To the best of my knowledge, Honeywell has sold over 27 billion yo-yo
thermostats since their inception, and none have ever gone bad. Should you
add a yellow wire? Why, what for and what would you connect it to?
Thermostat cables come with a variety of colors, so don't necessarily expect
colors to have definitive meanings. Your best bet is to contact a HVAC
professional and let him check it out



"Davej" wrote in message
ups.com...
I'm having some odd fan cycling problem on my Carrier furnace. It has
behaved oddly ever since it was installed. At the furnace controller
card no yellow wire is attached. The voltage on the Green wire is
varying and when it gets down to 18VAC the blower fan kicks off. The
red wire is a solid 27VAC.

I'm still using an old round Honeywell thermostat and I'm guessing the
mercury switch in the thermostat has gone bad. Is that a good guess?
When I get a new thermostat should I add a yellow wire?

Also, is the wiring diagram below essentially accurate as a typical
example?

http://xtronics.com/reference/thermostat_wires.html

Thanks!