My Trane gas furnace will light and burn fine. The blower will not come on in the heat position but will work when the air conditioner is on. What part controls when the blower comes on for heat. The furnace cuts itself off after about 1 1/2 minutes from over heating.

Depending on the age of the furnace,it could be a bad or stuck relay on the circuit board or bad combination fan/limit switch which brings on the blower upon tempature rise. What's the model number or age of the furnace?

Not sure of the model number but the unit is 20 years old. I see the fan/limit switch on the unit. Is the dial supposed to constantly rotate? The relay that I see functions when the air conditioner is turned on.

Do you own a volt meter?

Yes

Locate the combination switch. Check for 120 volts to and across the switch.