My American Standard electric furnace runs only when the thermostat is on the "on" position and not when on the "auto" position. So basically it's running manually. I replaced the thermostat thinking it was the problem but that did not help. Now the AC works fine on "auto." Can you help me?

Your heat sequencer or heat relay for the blower is not working properly and could need to be replaced. There is a wiring diagram on the back side of the blower motor access panel or somewhere else on the unit since all heating units come with one attached to the unit. With the use of an electric test meter you can track down the circuit causing the problem and test the part. Replace defective part as necessary.