New Alternator - Now Battery Light is on at Start-up Idle...

[86 911]

Hi all,

Car is a '76 911 with a '78 SC engine. A few weeks ago I posted that I had alternator problems. Well, I got a rebuilt alternator and put it in the car. It seems to be working well EXCEPT: the minute I start the car, the battery light stays on and I am getting NO voltage to the battery. This problem is resolved when I increase the idle to 1500 RPM. Once I do that, the battery light turns off and I am getting the usual 13.6 volts to the battery. The odd thing is that once the engine is running and I get the light to turn off, it stays off no matter how high/low the RPMs are. Even when the car is idling, I am still getting voltage to the battery as long as I rev the engine past 1500 when it initially starts. It's almost like I have to "jumpstart" the alternator to produce voltage by that initial rev up to 1500. Does it sound like the rebuilt alternator might be bad?
The belt tension is good and I am using 6 pulley shims as recommended. The alternator also has an internal voltage regulator. Thanks!

[Joe Bob]

The pulley size different? When I went to a single belt conversion on my 3.6...it did that....been so for five years.....

[KNS]

Have you had these symptoms since the moment you installed the relacement alternator? If so it sounds like the alternator.

[86 911]

Yeah.. I never had these symptoms before... and the pulley is unchanged. Well looks like it is time to pull the alternator and take it back to the shop that I bought it from.

[dlearl]

Or the internal voltage regulator. I'm on my second round of the exact same issue with my ML 320. It's the VR. Luckily, the ML's alternator has a separate VR (~$24 here at PP) vs the $600 alternator.

I think Joe may have something with the pulley size, though. Can someone verify if it's the same, or different 76 to 78?

[luskesq]

I don't know if this is relevant to your problem but with a Bosch upgraded alternator ('78-'83 3.0 SC) in my '75 911S, they recommend a 91ohm, 5 watt resistor at the alternator lamp to "excite" the internal regulator because it had a 2 watt bulb. Later models had I believe a 4 watt fixture.

Keith

[kerthunk]

luskesq has it right. You need to install a 91ohm, 5 watt resistor parallel to the alternator lamp. There is a Porsche tech bulletin that deals with upgrading the voltage regulator to an internal one. They also describe using the resistor. I had a tough time finding the parts, but finally got the bits from Mouser Electronics. If you need, I can take a pic of the one I installed to deal with this issue. Worked like a champ for me.

[Lorenfb]

"I don't know if this is relevant to your problem but with a Bosch upgraded alternator ('78-'83 3.0 SC) in my '75 911S, they recommend a 91ohm,"

That's correct for the Paris-Rhone (Valeo) 75 amp alt internal regulator alt,
especially if the relay used in the older cars is still connected to the alt light.
The original Marchal 70 amp external regulator does not require this mod.

[86 911]

I understand what you are saying but I checked voltages at the battery and the light seems to be giving an accurate warning. When I start the car and the light comes on, the voltage at the battery reads 11 volts. When the idle increases to 1500, the voltage jumps up to 13.6 volts and the light turns off. Therefore, I don't think the light is the issue.. I think the alternator is not turning on until I hit 1500. Very odd. Oh an it is a Paris Rhone alternator.

[Joe Bob]

So giving the motor a rev to get the light going out is a problem? Geeez.....gives me a stiffy and pisses off my neighbor that wears the foil helmet.....if it does the deed, have a beer and fugggetaboutit.

Talk to your rebuilder.......let him know about the issue. If you want to do the yank on the alt....go for it...to me it's a waste of time.

Kinda like smelling oil and leather in my ride when I get in.....my kids say....what's that smell? My answer is...'It isn't stale fruit loops...buckle up, hold on and shut up'....

[K Sykes]

That's what my car does as well.. for 10 years and 3 alternators. It's not a problem, just a quirk. If it bugs you, solder in a resistor. Otherwise, just know that's one more thing that makes it not a Buick.

[kerthunk]

The exciter circuit is not yet kicking in to tell the alternator to start passing the juice along. This is controlled by the blue wire coming off of the alternator and going through the lamp. Putting the resistor in there doesn't futz with the light itself, but it helps activate the exciter circuit sooner.

I had this issues when I put PMOs on my car. All of a sudden I had much less electrical load and so the exciter circuit wouldn't fire up until I blipped the throttle to 1500rpm or so. After putting the resistor in, it now kicks on around 700rpm.

Originally Posted by 86 911

I understand what you are saying but I checked voltages at the battery and the light seems to be giving an accurate warning. When I start the car and the light comes on, the voltage at the battery reads 11 volts. When the idle increases to 1500, the voltage jumps up to 13.6 volts and the light turns off. Therefore, I don't think the light is the issue.. I think the alternator is not turning on until I hit 1500. Very odd. Oh an it is a Paris Rhone alternator.

[kerthunk]

Here is the Porsche Tech Bulletin I mentioned above:

http://members.rennlist.com/emcon5/Tech_bulletin_1.pdf

In the right column on page 2. Look at the "Note" and then the instructions below it.

[Lorenfb]

"When the idle increases to 1500, the voltage jumps up to 13.6 volts and the light turns off."

It's a function of the replacement regulator being used in the rebuilt alt and the excitation
current required by that regulator. If the regulator used by the rebuilder is not the OEM
Paris-Rhone (Valeo), the excitation current most likely needs to be increased beyond
the typical OEM value by using a resistor in parallel with the alt light.

[86 911]

Ok, now I understand! You guys just saved me from lots of work. Thank you kerthunk for the link to the page. Would you happen to have a list (or a link) of exactly what you purchased from Mouser so that I can get the same thing and install it? If you can post a photo of what the resistor looks like on your wire, that would be really helpful. The photo in the link is a bit shady.

[ljowdy]

Why go the trouble of installing a resistor?? Start the car, give the accelerator pedal a short "blip" and the light goes out. I've been doing it for years !!!!

[kerthunk]

Hey Matt, happy to help. I wrote up a new post that explains the process.

Alternator light doesn't go out after startup

Originally Posted by 86 911

Ok, now I understand! You guys just saved me from lots of work. Thank you kerthunk for the link to the page. Would you happen to have a list (or a link) of exactly what you purchased from Mouser so that I can get the same thing and install it? If you can post a photo of what the resistor looks like on your wire, that would be really helpful. The photo in the link is a bit shady.

[Jdub]

I would be very sure the battery is at full charge and not decrepit, and that my ground for the battery and the strap under the car were in good shape. Lots of misery from crappy grounds attempted through older braided/covered cables. A weak battery can give the diodes fits in a alternator.

[E Sully]

Originally Posted by 86 911

When I start the car and the light comes on, the voltage at the battery reads 11 volts. When the idle increases to 1500, the voltage jumps up to 13.6 volts and the light turns off.

I would have the battery checked also. The battery doesn't seem to be getting fully charged. 11 volts is a bit low.

[Joe Bob]

You should test at rest, idle AFTER the light goes out and at 2500 rpms.

A slow light going out after start up is a common problem. IF you feel the need, put a resistor in. I haven't.....it's a quirk I just deal with.