SC is reading 13 volts at the battery at idle — how much time do I have left?

[david05111]

The battery light on my SC has been lit basically since I inherited the car and got it back on the road last year. I never have any issues starting it, so I’ve not really looked into it that seriously.

Since I was working on the car earlier today, I decided to test the voltage at the battery when it was idleing. It’s at roughly 13 volts. When I turn the headlights and high-beams on, it goes to around 12.4.

I know that it’s supposed to be at 14-14.3 volts if the alternator is running properly. That would seem to suggest that it’s on its way out.

Anyone have any idea what sort of timeframe I should expect? Is this something that has to be addressed immediately? Or could this be something that just can be managed for a while?

[Schulisco]

Most supposingly the alternator is dying. Resolve the issue asap.

The alternator on a 911 is basically the same as on other cars...and so the error lookup process.

https://www.hella.com/techworld/sg/Technical/Car-electronics-and-electrics/Starting-and-charging-system/Checking-the-alternator-41623/#

CAUSES OF FAULTY ALTERNATORS: CAUSE OF FAILURE
An alternator malfunction can have different causes. The cause is not always due to an internal alternator fault, such as a faulty winding, rotor, rectifier or control unit. Before replacing the alternator, additional components should be considered as a cause of failure and checked.

Prematurely aged or faulty vehicle battery
Electrical connections on the alternator loose or faulty
V-belt or V-ribbed belt loose or faulty
Belt tensioner or free-running roller damaged

Go and find out if your car still has the external volatge regilator or the later alternator version with builtin voltage regulator. On too low voltage mainly the voltage regulator is bad. An external VR can be swapped easily, they weren't available for years but now they're available again. If the alternator has a builtin VR then replacing the whole alteenator makes sense. You can also overhaul it by yourself.
More info:
https://youtu.be/K4f6Mu4GV-w
https://youtu.be/Nm1oGok5Ppo

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/20-Alternator_Replacement/20-Alternator_Replacement.htm

https://forums.pelicanparts.com/porsche-911-technical-forum/54271-alternator-update.html

https://forums.pelicanparts.com/porsche-911-technical-forum/709712-911sc-alternator-update.html

Thomas

[stownsen914]

You're probably right, but it's worth doing a load test on your battery before replacing the alternator. Easy enough to do, I think some Autozones etc. do it for free. Or just test the alternator.

[GH85Carrera]

One quick question about alternator replacement. Is there a torque value for the band that holds the housing in place? I don't want to crack the housing with to much torque on the Allen head bolt that tightens the band around the housing.

[Schulisco]

Originally Posted by GH85Carrera

One quick question about alternator replacement. Is there a torque value for the band that holds the housing in place? I don't want to crack the housing with to much torque on the Allen head bolt that tightens the band around the housing.

Depends on the size of the used nut, they may differ:
M6 6,5Nm
M8 12Nm
Taken from the tech spec book for the 911.

Thomas

[GH85Carrera]

Originally Posted by Schulisco

Depends on the size of the used nut, they may differ:
M6 6,5Nm
M8 12Nm
Taken from the tech spec book for the 911.

Thomas

Thanks

[john walker's workshop]

Might want to clean the ground points at the battery and trans to body. Both ends.