Generator light on at startup (‘75 Eldorado)

[benji808]

When I start my car after letting it sit overnight, the generator light illuminates. It then slowly dims for about 5 minutes before turning off completely. Just started doing this recently. Only happens after car has been sitting for a while.

1) battery and alternator seem to test ok- when car is off I get ~12.8v and when on I get 14.4-14.5v steady. Actually seems a bit high off but I'm chalking that up to imperfect equipment for now. Car starts and drives great- strong crank, etc.

2) my understanding of these lights (from an old post by TJ, actually) is that they get a + from ignition, then another plus from the battery when car is turned on. I have verified that the light does turn on when I turn the key without cranking (first + works), and continuity is testing good between battery + and alternator connection (second +). I have also cleaned connections at the alternator.

Any ideas? I suppose it could be a short between the alternator and dash, but that seems unlikely to me since it's only during that startup window (no blinking/intermittent while driving that I have seen recently).

If the alternator was on its way out, could that cause this? Like it works "better" after warming up or something?

[TJ Hopland]

Did you do a load test on the alternator or were you just checking voltage?

It sounds like the circuit is fine.  My guess is the alternator is just weak and can't keep up at first when the battery is demanding more juice to recover from starting. I bet if you took it to be load tested it would fail.

My 73 would do that about every 2-3 years. I think its the high underhood temps and low quality parts that are available today.  If you could find NOS parts it may not be a problem but with todays stuff they just don't seem to last. I eventually put in a 12si out of an 84 Cimmaron I think.  It bolts up except for the rear support bolt which doesn't seem to be a problem.  Its basically the same alternator with a different fan and improved cooling.

[bcroe]

That light can come on with a failing diode, in
either the main rectifier set, or the small trio. 
Not hard to swap. 

Another possibility is weak wiring.  Ideally the
12V distribution is the same voltage everywhere,
but of course it is not.  The design assumes the
variations will not be enough to light the GEN
light.  But a large shift in some path might come
around and start to light that light. 

if you have that sort of equipment, a scope on the
alt GEN light terminal will spot a failing diode
spikes.  good luck, Bruce Roe

[benji808]

Makes sense! Yes- I was just voltage testing. Thank you both for the info!

[TJ Hopland]

Is there any indications that its an original alternator or are there stickers or paint marks and maybe its not that dirty to indicate its a replacement?    If its a more recent replacement I don't think there is any reason to keep or repair it.  If it is original may be worth repairing but I'm not sure where you would go to get decent quality repair parts.    If you are not going for show quality I would give the 84 cimaron one a shot. I just looked and at least based on my memory that's the one.

[benji808]

Well, after going through the hassle of removing the alternator (seriously engineers- give me one more inch of clearance for that center bolt so I don't have to remove brackets and almost air pumps lol), it appears to have been a wiring issue- when I went to reattach the battery it started a small electrical fire that burned through the wire from the battery. Exciting times! Anyway, replaced that wire and it seems to be running well. Will plan to update if I have further issues. The reman alternator I got didn't fit anyway, so I just put the old (probably original) back in.

[TJ Hopland]

Did you not have the battery disconnected when you disconnected the alternator?  The fat wire is pretty much a direct connection to the battery and one of the small ones is also connected to that post so pretty easy to short it if the battery is connected.   

Tell us more about the reman you had?  What was it supposed to be the stock one for your car?  What about it didn't fit?   Or were there multiple options like 63 and 80 amp and you guessed the wrong (80
?) one?   I don't know where the larger alternators were used, I have never seen one other than in the parts listings. I think the 80 amp is a 22si which is a completely different case and mounting.

[benji808]

Battery was disconnected when unplugging/removing the alternator.

If I remember correctly everything was hooked back up, and I reattached the ground to the battery as a last step for the reinstall. I think the + wire from the battery to the alternator had corrosion or a break or something that caused it short out...always possible I overlooked a connection though.

In regards to the reman, the mount on the bottom of my original is split, so the pivot bolt goes through front alternator loop, through the block mount, then through a second alternator loop at the back before screwing in. Most of the ones I'm seeing at Autozone/Oreillys have a single "loop" at the front of the alternator that is not compatible with my engine mount. It is possible that mine having an AIR pump and being a CA car gives it a different layout, but I'm not sure. See below links- pay attention to the bottom mount setup.

Reman I got: https://www.autozone.com/batteries-starting-and-charging/alternator/p/duralast-alternator-dl7127m/333351_0_0

What I probably would need: https://www.autozone.com/batteries-starting-and-charging/alternator/p/duralast-gold-alternator-7157mn/388768_0_0

Assuming my wire change fixed the generator light, I'll leave it for now...if I have more issues I'll reassess.

[TJ Hopland]

Ya that was what I was suspecting the difference between the 63 and 80 amp alternators. Interesting your car has the bigger one.  What options does your car have?  If it was a CA car rear window defroster or heated seats doesn't seem likely.  I wonder what it took to get the bigger alternator?

[benji808]

Interesting! I didn't even realize they were different amps- I thought it was just a mounting style.

It does have the defroster (although mine is unused as previous owner did not use glass in the rebuilt roof).

Were heated seats an option? Don't think mine has it, but that would be neat!

[TJ Hopland]

One of my parts cars had the defroster but thinking about it now I don't think I ever looked close at its alternator so maybe that pushes it to the larger alternator? 

I don't remember what year(s) heated seats were an option but it was an option at one point. It may have been a one year only thing.