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I have a 78 Cadillac Seville that seems to be doing pretty

Customer Question
well, but from time to...
I have a 78 Cadillac Seville that seems to be doing pretty well, but from time to time the generator light has come on while driving. I feel like this happens more at night when I'm using more power, and the battery was past its time so I replaced it, but the light came on again last night. There are a few things in the instrument panel which haven't worked since I bought it ( fuel gauge, antenna possibly the cruise control although I do see the yellow light when I switch it onand last lastly the 8 track player which may be unrelated as the stereo works . I'm wanting to know it it seems likely that there is a larger problem common to all those or if the reason the generator light comes on is sperate and that that cause could be.
Submitted: 2 years ago.Category: Cadillac
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Answered in 28 minutes by:
9/17/2015
Cadillac Mechanic: Jerry Newton, Cadillac Technician replied 2 years ago
Jerry Newton
Jerry Newton, Cadillac Technician
Category: Cadillac
Satisfied Customers: 4,832
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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If the generator light is coming on while driving, this almost always indicates a failure of the alternator itself.

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Customer reply replied 2 years ago
I thought that would be a continuous issue if the alternator goes out? And why wouldn't my battery be dead by now? I see in service records that the distributer cap was replaced as part of a service originally pertaining low engine performance
Cadillac Mechanic: Jerry Newton, Cadillac Technician replied 2 years ago

The light is controlled by the internal voltage regulator on the alternator. Though the alternator is apparently still charging (at least on some level), it can still be faulty. Most commonly, we would see one of the three diodes fail in the rectifier bridge, which results in a charging voltage of just over 13v instead of the nominal 14v that it should have, plus if you use a scope you'd see that the failed diode is creating an AC current ripple in the electrical system.

What this equals is, even though the alternator is still providing enough voltage to charge the system now, this is usually an indicator that it's on its way out, a warning that isn't provided very often, so it's best to heed it when it occurs.

In lieu of scope testing to verify, I would want to be replacing the alternator on an educated guess. Much more often than not, a generator light = a faulty alternator. The question you have to ask yourself is: where do I want to be when this alternator finally dies altogether?

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Customer reply replied 2 years ago
I'll take your word for it and replace, but how come this only occurs at night? I was wondering if this model has photon sensors like the self dimming headlights cause even with nothing using the battery that I dont use durring the day it still comes on and only at night. The clock will not keep time and there is a funky way to the way it starts. When turning it over it will often start up almost too fast or more ubruptly than I've ever seen before and sometimes then bug down immediately after especially if put in reverse directly after starting it up. Does just fine if given a minute or so to come back to a normal sounding idle. Then it will often have a whistle like sound(im assuming is a squeeky belt) but it goes away with very low throttle or when accelerating/ driving at approximately how speed. Could this have something to do with the same issue?
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Jerry Newton, Cadillac Technician
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