First post here, but have been here many times. Went on a service call tonight, 1969 Armstrong furnace that the landlord let one of the tenants mess with. The fan is on all the time, but the heat cycles on and off. Tenant "claims" it's been like this for few days, so he says he changed the stat. But he hooked red up to B instead of R and removed the jumper. (I put it back and put R on R). checked low voltage wiring in furnace for consistancy. Still fan on all the time. Tenant says he changed the Fan limit. He hooked up the 120v L1 to the limit side, not the fan side (but swears he wired it the same). So, the heat cycles on and off but fan on all the time. We have 2 transformers in here, one I believe is used just for the fan limit. He could have also fried the roll out near the exhaust, an old honeywell.
I'll have to go back. Any ideas? p.s. wierd voltage on low voltage terminals, like you'd see with a short.