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I am hoping that the wealth of knowledge on this website can help me to diagnose what is going on and how to fix it and if I need to replace the alternator.

The symptoms:

1. If I drive without any electrical load (except for the ignition & the fuel pump), the charging light can't be seen coming on (actually it does very slightly and can be seen if I pull the light from the instrument panel)

2. If I drive with some load, like the headlight or the heater fan or the A/C motor on, the charging light comes on, the more the load the brighter.

I know most of you will tell me that the alternator is not charging the battery causing the more load the brighter the light. However, when I did some checking the voltage on the alternator (B+) and the blue wire terminal (D+) I am not too sure if that is the case.

As you know the reason the charging light comes on is if there is a difference in voltage between the alternator (B+) and the blue wire terminal (D+), so I did some voltage checking at the alternator at different RPM and loads and this is what I found out

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RPMAlt (B+) w/o loadAlt (B+) with loadD+ w/o loadD+ w/load
2500more or less the same as at 2000 RPM 

If I check the voltage for B+ on the battery, it is a little bit less but it is less than even 1 volt.

What this is showing me is that the D+ (blue wire terminal) voltage goes way up and the difference with alternator voltage is causing the light to come on. Also you can see that the difference in voltage  between B+ andD+ with load is also bigger than without load between the 2 terminals causing the light to come on brighter.

Any idea what is going on? Should I replace the alternator? If I don't need to replace the alternator how do I make the charging light not to come on?

I need the power of knowledge and experience here.

Thanks all

1957 JPS Coupe #3

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What puzzles me is not the voltage on the alternator as it is around 14.4-14.6 volt without much load and with load will get to that voltage at higher rpm which according to my mechanic means it is charging the battery. What puzzles me is the D+ voltage why it goes up that high (16.5-20.5 volt) causing the charging light to turn on dimly.

From an auto electrical site:

A possible problem with a warning light is that it goes out as normal when you rev the engine slightly but comes on again as the engine speed is further increased.

If the warning light simply glows dimly it usually indicates the problem is a high resistance somewhere in the warning light circuit.

Test this by switching on more and more electrical accessories once the light has started to glow. If high resistance is the problem, the warning light will become brighter as you do so.

Carefully work your way through the entire charging system looking for loose or dirty wiring connections causing the high resistance. Disconnect the wires and clean up the terminals if they are dirty. Don't forget to check whether the battery lead connections and engine ground strap are still good.

Remember also to check the alternator to engine mounting bolts or strap. These provide a ground passage from the generator to the engine and body, but because of their exposed position in the engine bay they can become oily and provide only a poor ground connection.

Check this connection by removing the mounting bolts or strap. Clean away all traces of oil and dirt then refit the mount, making sure they are tight.

Hi Gordon, thank for the suggestions on what to do I might do that although tracing wiring is not really my favorite things to do.

On Allan’s comments: “when the alt reaches minimum rpm's  to engage the charging circuit the ground in the alt is no longer present and the light goes out light.” how does that work? Is there a “switch” in the alternator that disconnect the ground?

There isn't really a "switch" in the alternator.  One side of the dash light goes to 12 volts via the ignition switch.  The other end goes through the D+ terminal on the alternator and connects through the "exciter" coil and then to ground on the other end of the coil windings.  Since it is grounded through that winding, it lights when the key is on and the alternator not moving.

Here's where the electrical "magic" happens.  

Once the alternator starts spinning, it begins to generate voltage within both the primary windings (another coil that charges the battery) and the exciter windings.  When the exciter windings generate as much or more voltage level than the battery's voltage, it magnetically isolates the dash light circuit from ground and essentially cancels the lamp out since it now has 12+ volts on both sides of the lamp.    12 volts coming from the ignition and 12+ volts coming from the exciter windings.  No ground, no light.  

As the alternator slows down and the exciter winding voltage drops because the magnetic field becomes weak, eventually it goes below the level of the battery voltage and allows that dash lamp to magnetically "see" ground through the exciter winding.  It's a progressive thing, like a dimmer switch.  The slower it turns the weaker the magnetic field and the more "ground" it sees and the brighter the dash lamp gets.

Hope this helps.  It almost seems like there's some black magic going on, but it's all magnetic fields, like a motor in reverse.

Last edited by Gordon Nichols

I'm the exception I like new wiring projects and finding issues in older stuff to resolve ~ On the dune buggy I did it had all new harness and one of those 5wire to 4 wire boxes for a single combo taillight on each side . The left rear was dimmer for tail, brake, and signal lights so automatically I go for ground issues and find nothing. Next I jumped out that 5 to 4 wire electrical conversion box and it still was dim.  Then I look at the front amber part lights that has a dual filament bulb,and one is brighter than the other... I witched the one wire to the smaller filament then the  taillight worked as it should. The 5 to 4 wire box being "electronic" was getting the wrong input signal because of the wiring was reversed for the filament in front light .   Electrical issues usually can be traced to fuses which we all miss, faulty grounds or wiring issues.

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