No Aternator Light But Charging
 

How can this be? When I replaced my alternator some time back, I noticed the light was not working but ignored it since it was charging better than ever anyway.

Would there be any the advantage fixing it?


Cheers,

Joe
74 w/ 86 3.2

Question is: what good is an alternator light anyway? Answer is, it tells you if your alternator is not charging the battery, and more importantly, it will light all the way up if your fan belt breaks. This is kinda important as you really need to pull over immediately if your fan belt breaks.

Doug,

That makes sense, although I do have a digital voltage monitor plugged into my cigarette lighter that keeps me up to date should it fail.

What has me stumped is the fact that the charging light is not working and I thought it was an integral part of the charging system.


Thanks,

Joe

I guess I am missing something here. The light is a "not charging light" in layman terms. If it is charging and the system is in balance, the light has no voltage to light up. That is good, right?

It would appear someone wired a resistor in parallel to the light. This is necessary when using the newer internally regulated alternator in place of the esxternally regulated alternator (as you would have with a 3.2 in your '76). However, IIRC the resistor was supposed to be wired in series so the light would still function normally. As Doug mentioned, the light should function as intended to warn you if the alternator/fan belt breaks.

I'm sure there's a write-up regarding newer alternators in older cars here on Pelican which would explain the resistor requirement and proper installation.

Jerry M
'78 SC

I am assuming that there is voltage present at the light but the filament open.

Good, yes, but has me concerned.

On one side of the lightt is 12 volts from the battery and on the other the regulator which when the engine is not turning puts all the current that goes through the light into the field coil to generate a magnetic field. When the alternator is turning, and providing current to the field coil, their is 12 volts on both sides of the lightt and it is off.

The field sometimes has enough residual magnetism in it to generate enough current to get the alternator running.

Ohhh - Was it not working before the change out of the alternator?

I don't think it was working with the old alternator either.

Do you think the light should be on now vs. off? How do you know it is not working?

Take a butter knife and put some duct tape around the blade.

Disconnect the battery.

Gently pry the oil pressure gauge out of the dash. Do not scratch gauge, dash or yourself.

Remove the bulb holder for the charge warning light. It's the one with a blue wire going into one side of it.

Replace bulb. Note presence of any other items on the bulb holder, such as jumpers, resistors, old editions of The New York Times, etc. Discard these items in as humane a manner as possible.

Reinstall gauge and remove tape from butter knife. Reinstall butter knife in silverware drawer.

Reconnect battery.

Turn ignition switch to "on." Bulb should cheerily light.

As Rick says, current flows from the battery to the ignition switch, over to one terminal of the bulb holder, through the bulb filament (making it glow with happiness) out of the bulb holder into the blue wire, back to the voltage regulator and over to the field and to ground and back through the steel body of the car to the battery. Current has a cold Spaten Optimator after that long trip even though it is accomplished at the speed of light.

If that doesn't happen, then the alternator field just doesn't get excited. As Rick also mentions, there may (or may not) be enough residual magnetism in the field to self-excite the alternator in the absence of external current from the blue wire. You may have to spin the engine to 80,000 rpm to charge the battery this way, but it may work.

In any case, the bulb is part of the charging circuit and the fact that is also serves as a status monitor on the health of the circuit, the health of the battery, a reading lamp and the world's smallest space heater earns it more respect than to be called an "idiot light."

OK - That is what I am missing. It does not come on when the key is in the on position rather than running.

Edit - Spaten Optimator - Ha - Had to look that up.

It does not come on if all the trio diodes show up for work and the motor is running. In that circumstance, the output from the trio diodes is sufficient to energize the field, and "balance" the voltage coming from the battery on the other side of the warning lamp bulb holder. When the voltage is equal on both sides (more or less) no current flows through the filament and the light goes out.

If one of the trio diodes stays out too late and doesn't make it in the next morning, this will be reflected in a faint red glow from the (don't call it an Idiot) light.

The alternator warning light topic has come up a few times. Someone had a fairly complex explanation, but the way the cars were originally wired if the alternator light was out no charge would be generated. Wayne has a tech bulletin that may be of help and another relevant thread.

http://www.pelicanparts.com/techarticles/911_alt_replace/Alternator.pdf

http://forums.pelicanparts.com/porsche-911-technical-forum/282104-alt-light-doesnt-work.html

I guess I am still a blind man with a stick looking for the edges of a room. All that is mentioned is that the light does not work and that the system is charging like a champ. So, my conclusion is that it should not be glowing but would be glowing with the key on but before the trio clocked in when the engine was started.

Again, I must be missing something.

The initial post didn't specifically state whether it was running or not running with the key in the On position. You are describing normal operation, but since he posted I think most of us assumed it was not running and Key On.

If the charging light does not light when the engine is NOT running and they key in the On position, then the system should not charge when the engine is running. He is stating his is charging, which is not the expected operation and the subject of the thread. Hope that helps.

+1

So,..the light bulb "may" have been out PRIOR to alt replacement? You just don't recall?

Something fricked in the wiring.....? If it's NOT in the circuit, then you're not charging (if wired acc'g to factory),..UNLESS someone's wired it differently...

I'd like to know the voltage measurement AT the bulb holder when in the ON position (NOT running).

When was the transplant done?

Doyle

Perfect. Thanks much.