Charging system help?

[n8marx]

So I'm driving to work this morning when all of a sudden my 3.2 Carrera starts spitting and sputtering and finally quits altogether with a dead battery. After arranging for my wife to bring jumper cables, tow strap and extra charged battery to me and my dead car we discover that the "new" battery starts and runs the car just fine. After driving home I checked the voltage at the battery with the engine running. 12.09 volts. Okay, alternator is not charging.

I have searched the archives and have come across more than one post stating that if the charging system light does not come on with the key turned, which mine doesn't, then the charging system will not work at all since the alternator's pre-excitation field current is routed through this bulb.

Question: Can someone better explain this? Could my whole charging problem really be as simple as a burned out bulb?

[ischmitz]

I am not sure if I can explain things much better. Here is a try:

In a nutshell the rotor is not a permanent magnet but an electromagnet with variable field strength. It produces a magnetic field by means of running a current through its coil. In a running and charging alternator the field coil gets its power from the stator coils by means of an rectifier array and the regulator.

If you rotate this electromagnet without any field it will not induce anything in the stator coils - No output.

To get the alternator to produce power you initially need to supply battery power to the field coil. This is done via the GEN light. Once this initial field powered by the battery induces some juice on the stator coils the alternator can sustain itself by using part of its output (D+) as field current supply DF.

Some cars do have a resistor in parallel to the alternator light and should charge even with a burned bulb.

Does this all make sense?

Ingo

[Jocke]

Check the ground-cable between the battery and the body, se if there´s in bad shape and in that case just clean up the contact-areas on it.

I think you find your problem there.

/Jocke

[n8marx]

Thanks. That makes sense. So the initial current through the bulb just get's the ball rolling, once the alternator is generating it's own current it doesn't need the current from the bulb so the bulb goes out.

It still seems odd that a $1.50 light bulb could bring my otherwise wonderful car to it's knees. Perhaps I ought to just accept it and move on.

I'll check the ground strap too.

[ischmitz]

Originally posted by n8marx
So the initial current through the bulb just get's the ball rolling, once the alternator is generating it's own current it doesn't need the current from the bulb so the bulb goes out.

Yep, that's a good way to put it. Here is a link talking about Aternator theory.

As Jocke said checking your ground and cable connections is always a good point to start.

Ingo

[Rot 911]

Originally posted by n8marx

It still seems odd that a $1.50 light bulb could bring my otherwise wonderful car to it's knees. Perhaps I ought to just accept it and move on.

The exact same problem happens if the bulb on the alternator warning light burns out on my high dollar MGB!

[Early_S_Man]

Kurt,

I guess the concept of 'high-dollar' depends on whether you are selling a car ... or replacement parts!!!

What does a replacement aluminum hood for an MGB go for, these days?

[n8marx]

Well, the light bulb was not the problem. The bulb was still good, just not lighting up when the key was turned on, or at any other time. Continuity from the bulb to the alternator was good and ground straps do not appear to be the issue. Wayne's book says the other options are a failed alternator or a voltage regulator for these symptoms. Sound right?

So the alternator is out of the car and will hopefully be checked out soon. The voltage regulator is internal to the alternator on my '86 right? Any competent auto electrical place ought to be able to check it out for me?

Any other ideas?

[n8marx]

Turns out it was a set of worn out brushes in my alternator. Sixty bucks to fix it. Gonna button it up and go for a drive as soon as I finish cleaning up the fan and shroud.

[Kemo]

Good troubleshooting. Im glad it worked out.