my 90 f250 wont start. the fuel pumps would turn on and take a load but didnt start so i changed the EEC and fuel pump relays. the next day the pumps didnt turn on at all i checked the filter and its clear. i even left the fuel line out of the filter to see if i turned the key it would blow fuel through but that didnt happen. people have told me to change the fuel tank selector switch because i have two tanks but i dont know what im looking for or where at. i know i have the electric switch on the dash but i cant find the mechanical one under the truck. what should i do?