1991 should have the Nippondenso alternator, with the regulator built into the PCM.
If you look at the back of your alternator, you should see either three or four wires, located with a moulded rubber block. The one coming out the end of the block should be the mains output lead, and the two coming out the side (about an inch or so apart, and the alternator studs have a grey shroud around them) will be the "field" leads.
What that code is trying to tell you is that it's not sensing that the field coils are recieving voltage from the regulator circuit, and it suspects a wiring fault somewhere in that circuit. Check the alternator connections first (since they're more likely to have trouble,) and give the wiring harness coming from the alternator a thorough visual inspection - the two field leads should eventually run to the PCM.
Pictures of your alternator (not wired) are up on my website under San Jose Generator, in the "Alternator Identification Guide."
If the wiring itself is sound, it may be that the field coils in the alternator proper have a fault, and it may want replacing. However, if you take it out for testing, I do not suggest you take it to a "chain" auto store - they won't test it at full output. Find a shop that specialises in alternators and starters (prefereably someone who does remanufacturing,) and take it to them for testing. They may charge you a few bucks, but it's a better test, and if there's a fault, they're likely to be able to identify it. Probably be able to tell you exactly what it is, from their test readings. Or, they'll test it at full current output and tell you it's OK (rather than just checking voltage, which is what the parts house benches do...)
