FAN is always on, everything was replaced

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Old 05-31-20, 05:30 PM
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Question FAN is always on, everything was replaced

I've been having this issue where my fan is always on. I replaced the blower motor, transformer, thermostat and control board. I assured the wires are on correctly with the honeywell st9103a manual. Even with the green disconnected from the control board the fan is running. Even with the new thermostat disconnected its running. I checked both limit switches and even tried to jump them each. What else could be the cause of this? Its an oil furance with AC if this makes a difference. Also I checked between Green and control on my board and theres around 40 volts(only when I call for cooling)
 
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Old 06-01-20, 06:49 AM
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Hi, not much to go on here, what type of equipment do you have? Mfg. mdl# would be great, also post a pick of the wiring Dia. If available.
Geo
 
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Old 06-01-20, 01:23 PM
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This is a 361AAN036075aabe, I believe from bryant however it doesn't say it. The control board is honeywell st9103a. I only have the wiring for the honeywell https://customer.honeywell.com/resou...s/69-0771b.pdf
 
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Old 06-01-20, 07:02 PM
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Welcome to the forums.

I've been having this issue where my fan is always on. I replaced the blower motor, transformer, thermostat and control board
You changed all those parts to fix this problem ????

The problem is more than likely an open safety switch/sensor.

The ST9103A also monitors a limit switch string, which energizes the circulating
fan whenever the limit switch opens.


Limit switch string opens.
Oil primary control shuts off burner.
Circulating fan is energized immediately at heat speed.
ST9103A opens oil primary control T-T connections.
Circulating fan runs as long as limit string stays open.
If there is a call for cooling or fan, the circulating fan switches from heat
speed to cool speed.

In the highlighted diagram below.... I show the wiring from the limit control. The generic diagram shows one limit switch..... your furnace may have more in that loop. That circuit needs to be closed for things to be normal.

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Old 06-01-20, 07:59 PM
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I've checked both limit switches. I have one up by the oil burner which tested to around .2 resistance with clamps. The lower at the blowerfan tested at .2 also
 
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Old 06-03-20, 04:12 PM
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Is there any chance it could be my oil primary control unit?
 
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Old 06-03-20, 06:06 PM
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As far as I know...... only an open limit string will keep the blower running. If the string is open..... you should hear the blower change speeds when changing from heating to cooling.

Go right to the two pins on the multipin connector. You should measure 0v between the two pins I labeled.

I checked between Green and control on my board and there's around 40 volts(only when I call for cooling)
I'm not sure what this is referring to.
 
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Old 06-04-20, 12:31 AM
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I just went down and checked those 2 pins on my board, I received a -5.5V reading, what does that indicate is bad? To test them I unplugged the big multiplug and the fan was still running while tested unplugged.
 
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Old 06-04-20, 12:59 PM
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Yes.... the fan will run with the multipin plug out.
Across those two points you need to measure 0v.
You'll need to double check the wiring and connections to the limit switches.
 
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Old 06-04-20, 01:53 PM
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You did mean to measure on the board and not the plug though right?
 
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Old 06-04-20, 03:51 PM
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Measure at the plug with everything powered up.
 
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Old 06-04-20, 08:13 PM
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Thank you, I checked the plug and that is indeed 0.0Volts
 
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Old 06-04-20, 08:28 PM
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Then the blower should stop and the system should be waiting for a call for heat.
 
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Old 06-04-20, 09:11 PM
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The blower keeps going none stop whether its on heat/cool/off even when the plug is on. Could this mean the new control board is bad?
 
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Old 06-10-20, 06:34 PM
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Anybody know anything else I could check?
 
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Old 06-10-20, 08:03 PM
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There are several of these threads running..... does your control box have an LED on it ?
Is it showing anything ?
 
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Old 06-11-20, 05:18 PM
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I don't have any LED on my board. The oil control however has an lcd and reads no errors
 
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