Ok, have a Goodman GMNT080-4b Furnace/Air handler. We bought the house last summer, and AC was fine for the rest of the summer, Heat worked just fine all winter. When we went to turn it back to ac, the outside unit starts as normal, inside unit started up, we get good cool air, for about 3-5 minutes, then the indoor blower cuts off. Turns back on a few seconds later. This repeats with shortening time of the blower running, till I cut it off.

The diagnostic light on the control board is on constantly, showing normal operation. It never blinks any codes.

Started out checking for anything burnt, loose, etc. And haven't found anything amiss. Went ahead and replaced the capacitor, with no change. Tested the motor with an ohm-meter, and all the values seem to add up:
Common to Run -- 2.3, Start to Common -- 3.4, Start to Run 5.5, so was assuming motor was ok.


So was thinking it had to be in control board, so rigged up an external switch to test the blower. Turned on switch, blower ran for 5 minutes straight, about till I thought I'd figured it out, then cut off. Flipped switch off, and immediately back on and blower started up again immediately. The only thing I can think of to cause it to switch off now, would be a thermal shutdown within the motor itself. Ran it again till it quit on its own, noticed there was a slight squeal or whistle just before the motor shut off. Immediately opened the panels to check the temp of the motor, and only 30 seconds after it shuts down the motor is only slightly warm to the touch, not near scalding like I would expect (I have a large fan that would burn you in comparison when it has been on for a while).

So any ideas on what I should look for next? If it really is the blower motor how can I make sure besides replacing it? If not what else is the likely cause?

Appreciate any help you can give.

 

It must be the motor since you wired it hot directly. FYI, that furnace is known to have heat exchanger problems. I would not sink too much money in it until it was checked by a professional.

 

These motors are designed to operate against static pressure.
Was this motor installed in the furnace during this test?

 

It must be the circuit board. Because the fan would not come back on immediately if it were a thermal problem. When the fan goes off, is the outdoor unit still running? When the blower shuts off do you still have 120 volts going to it?

 

That is the confusing part, yes before I resorted to trying the direct wire test, the outdoor unit would still be running while the blower cycled on and off. With it direct wired and the blower shuts off, there is still 120 volts being sent to the motor. I was thinking it was likely the control board, till it did that on me.

Yes the motor is still mounted in the air handler.

 

Jumper R and G to rule out a bad connection at the t-stat or board.

If the fan stays on, the board and motor are fine.

If not, check for voltage between the cooling terminal and neutral with the t-stat calling for cooling, the motor disconnected, and the power to the outdoor unit off.

If it doesn't drop out, the board is good.

 

Ok, hooked motor back up as normal to control board. Connected R and G from the control board together (that run to the T-stat). Fan came on, ran for 5 min, off for 30 sec, on for about 3 min, off for 30 sec, on for about another 3 minutes, then I cut the service disconnect when it stopped again. Same behavior as wiring the motor to run directly from 120 VAC.


.

Edited: Figured it out after re-reading the wiring diagram a few times.

T-stat in cooling mode, blower motor and condensing unit disconnected. 28.3 VAC between Y from the furnace, and common/ground.

So does that sound like the control board is good and the motor is shot?

 

Another bit of info: Like I said the furnace had worked all winter long, seemingly ok. So just tried another test, moved the fan speed that was used for heat (orange wire, med-lo) to the cool terminal. Did the R/G short and put power back. Fan ran for 15 minutes, no sign of shutdown, till I finally killed power.

So I guess that was at least long enough for the heat cycle to complete, before the motor shutdown, or at least as we'd noticed it.

I'm so tempted to leave it like that and turn on the AC tonight but better judgment must prevail.

 

Connect the black wire to the heating terminal and run the furnace for a few minutes.

If it still cycles, the motor is overheating and shutting down.

Ensure that the motor openings are clear and look for oiling ports - if they exist, add a couple of drops of electric motor oil. (How to Lubricate a Furnace Blower Motor | eHow.com)

If that doesn't work, the motor is probably bad.

Doing that may freeze the coil.

Try medium high. (I think it's yellow or brown - check schematic)

 

Yeah I was worried about that too, but worse if it started cycling again on its own later on. But I did try it with Medium speed (blue on this motor), it ran for 15 min fine without the cooling circuit on, so went ahead and tried it on cool. Let it run for an hour before I went to bed (and man it was easier to sleep after that) then shut it back down before I did just in case.

Will try that when I get home from work just to be sure. I'm pretty confident myself that the motor is bad or going bad now. At least according to the furnace parts list/documentation the motor is "permanently lubricated" or in other words has a breakdown timer

 

I think your unit has the older goodman heat exchanger which according to others on this board is prone to cracking and popping crimp rings. (new model numbers start with gks, gmh, gmvc)

I would pull the blower assembly and check the heat exchanger prior to investing in a new motor.