Alternator not charging....

[Justdamnlucky]

I own a 83 911sc, rumor has it, that if your charge light doesn't come on with the key on, and engine not running, that when it is running the system won't charge. Could this be true?For the life of me, I don't recall ever seeing the charge light on...

[Alan L]

You will prob get specific wiring help on the 911 Tech forum. But, yes - according to my SC wiring diag , it is part of the charge circuit - I think linked to some exciter/energiser function in the alternator.
Alan

[patina]

Bulbs go out occasionally as well...

[xbmwguy]

Yes the bulb needs to be a 3 watt if I remember correct. Has something to do with the exiter wire circuit. Pull gauge and check bulb

[Alan L]

Originally Posted by patina

Bulbs go out occasionally as well...

Yes, but I am pretty sure it is part of the circuit (not just a warning light). Dead bulb = dead circuit.
Alan

[JFairman]

Yup, the exciter wire is the thin blue one on the back of the alternator.
I think I remember reading the alternator light bulb is wired in series with the exciter wire. I guess that's why it gets dim if the charging level goes down.
Your 160 amp alternator is probably always charging.

[Alan L]

Because it is in series with the exciter circuit, when it is dead, the exciter circuit is dead. Which means the alternator cannot charge (I think). According to my wiring diag, the power feeds in to one side of the bulb, and then travels to the exciter circuit in the alternator. That is my understanding. And it is wired to a diode circuit which allows voltage to pass one way - when charge volts are less that battery voltage.
That is my crude understanding of the system. Hopefully someone will fill in the blanks.
Alan

[Ken911]

Originally Posted by Alan L

Because it is in series with the exciter circuit, when it is dead, the exciter circuit is dead. Which means the alternator cannot charge (I think). According to my wiring diag, the power feeds in to one side of the bulb, and then travels to the exciter circuit in the alternator. That is my understanding. And it is wired to a diode circuit which allows voltage to pass one way - when charge volts are less that battery voltage.
That is my crude understanding of the system. Hopefully someone will fill in the blanks.
Alan

thats pretty much correct. So if the diode burns out or the bulb your alternator will not charge. I spent weeks trying to track down my burnt out diode. In the turbo manual ther is a simple trouble shooting chart that if i would have followed i would have found it in a few minutes.

[Justdamnlucky]

Thank you all, for helping me to get headed in the right direction!
Now...
To figure out best way to check, and change the bulb. Go through the trunk, or, pull the gauge, and, the best way of how, with the least chance of damage.

[Ken911]

just pull the gauge out. the bulb plugs into the back. the Diode is inlne with the wire that plugs into the gauge. it's blue with a black stripe and very thin.

[Justdamnlucky]

Originally Posted by Ken911

thats pretty much correct. So if the diode burns out or the bulb your alternator will not charge. I spent weeks trying to track down my burnt out diode. In the turbo manual ther is a simple trouble shooting chart that if i would have followed i would have found it in a few minutes.

Hey Ken911, I've been trying to trouble shoot this for a while too... Could you pass on that trouble shooting chart for checking the diode?

[Ken911]

uh it's still in a moving box someplace packed away. moved several years ago and still have boxes full of crap. If someone here has the page you might want to post it there is a thread on here someplace that lists the correct trouble shooting sequence. i dont remember if it was in the bently manual or the factory turbo manual.

[Zaz]

The bulb socket/wire can also pull/fall out of the gauge housing. At least that was the case with mine when I realized after a year of ownership that I had never seen the alt light...pulled the gauge and it was lit back in the nest.

[franklinstower]

You could always check your voltage at the battery when running to see if you are getting near 14v or not. that is where I would start.

[Justdamnlucky]

Okay, so, the the bulb is good, but no diode is in line where the light is... I have charged the battery, and I did a voltage test with engine running at the battery, and there was nothing. Seems odd that with no accessories on that, the battery would drain so fast. Went for a short drive, and it would run out of juice and die...(Probably around 1 hours time) The car has 160,000 miles on it. Could it be brushes...voltage regulator... diode... short..? Hmmm...

[Ken911]

Diode could be bad. Check across it. The line going to it. If it's burnt out it'll be open. Got van just short across it the battery will charge.