Dimly lit Alternator Console Warning Light

[Mr. Chuck]

I thought I had posted this problem before, but can't seem to find it to up-date it, since I have now found and corrected the cause of the problem.
Problem: For several years have experienced a dimly lit alernator warning light.
Work done: Checked and/or replace Alternator / light wiring and connectors; without success. After input from dealer and other Vette Service shops; removed alternator, dis-assembled and check out varies electrical components (diode trio, rectifier, regulator, etc) Found diode trio had one open diode element. Replace diode trio and the problem is fixed.
Funtional analysis: The three diodes provide rectified DC voltage to one side of the warning light, while the other side of the warning light receives output DC voltage from the alternator. When the two equal each other light remains dark. In this case the diode trio output was low since one or more of the diode elements had failed. The use of an volt meter set for AC Voltage would have shown some level of AC / varing DC voltage at the alternator #1 pin, the Brown wire on my 81 vette to alternator housing. Hope this info may be helpful to someone out there!

 

[Vetsvette2002]

I thought I had posted this problem before, but can't seem to find it to up-date it, since I have now found and corrected the cause of the problem.
Problem: For several years have experienced a dimly lit alernator warning light.
Work done: Checked and/or replace Alternator / light wiring and connectors; without success. After input from dealer and other Vette Service shops; removed alternator, dis-assembled and check out varies electrical components (diode trio, rectifier, regulator, etc) Found diode trio had one open diode element. Replace diode trio and the problem is fixed.
Funtional analysis: The three diodes provide rectified DC voltage to one side of the warning light, while the other side of the warning light receives output DC voltage from the alternator. When the two equal each other light remains dark. In this case the diode trio output was low since one or more of the diode elements had failed. The use of an volt meter set for AC Voltage would have shown some level of AC / varing DC voltage at the alternator #1 pin, the Brown wire on my 81 vette to alternator housing. Hope this info may be helpful to someone out there!

Since the diodes are internal to the alternator, when you exchanged the alternators why didn't the new alternator fix the problem? Why didn't a performance check of the alternator reveal the problem?

 

[KANE]

Since the diodes are internal to the alternator, when you exchanged the alternators why didn't the new alternator fix the problem? Why didn't a performance check of the alternator reveal the problem?

I had the same problem.

Until I put in a blue bulb. Then, only when the alternator REALLY goes out will you see it.

Ghetto, but it works.

 

[Mr. Chuck]

Never did replace the alternator, torn down the unit, replaced the defective diode trio part, which fixed the dim warning light problem. Other defects such as bad warning connections / burnt connector contacts to name a few of other potential causes. In my case all these aspects were checked and eliminated as the cause of the light problem.

 

[goertz]

Wouldn't it make sense when the alternator light comes on, to have a look at the alternator before replacing the wiring and connectors for the light? Not trying to be a smart ass, I was just curious why you didn't suspect the alternator as the problem in the first place.

 

[Mr. Chuck]

Initial visual inspection of wiring and associated connector clearly showed degraded wire installation as well as broken connection housing and loose / worn-out contact pins that are normally hidden inside the housing which had mostly fallen away over years of engine heat. This was the most accessable fix and may have been adding an unwanted voltage (IR) drop across the contacts, which would have accounted for a voltage differential at the generator warning lamp. Analog volt meters (cheap ones) like the one I have don't have the ability to display small IR voltages developed across degraded connector contacts. Being an old Navy aircraft electrican, experience says look for bad connections first, especially in areas of high heat and vibration.

 

[goertz]

I see. Did the car always start? I think you can get away with one bad diode, and probably not have dead battery all the time.

 

[Tifton81]

I still have that problem with my 81. The light shows dimly but when I turn on the air or lights and lets say the wipers, the light gets bright and the volt meter drops and everything seems to slow or dim. It seems as though it does not supply enough power when needed? Any ideas?
Tifton81

 

[Mr. Chuck]

The diode that was bad was one leg of the diode trio, which only function is to supply DC of equal level to one side of the generator warning light. The diodes responsible for the main output / charging the battery are contained in the rectifier assy that mounts to the inside of the alternator rear housing.

 

[Mr. Chuck]

It sounds like you need to have the rectifier diodes checked as well as the diode trio. If you live in an area where local auto store / shops offer free electrical testing, their tests will check the output / load capacity to spec called for the alternator in your Vette and also the battery capacity under load. I am not sure if the test will tell if the diode trio is good or bad, you could ask if it does. In SoCal stores like Auto Zone and Kragen offer the free on car electrical test.