On Monday, December 7, 2015 at 3:33:07 AM UTC-5, Micky wrote:
I have an old furnace with a one-speed-at-a-time-based on the wiring
fan motor.

I once rewired it, using the schematic, to see if I could find a lower
speed, so I know the schematic is correct, but in my dotage, I can't
understand it!

It calls it a "direct drive fan motor" and it has 4 wires coming out
of it. It appears that either 2 or 3 wires are to be connected at a
given time, although that doesn't correspond to my vague memory.
The wires a

White Common, connected at all times
Black High, perhaps connected at all times, if you go by the
schematic
Blue Medium
Red Low

The schematic shows 2 wires connected to the motor and a iiuc a 3rd
optional wire.

What's unclear? It has three possible speeds. If there are only
two wires going to it, pick you speed. If there are three wires
going to it, then one of those is for heat and one for cooling.
Typically cooling is on high, heat would be on medium.