Furnace Blower ECM module questions

[sublauxation]

Pretty sure the ECM module on my furnace is on it's way out. Had a problem with it 6-7 years ago and it turned out to be a Thermistor.

At that time the repair guys would only replace the entire module for $450 so I replaced the $1.50 thermistor myself...which worked great until last week.

Now with the thermostat in fan mode the blower spins back and forth but never gets going. This problem was intermittent for a couple months but switching it on and off would make it run normal again.....until now.

The blower seems to work fine with a different ECM module I bought off ebay but that module isn't programmed for my furnace so it wont spin fast enough to push enough air. To me that would make is seem like my blower motor is fine and the rest of the wiring/circuit board is good (nothing on the board appears burnt out), the problem is somewhere in the ECM again, only this time that module is $750.

If the problem was just some small part like last time.....that savings would buy a lot of boat parts! Anybody have any ideas on what to look at?

 

[Scott Danforth]

start with the motor capacitor

 

[dingbat]

Yep..I’d start with the start capacitor

 

[sublauxation]

There are 2 capacitors in the ecm module, (at least I'm assuming that's what they are). Would it be a mistake to take the 2 off the working ecm (but not the right one for my furnace) and solder them onto the board of the currently non working board?

 

[Scott Danforth]

the motor starting capacitor on the motor, or wired to the motor.

not the ones on the board

 

[sublauxation]

Maybe I'm missing something but unlike a PSC motor the only capacitors are on the ECM board?

I've read elsewhere about it being a mosfet but haven't yet figured out for sure what that is or where they are?

 

[bigdee]

I can't even tell you how many of those variable speed motors I have seen crap out. IMO a over priced bell & whistle gimmick to slightly increase SEER. For $100 you can replace that motor with a simple and reliable split phase, multi speed motor. You will also need a 24 vac relay.

 

[Scott Danforth]

your PSC motor still has a motor start capacitor.

if your $11 motor start capacitor is bad, you get the issues you describe.

 

[Scott Danforth]

if you let us know what furnace you have, should be easy to point out where the capacitor is.

 

[sublauxation]

Furnace is a Bryant Plus 80V, but it's not a PSC motor.
I'm guessing the 2 round things top/right corner of the pic are the start/run capacitors.

The service guy I just talked with told me the fan has to be specific to the ECM module. Does that make sense at all? The motor on these is the "dumb" end isn't it....just doing what it's told by the ECM? So a 1/2 HP fan should be a 1/2 HP fan correct?

I ask because I already bought a 1/2 HP fan that works and they want about $350 for just the motor.

 

[Scott Danforth]

you have a variable speed motor. they have a tendancy to die....on a Saturday night, well after any place is open for parts.

my new AC system is non-variable speed for a reason.

 

[bigdee]

Furnace is a Bryant Plus 80V, but it's not a PSC motor.
I'm guessing the 2 round things top/right corner of the pic are the start/run capacitors.

The service guy I just talked with told me the fan has to be specific to the ECM module. Does that make sense at all? The motor on these is the "dumb" end isn't it....just doing what it's told by the ECM? So a 1/2 HP fan should be a 1/2 HP fan correct?

I ask because I already bought a 1/2 HP fan that works and they want about $350 for just the motor.

Yes a multispeed 1/2 hp with proper rotation will work fine.....and alot cheaper! A ECM motor does not use a capacitor. The electronic module that controls the motor has capacitors but they are used in a completely different way. The ECM controls speed by varying the hertz to the motor...basically it is a AC inverter drive. That motor model is X-13 made by GE and used by all brands. They have about a 50% failure rate over 5-10 years.

 

[sublauxation]

any thoughts on what would make one wobble back and forth but not start up?

 

[AShipShow]

As mentioned, you have a variable speed blower motor. They are a brushless DC motor. They actually rectify the AC into DC, and then use a motor controller IC to control transistors to switch on and off in a H-bridge configuration. The motor doesn't have start and run capacitors. The motor will have a sensor in it (most likely a hall effect sensor) to tell the computer the speed its running at.

All of that is basically to make the point to say, unless your proficient with diagnosing digital circuits, you might not have an option outside of replacing the board. If you want to take your chances, my guess is there is a hall effect mounted to a circuit board somewhere near the shaft. If you can read the PN on it, you can look up the data sheet and if you can find the pinout diagram, you can test it to see if its functioning properly.
The only other thing I could see failing are the transistors. Theres most likely 6 of them for the motor, you could also test them, but now your getting really involved lol.

 

[sublauxation]

Would it be transistors on the motor itself or in the module?

The motor seemed to work normal with the wrong replacement module I bought, it just didn't push much air which I assumed was because it's programmed to a different furnace.

 

[sam am I]

Controller/Inverter sounds bad, however to feel better about the motor before you buy a new controller brd, you can/should make a quik check on the three (orange, purple, green) phase winding for proper consistent resistance from one phase winding to the other (or--> grn, grn--> pur, pur-->or) and that there is not any phase winding's shorted to the motor case, etc.

 

[sublauxation]

Thanks all for the info. Motor checked out, looking like the control board.

I have a guy coming out Thursday who, at least over the phone, sounds like he wont do me wrong. I'll pay for his diagnostics and go from there.

 

[bigdee]

Isn't it odd that we spend $$$ just to save a few cents?

 

[sam am I]

LOL! My Dad is famous for spending a Dollar to save a Penny.

He'll drive 4 miles one way, use approx a 1/2 gal there and back of gas ($1.60) to some BS gas station that is a whole penny or two less a gallon to fill up with 20 gals ($0.20/$0.40 if the tank is plum empty) and think he's saves a "buck two 98".

I can't convince him he's losing money when the sign says the gas is $0.02 less than the other place...........,he says, "a penny saved is a penny earned" and I tell him.......Dad!, you're tripping over dollars to pick up that penny.