I don't think I worded my last question the right way so I will try again.I have a Intertherm central air & heat unit model # ASCO36BCRAA it is an old unit the product date code is 83JY-21-01 My problem is with the blower fan in heat mode the fan only comes on when I put it to the on position it will not come on when set to auto the heating elements and thermostat work fine.When I put it in cool mode the fan comes on and goes off fine in auto.So my question is why doesn't the fan come on and turn off in auto in heat mode?

Because the automatic blower fan control that does the job in the heat mode is defective. Depending upon your equipment it could be a

Temperature controlled fan relay

Fan and limit control

On electric furnaces it could be one of the heat sequencers

OR just a small fan heat blower control disc.

The part will have to be replaced before it will work properly.