Alternator troubleshooting help

[55rag]

Hello all,
I took my '85 911 out to get smogged yesterday and passed with no issues thankfully. I decided to take a cruise around town and went to meet my wife for lunch. When I came out to leave the engine wouldn't turn over, I turned the ignition off and the speakers started making noise. I called AAA for a jump and they put a meter on the battery and noticed that it was not being charged. Well I made it home and am now charging the battery. It came back up to 12.5v but drops to 11.5v with the engine running. My battery light is on with the ignition on and goes out with the engine running.

I've done some searching on the boards but did not find anything similar where the battery light was still operation normally. Is this a bad alternator or do I need to do further troubleshooting.

Thanks in advance for your help

[304065]

Your battery is shot, replace it before it kills the alternator. With the charge light off you know the alternator output voltage is roughly equal to the battery voltage-- so if both are too low, I think it's an old battery.

[55rag]

I was thinking if the voltage on the battery doesn't show 13.5v or higher while the engine is running then the alternator is not charging. Are you saying if the battery is bad I may not see any charge from the alternator with the engine running?

Thanks for your advice

[Rot 911]

Originally Posted by 55rag

I was thinking if the voltage on the battery doesn't show 13.5v or higher while the engine is running then the alternator is not charging. Are you saying if the battery is bad I may not see any charge from the alternator with the engine running?

Thanks for your advice

That is correct.

[55rag]

That's good news. I was afraid it was going to be the alternator. I currently have a big ol' Interstate in the car and the car sits more than it should. Any suggestions on a new battery (Optima)?

[Rot 911]

Originally Posted by 55rag

That's good news. I was afraid it was going to be the alternator. I currently have a big ol' Interstate in the car and the car sits more than it should. Any suggestions on a new battery (Optima)?

Yeah the stock size battery weighs in at around 60 lbs! I went with the Optima and you save around 25lbs. You will need a longer ground cable that you can buy where you get the battery. You will also need to make a mounting plate which is cheap and easy to do. Just do a search on optima and mounting plate and you should get the threads that have the template to make one out of a cutting board.

Just to be sure you have a bad battery, the auto store that sells Optimas should be able to test your battery.

[55rag]

I think I'll pull the battery and take it in tomorrow for the test. Did you get a red top Optima?

[Rot 911]

Originally Posted by 55rag

I think I'll pull the battery and take it in tomorrow for the test. Did you get a red top Optima?

Yep. Bought my first one 5 years ago and it died at like 2 years and 9 months. They gave me a new one free!

[55rag]

Cool, thanks for the info. I've used red tops in the past with my weekend cars and they always seemed to last a long time in cars that sit.

[911pcars]

Originally Posted by john_cramer

Your battery is shot, replace it before it kills the alternator. With the charge light off you know the alternator output voltage is roughly equal to the battery voltage-- so if both are too low, I think it's an old battery.

I'll respectfully disagree. The charge light goes off when the alternator generates more voltage than the battery. Thus, with a discharged battery, your alternator may be caput.

Make sure your battery is fully charged. Place it on a trickle charge for a couple of days, then have it tested by an electrical shop. Some places can even check the battery capacity in a discharged state. Once a healthy battery is installed, the charge light should be OFF while running with the voltage at or slightly above 14 volts, off idle. During the voltage test, switch the headlights and blower fan ON just to be sure the electrical system demands more from the alternator.

You should get a more complete view of the system after that.

Sherwood

[55rag]

Thanks Sherwood,
I have my battery on a trickle charger now and will post results tomorrow.

[55rag]

I think my battery is dead. I've had it on charge since yesterday evening and it is still not fully charged. From my experience with this charger/battery it would have been charged by now. I'll leave it on until tomorrow just to make sure. After that it's off to Autozone for a test and possibly a new Optima.

[55rag]

Well, the battery did finally fully charge last night. The voltage reads 13v but when I start the car the voltage was reading 16.7v so I shut it down immediately. I noticed the battery was making a squealing noise probably from the high charge. So now I'm baffled, why would the battery go dead while driving and then after it's fully charged the alternator is overcharging? Is the regulator bad and if so I think my alternator has an internal regulator so that would mean replacement.

[Rot 911]

Yes it is beginning to look like a regulator problem. 16.7 is way too high to be charging.

[55rag]

So is it true that an 85 has an internal regulator?

[Rot 911]

Originally Posted by 55rag

So is it true that an 85 has an internal regulator?

Yes

[tsuter]

I think I would have Autozone load test that battery.....before pulling the alternator..........

[55rag]

Such an emotional rollercoaster, haha.

I took the battery to get tested and they say it has a bad cell. They didn't have any Optima's in stock and they were charging around $200. I'm going to look around for better pricing.

Once I through in the new battery I'll run the same tests and see if it is still overcharging which I'm sure it will be.

[911pcars]

Good suggestion from Tsuter. If there's a Costco near by, try them. I think the red top is around $130.00.

You might discover the alternator is suspect as well, but let's start with a fresh battery, then monitor the charge rate from the alternator before we get ahead of ourselves.

Sherwood

[55rag]

I got the new Optima 75/35 in today and alternator output is 16.9v so I think I'll be ordering that rebuilt alternator. I know I'll be doing some other projects "while I'm in there" since I have a couple of oil leaks to try and sort out. May as well detail the engine also.

I appreciate everyones help with this.