Alternator light issue
 

Car is an 86 and have owned it for 6 years now. One of the first year projects was rebuilding the alternator. New bearings, cleaned the commutator and installed a new voltage regulator/brush unit (t was a Paris Rhone style). Right from the start, the alternator indicator light glowed very slightly at idle but pretty much goes out (I thought) above 1000 rpm. Output is great, voltage will remain at 14.2 even with all loads on but never more. It's pretty rare that I drive when it's dark but I did the other night. With the darkness it became evident that the indicator light is always glowing and even more so with the headlights on. Now the battery has always stayed fully charged so I have not been too worried about.
Still, it's not right so why is the light not going out as it should?

Using the alt. light as a diagnostic tool:

1. no light with ignition "on" not running
a. bad regulator
b. bad light
c. open slip rings
d. bad brushes

2. light stays "on" with key "off" - shorted diode/diodes

3. light glows brightly while running
a. open diode/diodes
b. shorted rotor/stator winding
c. shorted alt. light wire to ground
d. bad regulator

4. light glows dimly & gets brighter with higher RPMs - voltage drop,
e.g. @ alt. B+ post or @ starter connection

5. light flashes at low RPMs - bad slip rings

6. light glows dimly at all RPMs - open diode/diodes

Since you are charging the battery It would seem that you do not have an issue with the rectifying diodes. You have the internal voltage regulator (VR) and I would suspect the operation is the same as the external VR. At one time, I had the glowing generator light but I had the external voltage regulator vs. the internal. In my own case, the alternator light came on at idle and would turn off as the rpm increased. Eventually the light stayed on all the time and the battery would run down. The alternator failed due to heat damage to the exciter diodes (these are not the main rectifying diodes). I replaced the three exciter diodes with 1N4000 diodes at a cost of ~ $1.50.

How it Works

When the car is started, terminal D+ is connected to terminal DF inside the VR. This means that the alternator field is connected to +12 through the three-watt warning lamp: the current to light the lamp flows through the field winding to ground and provides the initial excitation for the alternator. As output voltage rises, it flows through the exciter diodes and appears on terminal D+. The voltage is then equal on both sides of the warning lamp and it goes out.

Voltage at D+ may be getting enough higher than the battery voltage to cause the bulb filament to glow. That may be the result of voltage loss/drop from the rear to the front when the electrical system is heavily loaded, lots of current flow from the alternator to the load center.

Some cars use a diode in the indicator circuit to prevent this, block any backflow from D+ to the battery.

With the car running at high idle, 2000 RPM, turn on all electrical loads and then measure the voltage across the battery posts and then the unswitched voltage at the fuse panel in the engine compartment. The latter voltage should not be greater than the first by more than 0.5 volts.

Thanks to everyone for their input. Sort of good news/bad news finding. The voltage difference front to back running with a heavy load is .2 volts. The bad news is the voltage is kinda high at 14.45 at the engine side. The battery voltage is the lower value. No problems with battery boiling or electrolyte losses. Nice bright lights though. Opinions on that high value? And still no solution to the glowing light.

Measure the AC ripple content at the engine fuse panel vs at the battery.

Open diode to D+/VR might have that effect, high B+ voltage, plus higher than normal ripple content on D+, which would help explain the "glow".

Did the light glow before you cleaned up the alternator.

There are 17 diodes to test in the alternator, that is where I would start

I forgot to mention, I did check for AC at the battery while it was running. Between zero and .1 And did it have this problem before the rebuild, not sure. When I bought the car in November, winter was on us early so I never had a chance to drive it further. Wish I had checked each diode, looks like that will go on the list for this winters tasks.

AC content at the rear?