Furnace blower motor wiring question

Aescaepulus · Jul 22, 2018

Hello, I am replacing a blower motor in a Sears Kenmore T9MPT075F14B1 air handler/furnace. The OEM motor is a GE 5KCP39NG, 1/2HP 1040RPM 4-speed (that I didn't know until I pulled motor from mount). To try to save time and a significant amount of money, I had purchased a "rescue" motor a Packard 43587 1/2HP 1075RPM 3-speed.
Right away I saw that the OEM motor has the following wiring pattern to the control board (Honeywell):
Black - Cool
Orange - Heat HI
Blue - Heat LO
Red - M1

With the Packard being three speed with "normal" wiring for Black-Hi, Blue-Med, and Red-Low, I am thinking I should STOP, return motor, and order the GE 4-speed.
OR
See if I can wire this differently with assistance. I don't know the voltage of HEAT-LO or M1 (or what M1 does, it could be nothing). IF M1 is just a parking stub for Red (no voltage), then would I be able to wire the Packard's Blue- medium to the HEAT-HI, and the Red-low to the HEAT-LO, and leave M1 empty? I'm thinking not, but this is my first motor swap.
Thanks for your assistance.

slwtypmark · Jul 22, 2018

You should have more confidence in yourself, that's exactly what you can do! M1 is just a parking stub as you suggested(You are now up for DIYer of the month!

user_12345a · Jul 22, 2018

When switching to a different motor, you need to check the temperature rise in high heat and low heat mode.

see: https://structuretech1.com/diy-furnace-test/

The red on the new motor may be a lower speed than blue on the old and you can overheat the furnace using a different motor set to the lowest speed.

It's better to have a 4 speed especially for a 2-stage furnace. More choices to get the airflow correct.

Aescaepulus · Jul 22, 2018

Thanks! I definitely didn't want to make an assumption and fry a $200 motor, they're kind of hard to return that way! I will research the Red-low voltage/fan speed to make sure I won't overheat the furnace come winter, I may chicken out and scrounge for a 4-speed.

Aescaepulus · Jul 22, 2018

Excellent link! I will think this over more, I was starting to worry about that as well. If I install the 3-speed now and have a problem, I can't return the motor as it's a DIY job. I've asked my vendor to check if they have a 4-speed they can credit the 3-speed toward if I return it.

Thanks guys for the quick replies.

user_12345a · Jul 22, 2018

You won't fry the motor if wired right - a question of overheating the heat exchanger using the red lead on the new motor.

Furnace blower motor wiring question

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