Generator/alternator dash light is weird on ‘78 Eldorado

[Tornike]

Good weekend morning, fellows. Back to my ‘78 Eldorado, I’m trying to see if there is something going on with the alternator.

When I put an ignition switch on, the generator bulb on dash lights up bright along with brakes and oil pressure lights. When I start the car all the lights go away, but the generator light dims so that it’s hard to see on daylight, but it’s still on.

I know from the manual that if generatir light comes on than the alternator has a problem.
I can’t decide whether it’s a faulty bulb controller on the dash or my alternator is actually bad.

I measured the voltage from the front cigarette lighter. When the car’s off, it gives me little over 12v. When it’s on it gives me 14.3v.

I have put a new Valuepower 78 600 CCA battery 10 days ago since I got the car. I start the car daily for 5-10 minutes and have never had problems so far. I intentionally left the doors open yesterday for about 10mins with interior lights on and engine off, but the car fired up just fine. Now when I had my headlights on and tried to start the car that way it hesitated for half a second, but started normally.
When I put the AC on while engine running I can see it affects the lights just a bit but interior lights come back at the same brightness after a millisecond. However, when I turn the engine off the interior lights dim slightly. Everything in the car operates well electronically at normal speed. Trunk closing mechanism also works well when car is off.

Attaching photos of my dash taken in the evening that show the difference in brightness. First shows just an ignition on. Second is when the engine is running.

What are your thoughts on that? Any other way to diagnose the alternator?

[cadillacmike68]

Get an amp meter and test the output to the BAT line on the alternator.

[Jeff Rosansky CLC #28373]

Also check the alternator belt. If that doesn't work clean all the contacts on the alternator--disconnect the battery before you unplug anything.
Jeff

[bcroe]

I have seen 2 causes of the dim GEN light on these internal reg
alternators.  Normal operation is ign 12V is applied to the bulb,
current flows through to the alternator to energize it.  Once the alt
starts up, the small diode trio takes over the job of powering it up,
with 12V on both ends of the bulb it goes out. 

If a main rectifier diode opens up, the output winding voltage will
not be clamped to battery voltage for 1/3 of the time.  Spikes of
higher voltage will be fed back through the bulb, partly lighting it. 

The other cause is voltage drop in weak wiring.  Voltage of course
will be highest at the alternator.  If there is excessive voltage drop
in the alternator output wiring, and more from there to the ignition
circuit (made worse by loads turned on), there may be enough
difference between the alt stud and the bulb feed to cause enough
bulb current to dimly light it.  This reverse bulb current can be
stopped with a diode in the alt lead to the bulb, but better is to
improve the wiring.     Bruce Roe

[Eric DeVirgilis CLC# 8621]

I had a similar issue in the past with a few 425 engine Cadillacs in which the GEN lamp remained illuminated when running with normal charging and voltage output. Problem was due to bad diode as bcroe suggested.