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QUESTION:I have been back twice now to this customer's house and I'm glad its 90
degrees already and she doesn't need heat.
I replaced the circuit board a year ago and worked fine. This board was
reporting a pressure switch stuck closed (false) so I ordered board.
Now Coleman has come up with a replacement board for this furnace with
added diagnostic functions. The kit includes circuit board, adapter pigtails
to modify from 12 socket to original 9 wire plug, a second pigtail to modify
the shape of 4 wire plug, and a jumper wire (to isolate the three rollout
switches in series) to a new diagnostic circuit. I followed the
instructions exactly and the furnace won't light. The board reports bad
limit switch (false). I jumped 24v to one side of this new circuit and it
fires normally on demand. then when the stat is satisfied it shows the limit
fault again and the blower and venter motor run forever.
I'm at a loss and wonder if there is a new service bulletin on this kit
and I was sent the old one?
ANSWER: Try replaceing the limit. I know your VOM shows the suckers closed, but it's
a 5yr old limit and that bimetal could be weak and opening during the off
cycle from the residual heat in HX, you get down there and rip the wires off
to check, it jiggles the bimetal and snap it's closed by the time you get
your probes on to check it.
Before faulting the board eliminate any other possibility; whats the gas
pressure/ are we overfired/ whats our rise/ blower wheel full of /
furniture parked on the returns/ vents closed off/ electrostatic filter?
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