Carrier HVAC FAN in air handler continuously is ON


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Old 05-27-24, 10:56 AM
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Carrier HVAC FAN in air handler continuously is ON

I have a carrier FX4DNF037L00ACAA which the fan constantly running and does not go off. ( note: the A/C is cooling fine but fan never goes OFF)

In order to diagnose the issue I have performed the following:
  1. replaced the thermostat with the same model but fan still keep on running.
  2. When thermostat is removed, the fan goes off but in the air handler UV light stay ON in the air handler.
  3. While thermostat is connected and set to OFF position: At the air handler removed the WHT pin 2 and BLU pin 4 wire while common brown wire is connected at the fan motor (endura FM19, 1/2 HP 230VAC) which is used to set the fan speed, the fan stops running but UV light stay ON.
  4. With the same setting when either WHT or BLU wire is connoted to the fan motor, will continuously run and UV light stay ON.


Based on the above data, it seems like the thermostat and the wiring to the thermostat and ECM fan assembly is ok. So what is the issue? Fan relay? Anything else? Where can I find the wiring diagram of the my HVAC?
 
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Old 05-27-24, 07:56 PM
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The wiring diagram should be on the unit. Check the back of the service door.
Are the white and blue wires you are describing directly on the motor.... speed taps ?

If you remove those and the motor still runs.... there is a motor problem.
How old is this air handler ?
 
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Old 05-27-24, 09:49 PM
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Thanks for your response Pete,
unfortunately the wiring diagram does not show connection to relays limit switch and etc.
At the ECM motor while the brown (GND) wire is connoted, if I disconnect both WHT (pin2) and BLU.(pin4) then the fan stops working but if I connect either WHT or BLU wire to the speed control connector, the fan starts running. As a result, I assume the ECM and FAN motor ars ok and there is something else such as fan relay or there is another DC based relay in the circuit is faulty part.
 
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Old 05-28-24, 11:47 AM
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Ok.... do you have a meter ?
Those are speed tap lines.
You need to measure from each one to C or air handler ground to see which one is active.... has 24vac on it.
 
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Old 05-29-24, 03:11 PM
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sorry, due to high heat in the attic, had to wait and continue the troubleshooting early in the morning.
while thermostat was set to A/C position:
the voltage at speed tap connector pin 2 ( BLU) was 26 vac and pin 4 (WHT) was zero volt.
changed thermostat setting to OFF position:
the voltage at the speed tap connector pin 2 (BLU) changed to zero and pin 4 (WHT) remained at zero volt.
while the high voltage were connected to the FAN MOTOR, I checked all the speed tap pins and all were zero volts relative to GND ( note: BRN (pin C) , BLU and WHT wires were disconnected during this tests)
 
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Old 05-29-24, 06:14 PM
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I don't see where the low voltage is proven okay.
If you remove the stat from the wall and the fan turns off, it isn't the motor or a relay. ( there is no relay on that unit )

Did you replace the subbase when you replaced the thermostat or just the top portion?
What thermostat do you have?





 
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Old 05-29-24, 07:17 PM
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I did not replace the base of the thermostat just swapped the top section with another one which is exactly the same model. I assumed the base only brings wires to the connectors and there is no active part on it.
 
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Old 05-31-24, 04:55 AM
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The mounting screw can short R to G behind the stat.


Remove the low voltage wire connected to air handler "R" that runs to the stat.
If it doesn't run, you have a low voltage problem.
 
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Old 06-02-24, 10:02 PM
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I removed the thermostat and traced the G wire but did not find any problem.
reinstalled the thermostat and check the voltage at the G wire at the fan motor in the air handler pin2 BLU wire.
thermostat set to OFF position: 0.00 Vac
thermostat set to FAN position: 26.53 Vac
thermostat set to A/C position: 26.53 Vac
so with the above results, there is indication the G wire is not short to R wire and following the command from the thermostat.
so far the only way the FAN can be stop working is to unplug the thermostat and as a result a pull down resistor which is connected to the BLU wire and 24 Vac return (GND) BRN wire in the air handler area to provide zero volt to the speed tap pin 2 wire.
 
 

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