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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8049
Experience:  Furnace, A/C and heat pump specialist.
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Ok, here goes. I have a Bryant 350MAV furnace. When I first

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Ok, here goes.

I have a Bryant 350MAV furnace. When I first turned on my AC system this year it was having problems with the lines frosting over. A friend of mine who is new to the HVAC world did some testing. He determined that the blower motor was over amping and shutting down. Once we replaced the motor, the system would run long enough that he was able to hook up his gauges and verify that there was sufficient freon in the system. For the next two weeks the AC worked like a champ. Last week it was cool enough that I had the house opened up and the air off. Today I turned the air back on and the problem has returned. I replaced the thermostat because it was old. That didn't fix it. After doing some online reading I tried jumping the Y and R wires at the circuit board. It ran for about 10 mins then shut off. Then the outside unit came on but the blower did not. I then turned everything off and am here typing.

Any thoughts, suggestions, etc?? Any other tests I can do to narrow down the problem? A local supply store has a replacement board, but I would like to be fairly certain that is the problem before spending $240.

Thanks,
Tim
Hello.Sorry you are having problems. My name is XXXXX XXXXX I will do my best to help.Please wait to leave rating until we are finished.Do you have a volt meter for dsome troubleshooting. We need to know if the board is sending the 120v out to the motor.
Customer: replied 3 years ago.

I do.

Great. Now let me ask thi. If you touch the R and G wire together at thermostat, can you make the blower come on?
Customer: replied 3 years ago.

Yes, it does.

OK now lets go to Cool and check between Neutral and Cool terminals on board where fan motor plugs to board and see if we get 120v between those two
Customer: replied 3 years ago.

Ok, I'm at the board, but not sure exactly which terminals those are. What letters correspond with those?

Follow fan wires to board, One will go to terminal marked COOL. (Probably black). Then white will go to one of the terminals marked n (Neutral).Between those 2 wires we should be getting 120v

 

Customer: replied 3 years ago.

I'm not getting a reading. Not sure I'm getting good contact. Can I remove the wires from the board, or do they have to be connected?

You can either remove the wires or slide them back a little to get the probes on the terminals. They don't have to be connected to get the 120v
Customer: replied 3 years ago.

I'm getting nothing. I double checked against a nearby wall outlet to confirm my multimeter is working.

Ok. Now gently tap on the small black relay boxes integrated into the control board and see if we get a reading. Often those little relays will stick open or closed.

 

Customer: replied 3 years ago.

I unplugged the common and cool leads, tapped every black box, and still get no reading.

I feel pretty confident your issue is going to be in the board at this point
Customer: replied 3 years ago.

Ok, that's what I needed to know. I'll get the new one tomorrow. Thanks.

Thanks. Please come back and follow up with me on this.
Customer: replied 3 years ago.

Just for clarification, when I was doing these tests the furnace had power, but the system was not running. Is that how it should be while checking the terminals like I was doing?

Power On and thermostat calling for COOL
Customer: replied 3 years ago.

Well that changes things. My thermostat was set to off. i will set it to cool and go check the terminals again.

LOL. Let me know if anything different, we will go from there.
Customer: replied 3 years ago.

Do I want to drop the thermostat setting so that the blower is actually running while testing, or just have it set to cool?

Yes we want it lowered to the point that it should bring everything ON.
Customer: replied 3 years ago.

Well now we have a different ballgame. With the system actively running, the common and cool terminals are showing 119.5v.

Is blower and everything now working?
Customer: replied 3 years ago.

It's working, it's just a question of how long it will run before the system frosts over. The blower and everything will come on initially, but then the blower shuts off leaving the rest of the system trying to cool the air. Soon the lines (inside and outside) frost over.

AS soon a sblower stops,and thermostat is still calling for it to be on, perform the check we did earlier at the board. That will tell us if issue is with board or the motor or capacitor.
Customer: replied 3 years ago.

I thought of that. I ran downstairs as soon as the blower quit. The rest of the system was still trying to cool. I got no reading on my multimeter.

This sounds to me like an intermittent issue with the control board. You will most likely need to replace the board.
Customer: replied 3 years ago.

Ok. I will proceed as I had planned in the morning. Thanks again.

Please follow up and let me know.
airheatman and other HVAC Specialists are ready to help you
Customer: replied 3 years ago.

New circuit board installed earlier this morning. So far the system is working well. Hopefully it will prove to be finally fixed. Thank you for your assistance.


 


Tim