Another battery light on and 16+ volt gauge thread - repeatable at near limit revs

I've read a lot of similar threads via searching, and I think I know the answer is "new alternator", but want to just check on any opinions before I add that to the list of things to replace. So last week I had the battery light turn on (along with ABS light) and the battery meter spike to 16. I slowed down to a pull off the road, and the lights turned off and the gauge went back to 13.5. Yesterday I was out driving the canyons and in middle of passing at high rev, the same thing happened, lights and maxed voltage. And I noticed the AC turned off (could see the Snowflake was gone) and the fan cut too. I let the car coast and the warning lights went away and the AC and fan turned back on. When I was home, I revved in idle to a high RPM and sure enough the lights came on and the gauge spiked at 16. Taking foot off gas, it resets itself. So I'm guessing I need a new alternator, but what is strange is the Carfax shows the alternator changed in 2019 (yes carfax could be wrong, a crappy part could have been used, etc). But could it be something else? I tested battery - 12V on nose. When car is running 13.5 according to the multimeter, interior shows closer to 14. I recorded the multimeter while revving engine and at 6000 RPM all the dash light brighten quickly, then dull as battery light comes on. Multimeter shows the same spike in Voltage, so the regulator inside the alternator is likely shot. So many threads said the voltage regulator is built into the alternator, but i just found one where they clearly show the regulator can be changed independently. not sure why the differing advice?

Well, I would also check all cables and grounds first.

Yea "extra" voltage is the regulator. High voltage can damage a lot of stuff so i wouldnt run the car till fixed

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Yes, going to go through and do that just in case. Yes, off the road till resolved. I'm going to pull the regulator and replace it first to see if that resolves it.

Alternator removed. It is a Bosch, but it seems it might have been removed in past. There were two indents on the bushing where someone might have knocked it free. I removed the regulator, and it visually looks find, bushings seem to have good amount of life left, but that's about all I can ascertain. Is it possible anything else could cause the voltage to spike like that before I just replace the regulator? I visually checked the alt-starter cable, the ground strap rear-right, the other cables to positive engine terminal.

Not saying this is the cause, but you can check to see if the alternator is producing any A/C voltage. Anything over half a volt A/C creates problems. More than half a volt would indicate a bad diode.

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Some say you would need to actually pull those cables off (after disconnecting the battery) to see if there are connection issues. Sometimes a cable looks connected just fine but isn�t. Normally, a wire brush and some contact cleaner can help before re-attaching. I cannot think of anything else that would cause voltage to spike like that besides the regulator.

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Wouldn't have thought of that. It's all pulled out right now, and waiting on new regulator, but can check that when reassembled. Assume just switch the Multimeter to AC mode?

Yes, when you get your 911 running again just switch meter to A/C volts and connect at jump point in engine bay.

you might want to thoroughly check the signal wire for poor fit and or broken wire .That wire drives the abs -idiot light etc and an intermittent connection would result in the conditions you are experiencing.

Electrical engineer here. It's your regulator or alternator. Since you have no issue pulling either, change the regulator first and if the problem comes back, change the alternator and regulator. And do not run with the voltage spiking to 16!

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What's the signal wire? Yep, not running it. Regulator comes today. Hope that was all it was!

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The single small gauge wire with the connector that snaps in. Typically also called the exciter wire. Its not your issue. That wire tells the regulator to supply or not supply power from the alt based on its voltage.

Well new regulator arrived. It's got a different model number on it from the other. And it doesn't match what was on the pelican parts photo (or really any similar site's photo of it). I searched by the model number and it does link back to the same one I have now. The brushes look almost exactly the same length as my current, so my current one must be not that old (per the 2019 alternator replacement I saw on carfax). We'll see what happens!

Look, I would have been done with this. This involves too much trial and error. You can get a new Bosch alternator on Amazon. I would just replace the entire thing and be done with it.

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Wire serves two functions -supplies 12 volts to activate alternator with key on and is the engine is running signal ( positive12 volt ) for abs climate control etc .May not be the root cause but if abs light came on that signal was dropped.

Or because its outputing 16+ volts and it did not like it. All kinds if things happen in these cars with low and high voltage but 1 very consistant fault is an abs warning.

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valid point -

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Also a valid point, but now I just want to know if it was the regulator or not

Well new regulator didn't fix the problem in fact it seems to have made it worse. Car idles just over 14v now according to the dash gauge, and when driving it's 14.5 - 15v. Mutlimeter at idle shows close to 15v at idle. Drove the car around the block and accelerated the revs up and again it's jumping to 16V and ABS/Battery light on. Going to get a whole new alternator now and hope that resolves is, but now wondering if the prior owner changing the alternator 4 years ago had any connection...

If you decide to replace the alternator, the 997 model fits perfectly, is sometimes cheaper, and has a higher current capacity. https://rennlist.com/forums/996-foru...l#post18179480

Well, that's what we are going to find out...whether it was worth the time or not! I would have and did replace the whole Bosch alternator so I wouldn't waste my time, but it turned out, I wasted my money because it was the damn corroded starter/alternator wire.

