Electrical Woes: Alternator not Charging

[euro911sc]

Before you say get a new alternator... I did... and I had the old one rebuilt... same problem. So...

• Battery is an Optima Red. I can charge the battery up and the car runs like a champ: 2 days at homestead running 30 m sessions while charging the car between sessions yielded FUN FUN FUN
• I took the old alternator to an alternator shop. Called back a few days later and the old timer said he saw immediately what the issue was and fixed it. Put it in and system doesn't charge.
• Spoke to a top line mechanic, Klub Sport, he mentioned that 'those' alternators are iffy at best to repair. Bought a NEW alternator, Valeo. Installed. No charging.
• I have checked all the major grounds in the car.
• The charging warning light does not come on.
• Loss of charging seemed to be instant and not a slow decline over time.

How I test the charging system: Charge the battery, measure/note voltage across battery (12.49), start car, measure voltage (11.95), turn high beams on, measure voltage over time... ie: watch the voltage decrease real-time (it dropped to 9.3 in a few minutes before I shut the car off)


I have pulled out the electrical diagram and the systems seems very simple as far as the battery/alternator is concerned. There is a Red/10 wire that goes from B+ to the starter at the same terminal as the big honkin' Blk/25 cable from the battery. I believe this is how the alternator charges up the battery.


My trouble shooting ideas:

1. Since the car will run and start just fine with a charged battery the issue must be in the wire from the alternator to the starter? This would be part of the wiring harness on the engine.
2. I can test the voltage coming out of the alternator if I follow the other Red/2,5 wire from B+ that goes to Fuse 24/25A that leads to the rear defogger relay. This fuse is on the rear panel. Which of the 3 is it? top or bottom one? If this voltage says 14v then that will confirm that the alternator is working properly, no? If it says 0v what then?
3. Is it possible that the battery cable is done? Ie: the battery has enough CCA (800+) to start the car, but the alternator at a meager 75A cannot overcome the resistance to charge the battery? This is a long shot given the sudden loss of charge ability.

This is a bummer since I was supposed to be at Sebring for the whole weekend...


Ideas? Thanks for your help in advance!


-Michael

[JJ 911SC]

Once you start the car, the voltage on the battery should be up not down.

Have you got an External Voltage Regulator?

If so, I would suspect it.

[luskesq]

This may be too simple but have you confirmed that the bulb for your alternator light is good? It needs to work to "excite" the regulator, thus resulting in the alternator putting out a charge (although I'm no expert in these matters).

Keith

[pete3799]

Originally Posted by luskesq

this may be too simple but have you confirmed that the bulb for your alternator light is good? It needs to work to "excite" the regulator, thus resulting in the alternator putting out a charge (although i'm no expert in these matters).

Keith

+1

[kerthunk]

Originally Posted by luskesq

This may be too simple but have you confirmed that the bulb for your alternator light is good? It needs to work to "excite" the regulator, thus resulting in the alternator putting out a charge (although I'm no expert in these matters).

Keith

+1 Before you start the motor, make sure that the alt light is on with the key in the on position.

[prebordao]

So, external regulator or not ?

[euro911sc]

Its an internal regulator.

I didn't even consider the bulb... meh its a bulb... Damn that F****** bulb! Who knew it was an on/off switch for the alternator! WTF? The whole car can be disabled by a single bulb? The light does not come on with the key so I'll swap it out with a spare and see if she charges. It always glowed at idle and I thought 'Damn! I finally solved the glowing at idle problem I have had for 17 years! Cool!' *Face Palm* Seems I am now in possession of a spare alternator...

Lets hope its the bulb or I'll be calling you all wankers for getting my hopes up!

[Josh D]

I just recently replaced battery and a bad alternator. Following replacement, all the lights, including the dash lights, were brighter.

Several days later the Alt light doesn't come on. I'm thinking WTF? Bad alternator rebuild?

Nope. The fresh alternator was too much for the old bulb and it killed it. I was relieved when a new bulb took care of that problem.

[KNS]

Is it the same type of bulb as the other dash bulbs?

[luskesq]

An '82 I believe is a 4 watt bulb. I have a '75 that takes a 2 watt, same as other dash lights except I put a 91 ohm resister in parallel on the light because I have a new alternator that is internally regulated.

Hope for you it is the bulb!

Keith

[JJ 911SC]

QUOTE=luskesq;6753250]An '82 I believe is a 4 watt bulb...[/QUOTE]

Correct

Pelican Parts - Automotive Parts and Accessories - Porsche & BMW

[KNS]

Thanks for the info on the bulb - probably wouldn't be a bad idea to keep a spare.

[JJ 911SC]

Originally Posted by KNS

Thanks for the info on the bulb - probably wouldn't be a bad idea to keep a spare.

If i'm not mistaken for the 3.2, the light bulb is not part of the charging systems

[euro911sc]

HAHAHA you guys are lucky! Oh and very helpful too! Bulb was blown... replaced and now I have a nice new charging system! YEAH! Now I can charge off to Sebring for the weekend without fear of being stranded on some 2 lane highway in the middle of no-where central florida... if you have been there you know what I am talking about!

As a side note, I now have a rebuilt alternator with 0 hours on it

Thanks again for the tidbit of knowledge that had escaped my 17 years of ownership with the car!!

-Michael

[kerthunk]

Good to hear it is working! Enjoy Sebring!

[luskesq]

Aren't simple fixes great! Glad for you and have a great weekend.

Keith

[kodioneill]

Spoke to a top line mechanic, Klub Sport, he mentioned that 'those' alternators are iffy at best to repair. Bought a NEW alternator, Valeo. Installed. No charging.

OOOPPS! He should've known that it could've been, and most likely was the bulb.

[Lorenfb]

"Spoke to a top line mechanic, Klub Sport, he mentioned that 'those' alternators are iffy at best to repair."

The '82/'83 911SC Paris-Rhone (Valeo) alternator is a marginal alternator and unreliable
compared to the earlier 911SC alternator (Marchal).

[prebordao]

BTW, what was the alternator on a 74' ? Marchal or Motorola ?

[Lorenfb]

"what was the alternator on a 74' ? Marchal or Motorola ?"

They both look the same and were both used (2 alt sources),
another weak Porsche alt.