I have three wires running from the furnace to the electric air cleaner, black, red, and white. I want to run the fan on low, and high when there's a call for heat. Black and red wires have 120 volts. The red is connected to the black wire from the air cleaner and black is not connected. Where do I hook the black wire? This is an olsen gas furnace 25 years old. I can't get the furnce to run on low when switched at the thermastate to on. Its running on high when there's a call for heat. I want the fan to run on low and when there's a call for heat switch to high?

As old as you furnace is, it may have 2 limit switches, a high and a low. The low limit turns the fan on when it heats up soon after the gas comes on. Often it controls a relay. You need to find the switch and relay if any. Likely the red wire was originally connected there. Connect the black wire there on the load terminal now.

Before you do that, you might find the high limit switch. It is what keeps the furnace from working on the low blower speed. If you clean or replace it, you may be able to go back to the original wiring. You may find the high speed on the blower drafty.