[turbo6bar]

Sounds like the low or medium speed on the blower motor is kaput. When you flip the fan switch to ON, you're triggering a higher speed that's used for cooling.

If you can read the schematic and read the wiring, try changing the speed. You're looking for the male spade on your circuit board marked HEAT. You'll want to throw another color (representing another motor speed) onto that spade. If the furnace then works properly, then you've isolated the problem to the motor. The blower motor can be changed with little trouble assuming the blower wheel isn't frozen to the motor shaft (doubt it, though). jurgen