Ford Repair: Alternators (sadly a 1991 aerostar), electronic fuel injection, correct assumption

Question
Sorry, since it's not during your time period, but alternator technology is, so this is just a down and dirty check of my problem solving.

Battery idiot light is lit.  Alternator gauge reads 8 volts and never exceeds that with higher engine revs.  I assume that A) the alternator is either not able to produce more than 8 volts, or B) the voltage regulator is not allowing the charge through to the battery.

With that assumption (and knowing that an alternator can put out 70+ amps) I figure I can put my AC/DC meter on the field output and ground and see what the actual output of the alternator is, thereby determining whether it's the alternator or the voltage regulator.  Is this a correct assumption?  I mean, is this the testing method you would use?  I'm not really certain at which point along the way the alternator gauge is reading from (meaning pre voltage regulator or post).

Then, of course, it's going to be up to me to figure out whether I have an internal voltage regulator on the alternator or not, but if you can point me in the right direction on testing, that would be one heck of a lot of help.

Oh, the Aerostar is a 4.0L V6 with electronic fuel injection and electronic 4 wheel drive.  Also has about 152,000 miles on it, so I kinda expect to see the external parts starting to need to be replaced.  Otherwise the baby runs great.

TIA for you time,

Roger W. Norman

Answer
Mr. Norman,

First off your vehicle has an internal voltage regulator. The easiest way to test, with everything TURNED OFF, check the voltage of your battery...which should be about 12.4 volts. Anything under 12 and your battery is no good. Next start the vehicle up and voltage should jump up to 14-14.4. If it does not, you have a bad altenator. Unfortunately to just change the voltage regulator will cost just as much as replacing the altenator. Make sure that your serpentine belt is tight and all connections are clean and also tight going from the altenator to the battery. Also make sure that your battery cables are in good condition as this will also cause low voltage. Hope that this helps and if you need further assistance, please do not hesitate to contact me. Good luck.



Ken