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Discussion Starter · #1 ·
Hi,

My mother has the subject furnace in her house with model number CMPE075U3. Last week here blower under the house quit working. I went over and replaced the capacitor for her and all seem to be working fine. Then when she went to put the fan on auto, it would not come on anymore even if she switched back to the on mode. If she left the system off overnight and turned it on again in the Fan On position it would run again. I went back under the house again and removed the access panel to the control board and found this one red wire hanging. According to the schematic it goes to "M1". Anybody know if this has anything to do with her issue, if not I'm not sure what to look for next. I also noticed that whoever worked on it last swapped out a 5 AMP fuse with a 10 AMP fuse. Any help would be much appreciated.

Thanks,

Steve
 

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Follow the wiring diagram on the back of one of the covers. Probably M1 is motor "One" and may need hooking up.

It is supposed to use either a 3 or 5 amp fuse. The circuit board or transformer will blow before that fuse goes so it is useless. Maybe that was all the guy had but I would replace it.
 

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It looks like the black motor wire goes to Cool and red may be used for heating or parked/unused on the M1 terminal which is dead and goes nowhere. Then you would have the blue wire for heat.

If it runs when turned ON intermittently then it is either the thermostat is not sending a G wire signal or the cooling fan relay on the board is sticking.

Try turn the fan to ON. If it works try it several times until it does not. Then go downstairs and see if you are getting 24 volts between G and C on the board.

If not then the thermostat is bad. If you are then lightly tap the black plastic relays with the butt end of a plastic screwdriver. If the fan starts then the relay is sticking and you need a new board. Not a uncommon problem.

Maker sure there is a fan wire on heat and cool.

May use red as parked, blue as heat and black as cool or a different setup depending on how big the ductwork is, airflow restriction or size of AC.
 

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Discussion Starter · #7 ·
Would you still see voltage at the air handler across the green and red terminals if the fan is running? I ask because I did check the voltage with the two probes in one hand and pushing the safety switch with the other and wasn't seeing any voltage.
 

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Red is hot and C is common or neutral in laymans terms.

In theory you may get 24 volts from R to G as it then goes thru the cooling relay back to C.

Use duct tape and tape the switch closed.

Check from G to C.
 

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Discussion Starter · #10 ·
Just wanted to follow up. I checked th voltage between G and C and the value was something like 8 milliVolts. Swapped it the thermostat and so far everything seems to be in order. Thanks for all the help.
 
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