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Old 10-03-2011, 05:07 PM   #1
Richard and Rhonda
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Default Dometic Basement Air Blower Motor Failure

I am posting to provide info that may help someone in the future.

The blower motor failed on one of my basment units. From the inside, the result is either no cold air, or a tripped circuit breaker. From the outside the unit makes a loud buzzing sound when it trys to start.

Here are the things I learned in the diagnosis of this problem.
If you hang an ammeter on the blower motor or the compressor motor it appears they are both pulling maximum amps. I eventually used a very complex technique to determine which one was locked. I put my hand on each motor. The buzz was definitely in the blower motor.

The problem with my motor, and most others that are referenced on this site, is that the bushings simply wear out, causing the motor to drag.

However, to be a little more scientific, here are some reference numbers you can use to check both the blower motor and compressor motor before you tear into the refrigeration system.

The resistance readings for the compressor motor were 0.6 ohms black to white, 7.5 ohms white to red, and 6.9 ohms red to black.

The resistance readings for the blower motor were 9.2 ohms black to white, 66.2 ohms white to red, and 57.2 ohms red to black.

The current system is wired to use a run capacitor of 20 mfd for the compressor, and 7.5 mfd for the blower. It also uses a start capacitor of 88-108 mfd wired in parallel with the compressor. The start cap uses special resistor that is temp compensated to disconnect the start cap after the motor is running. This little resistor, made by Ceramyte, part # CM O 305 C20 is kind of old school technology. I replaced it with a standard hard start capacitor made by Supco, available at any HVAC supply place and/or Amazon.com. The Supco is self contained on the disconnect.

I did get a replacement motor from Newell. My attempts at finding a suitable replacement through all the typical sources were not successful, and Newell's price was in line. I did take the old motor apart of course to see the failure. I will try to replace the bushings, however the motor shaft also appears to be worn, so I don't think it would last very long even with new bushings.

As far as replacing the unit, I initially tried to get the motor out of the blower housing. Wrong! It was much easier to remove the entire blower housing, four screws.

Hopefully, this will help you diagnose a similar problem, or better yet keep you from ripping out a perfectly good compressor only to find that's not the problem. That's where my head went when I heard the loud buzzing coming from the unit. Locked compressor. Then a tripped breaker. More locked compressor evidence.

The quick and dirty dead give aways on diagnosis were: no air blowing out of basement unit, the blower squirrel cage not turning, and the buzz originating at the blower motor.

Seems simple in hindsight, eh?
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Richard Rhonda Ty and Alex Entrekin
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Old 10-03-2011, 06:25 PM   #2
busnut
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I recently performed the same task. Also tried to remove blower motor, but gave up and remove the blower assembly.

The reason the motor failed was the bearing went dry. In fact the motor mfger says to lub them every two years. Ya right!! But I may remove the other three and relub them this winter.

Anyway had no luck in finding a new bearing and the shaft was slightly worn also. Anyway the bad bearing was on the blower end, so I removed the bearing on the other end and swapped them around. Did it work? Appears to have, but I put in a new motor anyway. I will keep the garyrigged motor in case I need it.

It is my opinion the this motor should have had needle or roller bearings, but it appears price won again and they used a bushing bearing.

As luck would have it, it was A/C number four which is mounted in the bay by the entrance door back in the middle of the bay. Lots of work to get this fat body back in there.
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Old 10-04-2011, 11:32 PM   #3
Richard and Rhonda
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Well you just earned a big time atta boy.

Never thought about it that way. Say you are in the middle of Arimexico and it's over a hundred. You lose the blower motor. A ha, pop off the end caps, reverse the bearings, add a wee bit of grease, and you have AC until UPS delivers the replacement motor.

Cool, in more ways than one.
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Richard Rhonda Ty and Alex Entrekin
1995 Newell # 390 DD Series 60, Allison World Trans
Subaru Outback toad
CoMotion Tandem
Often wrong, but seldom in doubt
Rhonda's chronicle https://wersquared.wordpress.com/
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