Here this will help.
This is from a 1993 EVTM but others years are similar. If you are serious about testing this thing get a an EVTM and the Ford shop manual (Not Chiltons or Haynes). It has more than you ever wanted to know about testing and troubleshooting. It is a PITA for me to scan them in so you get one page. there are 15 pages of testing in eth Ford shop manual.
A few thoughts. Some of these have already been said. Remember there is a lot of my opinion in this. Not all agree.
This assumes all of your battery cables, and all you connections at the starter relay are clean, tight, and free of paint or corrosion.
-The ALT Grounds via the case. (Probably good)
-So the case must make good contact with the bracket. (Probably good)
-The Bracket must make good contact with the block. (Probably good)
-The block must be grounded to the car and battery. (Normally iffy even brand new from Ford.)
-The block has a ground cable on the driver's side on the timing cover that goes to the battery. Check to make sure this is clean and tight. (Not a good location for a ground IMHO, check for corrosion)
-The block goes to the frame in two places. A ground strap from the Trans tunnel to the pass side head and where the cable I talked about in the previous step goes to the inner fender near the battery. IMHO these grounds are the absolute minimum from Ford. Any deterioration at all of any kind and they no longer can do the job 100%.
Onto the ALT wiring.
Since I don't have the spec for the wire the amp ratings are a guess.
First the BK/O wires.
There are two of these wires. They start from the starter relay as a Green fusible link. Then then split off and travel to the ALT. IMHO these wires are once again marginal to do the job when new. They see a lot of heat from the engine, and the load they carry, and deteriorate over time. They are prone to catching fire especially near the alt when the movement of the engine cause the most strain. It is not unusual to find a hot spot in these wires.
-If you replace these wires you must provide a fusible link or fuse. Other wise any shorts in the alt will result in a fire. The battery will discharge at full amps into the alt via these wires until it is discharged or it melts something. You want it to melt a fusible link or the fuse.
Next the Y/W wire.
This is the most important wire IMHO. A lot of people inadvertently cut this wire or hook this wire up improperly. This wire's purpose is remote voltage sensing for the regulator. Wherever this wire is hooked up is where what the ALT measures and provides voltage for. Since Ford uses the starter relay as the main junction point this is where this wire should be hooked to. So stated another way IMHO this wire is what allowed Ford to use the bare minimum wire size to carry the load since it will increase alt voltage to overcome any voltage drop. Obviously there is a limit to the voltage increase.
My example is if voltage falls to 12 volts at the starter relay then the ALT increases it output to compensate. Having the voltage sensed away from the ALT also allows some dampening effect to allow for a more stable output. It does make the ALT slower to respond to changes but the more stable voltage is worth it. Electronic devices like stable voltage.
Don't cheat and hook up this wire anywhere but the starter relay.
BTW the wire also provides the field.
Onto the LG/R wire.
This wire "turns on" the voltage regulator with the key. It also goes through the cluster. Normally people who remove their gauges cause an open in this circuit. If power is not applied here the ALT does not charge.
The W/BK wires
This is feed back from the Generator portion of the ALT to the voltage regulator.