Help! Bad battery or bad alternator? How do I tell?

I just came back from a 30 mile drive, partially on the highway, and my battery voltage was around 11 volts, even when I was driving on the highway. When I came home, my digital voltmeter showed 10.9. Battery is an H7 Duralast Platinum AGM, which is now nearly 4 years old. So, do I just have a dead battery or is the alternator shot? Thanks!

Alternator most likely. Test it at the alternator leads for accuracy while running if you can (I've never done it on a 996 so I don't know how easy it is to reach).

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If the voltage at the battery termimals is just 10.9 volts that's a sign the alternator is not producing electricity and the car is running off of battery power alone. You are probably lucky to make it home on probably just battery power only. What you can do is shut off the engine and let the car sit overnight. It is probably a good idea to put a battery charger on the battery to make sure it is topped up for the next day. The next day then start the engine and let it idle while you check the voltage at the battery. I suspect it will be 13.5V or higher nearly up to 14V, which is what I have observed with the battery voltage gage in my Turbo, and is normal. If you let the engine idle and the voltage begins to drop and gets under 13V that's a good sign the alternator is as it gets hot is unable to supply sufficient power. You can remove the battery and take it somplace and have it tested. If it passes then the alternator only comes in for more suspicion.

drive to your local auto parts store and have both battery & alternator tested.

Thanks guys. Looks like an alternator is quite pricey. Take a look at the link below: https://sonnenporscheoemparts.com/pa...ion=ALTERNATOR Any idea how many hours of labor replacing it entails?

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4 hours according to this DIY by Pelican: http://www.pelicanparts.com/techarti...Alternator.htm

I dont agree, more likely battery is bad. Go to auto parts store and have a test.

Voltage regulator?

Ive replaced alternators on cars in the past but never on a porsche but its usually a pretty easy to do job. but i would order a rebuilt alternator and save yourself about $1000 over buying a brand new one.

If your car is not producing the correct voltage when running, I would suspect it's the alternator and not the battery. As noted, have the battery checked. (you can also check the charging voltage with a durametric if you have one) If you have the alternator checked as well and they say it's bad, be aware that the voltage regulator is also a wear part. Sometimes, replacing the regulator remedies the issue. However, it still requires the alternator to come out. If you are inclined to wrench, this is a pretty easy one. I just replaced my voltage regulator on my 997 and the process didn't take 4 hours. Maybe 2 total including the new belt. The voltage regulator was indeed the culprit - and it was only $37. New alternator is pricey, but you can find a reman for a lot less. I've seen a few threads here on it. -td

Well, if you are short on diagnostic equipment, you can have the battery tested at any auto parts store, as kromdom said. 4 years is a full life lived for a battery in a 996. If it isn't going bad, it is already living on borrowed time. So, if you want to try to diagnose by replacing parts, then I'd start with a new battery because even if it is still good, it won't be for long. This will take the sting out of the parts price if it is the alternator - OEM Bosch alternator reman for $479, minus another 10% discount if you use the promo code RENNLIST, making it $431 from Rennlist sponsor Warehouse33auto.com: http://www.warehouse33auto.com/parts...905/6/0/2412/0

The latest tip in case you guys are not aware https://rennlist.com/forums/996-foru...nto-996-a.html New Bosch AL0815N is the best deal in town. $275 shipped from amazon. It's for 997 but it fits 996 as well.

That is a great deal, plus the alternator is 150 amps versus the 120 amps you have now.

Searching for my '02 996tt, it appears Porsche has already changed the replacement to the 997 alternator. I would think the M96 would get changed soon to the 997 part.

So, off to Advance Auto I will go this morning to have the battery and alternator checked for free. Based on what everyone is saying, it sounds like the alternator or the voltage regulator. I noticed the Bosch regulator new on 2 web sites is around $355 after return of the old one, which is a lot less than the Porsche branded one (no doubt made by Bosch). With the non-Porsche alternator, I'll have to go to an independent mechanic, but there's one nearby that I've used occasionally who is good. I hope he can figure out whether it's the voltage regulator. I'll report back with more info. Thanks again.

I went to Advance Auto today after charging my battery overnight. When I left my house, the voltage was at 13, but dropped to just over 12 by the time I got to Advance Auto (1/2 hour drive there). The battery tested okay (not great due to be drained down as I drove) and the alternator seemed to not be providing enough charge. I asked the tech if he could tell whether the problem was the voltage regulator and he could not. I noticed on Pelican Parts that there were several posts indicating that replacing the voltage regulator took care of the charging issue without having to replace the alternator. Is there a way that a shop with a diagnostic tester can plug that in and figure out whether it's the voltage regulator or the alternator? Otherwise, I could end up paying to R&R the alternator to replace the voltage regulator, only to have them R&R the alternator with a new one. A bit of a gamble.

No, no shop/equipment will be able to tell you whether it's the regulator or something else inside the alternator (without removing the parts). Also keep in mind a bad y-cable can also give you similar symptoms. The only way to be sure is to perform a proper diagnosis. Without that, anything will be a gamble.

As Ahsai said, your garage will be the one that can tell you which part needs replacing. In my case the voltage regulator was bad.

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I can vouch for this one as I installed one in my car a couple of months ago. I too was going through a bunch of unusual issues where my battery would die but it appeared as if my alternator was working properly. Well it wasn't doing so on a consistent basis. I replaced mine in just a couple of hours, it wasn't a bad job at all. The worst of it was working in my garage during the heat wave we had here in the Carolinas. But the task itself was fairly simple and straightforward. I also ordered and replaced the ground cable at the same time so I wouldn't have to worry about it later. Just for reference I got a $1600+ quote to get it fixed so that's what drove me to do the DIY route. Saved $1300 in the process!

I checked with a Porsche specialty shop in the area and they quoted $200 for labor (approx. 2 hrs) plus around $300 for a Bosch rebuilt alternator. As to the Amazon new Bosch 997 alternator, I see that your car is a 2001 with a 3.6 engine and mine is a 2001 with a 3.4 and Tip, so this is not likely to be the correct alternator for my car. Thanks for the tip though.

Actaully in the renntech link I sent, that alternator fits multiple 2001 996s so it should fit your car. However, since you are not DIYing this, I think it makes sense to go with what the shop recommended.

It could be the bad voltage regulator and you can order the part alone. Pelicanparts has a good DIY on the subject. I changed mine out because I had dropped the motor and access was easy. the part cost me like $25 I think. Here is some info if you choose to go this route. Bosch regulator: Bosch interchange part number: F00M-145-350 (which supercedes PNs: F-00M-145-200, F-00M-145-209 and F-00M-145-225) Porsche part number: 000-043-206-37-M14

I got the alternator replaced with a Bosch remanufactured unit by a Porsche specialty shop on LI. They charged me $210 for labor (1.5 hrs) and $290 for the alternator, which seems to be the going price locally and online. I got quotes from 2 other Porsche indy's for $1150 and $944 all in. It pays to shop around. I would definitely recommend the shop that I used. Clearly, the mechanic has done this lots of times. Looking at the Pelican Parts instructions, this is not that hard a job and I could have done it myself -- probably in 3 hours, but I would have only saved on the labor, and $210 was not that bad.

Sounds like a good deal to me!

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The million-dollar question: What's the voltage at while driving since replacing the alternator?

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I didn't have my digital voltmeter that plugs into the cigarette lighter with me when I picked up the car (I highly recommend this little device), but going off the car's voltmeter, it was a tick below 14 volts.