They make code readers that are inexpensive for OBD-I, which the EEC-IV is. However, you don't need a tool to scan the computer for codes. Just go to
http://fordfuelinjection.com and click on "Getting Codes" to find out how it is done. The diagnostic ports are under the hood on all trucks/vans before '96. The first OBD-II computers were EEC-V, and first appeared in trucks/vans in '96, which have the diagnostic port under the dash.