I installed a new alternator, the old one crapped out, started it up, everything fine. A day later I go to use it, and since it is pre-dawn I needed to use the headlights, after a couple blocks the volt meter said about 12V after reading a healthy 14+.
So I go home and use my other car.
Check it out, and it was outputting 14+V again. Only have used car volt meter so far.
So again I tried to drive it the next day, same thing, low V, ~12V
A day later I tried driving it during the day, no problem, 14+V

I'm about to get out my VOM, but wondering if anybody has encountered this or has any ideas.
My passenger side headlight motor is disconnected, hand cranki t up and down for now otherwise everything is OEM and working except radio, which has been out for some time.

Can be anything...
Iffy ground(s).
Dash gauge lying.
"battery" light not working or side plug on the alt has problems so alt doesn't make power.
Battery, Alt or both have bad cables.

See https://web.archive.org/web...cast.net/~fierocave/ Electric section several pages in general & charging for a start.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Startup in garage, reads 14+V
drive with nothing on, still 14+V

turn on headlights, drops to 12V, turn on heater, drops to 11V
turn off heater or headlights, jumps up to 12V, turn off other, jumps to 13V, will slowly go up to about 13.6V

if I drive with nothing on, seems to stay at 14+V

headscratching
btw, when the old alternator died, first it started reading 12V and idiot light came on when running without headlights or heater (daytime, warm out)
I drove home about 7 miles
when I went to start it two days later battery was dead, after charging the battery and attempting to start, I heard something fry, everything went dead, and no juice to anything, I thought maybe a fuse or fuseable link went but after installing alternator and checking fuse box everything works except the current situation

I also have my old alternator, I'm thinking about taking it apart and seeing whats wrong with it, maybe I can fix it instead of tossing it
my guess though, is that diode bridge is toast, or worse, if anything worse can be wrong

[This message has been edited by ZaraSpOOk (edited 12-16-2023).]

quote Originally posted by ZaraSpOOk: I also have my old alternator, I'm thinking about taking it apart and seeing whats wrong with it, maybe I can fix it instead of tossing it my guess though, is that diode bridge is toast, or worse, if anything worse can be wrong

The older SI alternators are easy to rebuild. The newer CS alternators are quite a bit more difficult to service... but having said that, it's a good idea when replacing a SI alternator to upgrade to a CS unit.

quote Originally posted by ZaraSpOOk: Startup in garage, reads 14+V drive with nothing on, still 14+V turn on headlights, drops to 12V, turn on heater, drops to 11V turn off heater or headlights, jumps up to 12V, turn off other, jumps to 13V, will slowly go up to about 13.6V if I drive with nothing on, seems to stay at 14+V

Because you very likely have crap grounds, power wires/cables or Both.
Clean/fix all grounds bolted/screwed to engine block etc in engine bay as good start & use silicon or permatex green brake grease to keep out "water."

I did indeed find corrosion on my added GND wires, but problem still exists, and that brings up another question, what are others doing to add gnd wires?
When I last did a cradle drop I changed my battery to the one that has both types of battery posts. I am still using the + side battery post, but now use the top post as my GND to engine block, the clamp has two extra posts that I added a gnd to the deck lid bolt just above the bolt that has a gnd to the engine block as original, and another to the inner fender lining (mostly because it was easy) but I think to the engine block would be better. Now that I think about it, adding a wire from the - side post to engine block would be even better. Maybe another wire from the battery - terminal to the alternator gnd post?
open to ideas

I noticed some people have added an extra wheel to give more "grab" to the belt/alternator, is that really an issue?

Also, I think it was Ogre who said not to run the car without a battery, I am surprised it will even run given it basically serves as a giant capacitor, I wouldn't think the engine would run all that well without one.

quote Originally posted by ZaraSpOOk: I noticed some people have added an extra wheel to give more "grab" to the belt/alternator, is that really an issue?

It is for anyone who's ever had a squealing belt... which might not be a problem for 2.8 Fieros that never see rain.

Fiero Belt Tensioner by Dodgerunner

[This message has been edited by Patrick (edited 12-23-2023).]

quote Originally posted by ZaraSpOOk: Also, I think it was Ogre who said not to run the car without a battery, I am surprised it will even run given it basically serves as a giant capacitor, I wouldn't think the engine would run all that well without one.

Think Your confused w/ by removing battery cable end(s) w/ engine running to "test" the alt.

Never do that!

Can cause power problems that can/will fry ECM & more.