Need help with a Gremlin! Please!

Problems related to residential installations.

- Need help with a Gremlin! Please!

Postby john57classic » Mon Jan 13, 2014 10:07 pm

Thanks yes the 24V I now see tells me the transformer is working. The - AC reading on the the T'stat is exactly the same as what it displays when the power is switched off to the furnace. I'm still researching this T'stat but the batteries, which have been changed, are there primarily to maintain programming although they also keep the display alive. As I said I'm still researching but I'm pretty sure this T'stat requires the 24 V to function and when it says -AC it is not seeing that voltage and so is not functioning which it is not, there is no call for heat or even a temperature read out. You could be right and there could be a break somewhere in the wire and I should be able to check the continuity and find if that is the case but since the 24 V power only gets to the T'stat by passing through the control board and given the weird evolution of the problem I 'm betting I'm out for a new board. Thanks again
john57classic
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- Need help with a Gremlin! Please!

Postby Freon » Tue Jan 14, 2014 2:50 pm

Measure the voltage between R and C terminals at the thermostat base (remove the cover). If there is a bad connection this voltage will come and go. Wiggle the wires. There should always be a constant 24 volts AC between the R and C at the thermostat.
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- Need help with a Gremlin! Please!

Postby john57classic » Tue Jan 14, 2014 7:40 pm

Thanks again I went back up and checked the furnace board as well as taking the cover off the T'stat. The voltage b/w R and C at the T'stat barely registers maybe 2V but it does move the needle. Voltage between R and C on the control board (other end of wire) same things barely moves the need but there was 24 V here a few days ago. Voltage across the transformer is 24V so it's either the wiring between the transformer and the board which is part of a main harness or I need a new board.
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- Need help with a Gremlin! Please!

Postby Freon » Fri Jan 17, 2014 4:06 pm

Next time this happens and you have 2 volts at the thermostat, go to the furnace and disconnect the R wire at the control board before measuring the voltage. Although unlikely there may be something in the wiring between the control board and furnace, or the thermostat itself, that is dragging down the voltage.

If, with no R wire connected at the control board and a reading of 2 volts, more or less, between R and C at the board AND 24 volts at the transformer secondary, then I agree the control board or harness would be suspect. Check it for any signs of cold solder joints or loose connections/traces especially where the harness connector is soldered to the control board.
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