# Trol a Temp MST Damper Motor



## arthrodoc (Dec 26, 2010)

I have an oil fired furnace - 1 zone is controlled by 2 trol a temp damper motors through a forced air system. The motors are master and slave to the best of my knowledge and control 2 dampers. The thermostats are working but not turning on the fan or calling for heat from the furnace. I have been told that the master MST damper motor is not working as it is always in the closed position. It is receiving 24 volts and the arm moves when jumped but goes back to the closed position.
Question - I am told that if I change the damper motor the system will work and the thermostat will turn the furnace and fan on - do these motors feed back to the other parts of the system? Do I need to change both, as the other slave unit seems to be in the open position continously since the problem started.


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## beenthere (Oct 11, 2008)

Is it wired up as a 3 wire, or 6 wire.

It makes sense that the slave is open, if only the master is not working.


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## arthrodoc (Dec 26, 2010)

it is wired as 3 wires to the damper motor 1, 4 and 6 and number s 2 and 5 are jumpered - both motors are wired identically


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## beenthere (Oct 11, 2008)

If X and Z aren't used. then its not controlling the furnace. Or the other damper.
X would be to energize the furnace.

Your zone board may also be defective.


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## arthrodoc (Dec 26, 2010)

the zone board goes to relays housed inside the fan unit - they seem to click on and off - could a relay be bad - the zone board is sending 24 volts to the damper motors - how do i evaluate the zone board?


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## arthrodoc (Dec 26, 2010)

the zone board is a mastertrol mini-zone


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## beenthere (Oct 11, 2008)

See if its sending 24 volts to open the damper. If it isn't, then either there is a problem with the board, or the thermostat.


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## arthrodoc (Dec 26, 2010)

both damper motors are receiving 24 volts


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## arthrodoc (Dec 26, 2010)

jumping the thermostat doesn't help either - the hvac man said the damper motor would feedback to turn the fan and furnace on and off but I am highly doubtful and believe that the problem is elsewhere - either the control board or one of the relays that it controls - he said no because the damper motor is receiving 24 volts and not opening and closing - i shorted out the damper motors and they don't work as they should - shorting when in closed position moves the arm but it ends up in the closed position, not the open position as it says in the instructions


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## beenthere (Oct 11, 2008)

Those are power open power close motors.

If they are wired to operate the furnace. Then there should be a wire on #3 terminal.


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## arthrodoc (Dec 26, 2010)

Based on the response, it seems the problem is in the master control board or one of the relays that the control board operates. Since the control board seems to be sending 24 volts to the damper motors, could the problem be a relay ?


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## beenthere (Oct 11, 2008)

Could be. Did you check any of the relays.


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## arthrodoc (Dec 26, 2010)

don't know how to check them - i took then off the back wall housing and they look ok but haven't replaced them - how can i check them?


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## beenthere (Oct 11, 2008)

See if they are closing when they should. use your meter, and see if teh contacts are sending out voltage or not when they should.


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## arthrodoc (Dec 26, 2010)

24 volts correct - they are white-rodgers 90-340 relays - do you know which terminals i should check?


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## beenthere (Oct 11, 2008)

Are they the type 91's?

The 2 outside terminals are the normally open ones for both poles. Make sure they are closing. And the center post is the normally closed.

The 2 terminals with the thin copper wire are the coil terminals.


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