This problem just started. Hile furnace w/ST9120B fan board will start up normally but blower no longer engages. Limit switch then of course shuts it down after a period.
Previous posts do not really address a solution or diagnostic check to this issue. I do understand electrical/electronic circuits somewhat. This furnace originally installed with an AC untit but the AC was uninstalled and furnace put in another location and worked fine for a while. Modifications were made to board to adjsut for AC removal.

Thanks for any help

Try turning your Thermostat to FAN ON position... see if fan comes on... if not... you may have bad capacitor or bad fan motor...

How many leads do you have to the motor? Often the fan on position uses a different relay and motor winding than heat. Can you get at the connections on the fan board to check stuff? Power often goes from the transformer to the lower limit switch to the coil of the heat fan relay and back to the transformer. With 24 volts at the coil terminals, and 120 at line, the load should have 120 volts too. Often if you have power to a good motor and the capacitor is bad, the motor will hum and even take off if you give the fan a spin.

Guys,

Thanks for the input. Here's what I've discovered. There was voltage to the blower motor. I pulled the start capacitor,metered it, and it would charge and discharge. There were 4 connections on motor. According to diagram one was Common and the other 3 were 'low-med-high'. From 'C' to Low metered 'zilch'. 'c' to med-high mesured close to dead short on 2 meters. It actually was around 2-3 ohms on analog meter and I can never get a reading that low on digital meters. I know motor windings are low but would 2-3 ohms be enough (I'm guessing no @ 110volts AC)

The open 'low' reading I'm guessing is an open winding. My semi-educated guess at this point is a bad blower motor. What do you guys think?

Thanks again for any assistence in this issue.

If you Check from common to any speed and its open you have a bad blower motor. If the motor was over heated due to a bad run capacitor all speeds of the motor would read open from common. I hate to say it but you have a bad motor. The good news is a universal motor will work with it at a cheaper price than the factory motor. Take the old motor to a local supplier and they will fix you up. Ask for the new run capacitor that comes with the new motor.

Thanks to T-Top. I have ordered an A-O Smith online that was dead-on in specs. As a replacement. It's a Smith that is OEM in blower now. Shop around if you need replacement motors. It ranges from $348.00 for a OEM exact replacement to the $69 I'm paying! FYI I'm going to replace the capacitor also even though it seemingly charges OK. Could be breaking down after being loaded for a time.

Thanks to all