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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.
You can also run a search on here for "pulling codes"
Yours more than likely has the 3-digit codes, and all you need is a paperclip to check the codes.
If not, go to Advance or AutoZone & they'll pull the codes for free.