heat speed AND "constant" fan speed outputs. Most older control boards only have 2 motor outputs, heat and cool.

The extra speeds are for flexability . Say you wanted your heater to put out hotter air. You would choose MED- LOW or LOW speed on a 4 speed motor . Say you want to dehumidify a bit more , you would use a lower speed for cool.

But in reality , the differences between speeds is is not too much to be concerned about. you will not hurt anything by choosing Medium speed on a 3 speed motor when your old motor was hooked up to med -low.

Don't worry - -- Be happy

[Qball]

Ok, I just received my new motor. It's an AO Smith 3 speed. The problem I have is; There's no white wire. I have a Yellow, Black, Red and Blue. There's the two for the cap plus there's a set of shorter wires: an orange connected to a brown and a white connected to a purple. It's like it's looped back into the motor.



What goes where?

edit: Sweet Georgia Brown. I get so worked up over nothing. I finally bothered to look at the label on the motor. All is well. Stupid rookie.

Edited by Qball, 04 August 2011 - 08:40 PM.

[RegUS_PatOff]

.. Sweet Georgia Brown. I get so worked up over nothing.

[Qball]

Ha ha!! Thanks for that. It's fixed! Thanks for all the help everyone! I couldn't have done it without you.