I track a 2013 Boxster S. Not sure when it happened after an event noticed the voltage charge was 15.8. Never dropped below 15.2 at idle. I called around to see if anyone still repaired/rebuilt alternators. Found one hole in the wall shop. Told him I tracked the car. He suspected a loose field winding wire in the stator that shorted the regulator causing it to charge at the max voltage from the high RPMs. It had cooked my battery. Luckily my Indy warrantied the battery and I had a new alternator installed. There are some crazy cheap supposedly Bosch alternators on Amazon. I got my Indy to cut his price but he said he could not match the Amazon price.

Well in the continuing saga... I replaced the original voltage regulator and the issue with the 15v went away. It starts at 14v, then when warm running at 13.5-13.9. And doesn't creep up under acceleration like it did with the new regulator. 16V issue however is happening at lower engine RPM, had it happen at just 5000rpm, so something very odd is going on. Reading the ground straps can cause gremlins, as well as the y-connector to alternator. I know I should probably just buy a new alternator, but why would both regulators fail to keep voltage below 14v. Even if the alternator was bad, isn't that their one job? So I wonder if the alternator isn't the problem. Though it is definitely connected to engine, aka belt speed. Trying to avoid 996-CAB's "... but it turned out, I wasted my money because it was the damn corroded starter/alternator wire."

Your current voltage levels indicate your Y-cable is fine. It only decreases voltage. Start up at 14.+ is normal, as is 13.8+ when warm.

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Yes, 13.8 is what it was before I tried the new regulator (which upped idle to 15v and 15+ when driving). Going back to old regulator, which was supposed to fix the 16V surge at 6500 fixed the 15+ issue, but the 16+ is still there. Though now it happens at 5000 rprm. I read in other threads that the ground and y-cables could cause issues like this, but you're saying those only show a decrease in voltage when bad? Is the altternator surging SO high that the regulators can't keep up? Is that even possible?

It isn't the Y-cable or the grounds. Those only make voltage go lower. I would replace the alternator AND the regulator. Something still isn't right with the combo you have.

Have you got a multimeter? Measure DC voltage from the alt output terminal to the alt body. For kicks check AC volts here too. Then measure DC voltage at the battery posts. Report the values.

Quotes source post: 18959499

Thank you for repeating this. It seems to have gotten lost in the noise here. 13.8 to about 14.5V are normal charging voltages. Most importantly, bad grounds, or bad cables, are resistors, and they cannot create higher voltage, they can only reduce voltage by turning current into heat. The only source of voltage on your car is 1) the battery, and 2) the alternator. The battery will not hold 16V for long (if charged to that), and if it's been at 13-14V, it can't go up to 16V all on it's own. Ergo, it is an alternator problem.

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An alternator is fairly complex. It has an excitement circuit, and it uses its own power to generate an electromagnetic field. If there are shorts anywhere in the system, you could get very strange results with not only the voltage and amperage, but also the frequency of the unit. Who knows how the regulator handles those types of issues.

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Yes of course, not sure lost in noise. My replacement regulator was showing 15-15.5v. When I put back the supposedly bad regulator, it went back to normal 13.5-14.

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Yes i've been monitoring via the battery terminals. Video recording the surge to 16v. How do i monitor in the engine? I see the positive terminal box, what do I connect the negative too (yes I assume any ground point, but a specific point would help )

Since going to all the trouble of reinstalling alternator why not check for a/c volts? takes about half a minute to open engine lid, flip open jump point. Set meter to a/c volts. Attach positive probe to jump terminal. Negative probe to hood latch. Let engine idle during test

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Touch the plus meter lead to the alternator output terminal and the minus meter lead to the alternator's metal body. Check both AC and DC. If you see any appreciable amount of AC there's a bad diode in the alternator and it needs to be replaced or rebuilt.

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Ok I did that. DC it reads 13.8V. AC mode, it reads 5V (I think.. see image below, red probe indicates where it was at, set on the 50 setting). (Someone else suggested connecting to alternator's output but I wasn't sure where that was.)

Okay, I�m not familiar with your meter. Looks like the a/c scale when set to 50 probably multiples the reading of the scale by five. If so that means your alternator is producing around thirty volts a/c. That of course is not what you want. Even if that scale indicates six volts that�s again not correct, or even .6 or six tenths that�s again too much. For comparison I checked my alternator and my meter measured 0.1 volts A/C. Time for a new alternator. I think an earlier response to your post gave a link to an alternator that is used on the 997. They produce more amps if you need extra or just buy the standard one for your 996.

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The Bosch AL0815n (997 version) can support higher current draw, but also, at least at the time I looked, it was quite a bit cheaper than the AL0812 (996 version). ​​​​

Well I went in circles and ended up ordering the 997 ver. AL0815n on Amazon as it was from Bosch brand new -- $400. BUT just searched again and Amazon had an AL0812x for $126 which was "used" and sold from Amazon.com LLC Services, but that seems questionably cheap. So I thought I'd risk it, and cancled my AL0815n ordere -- but then thought, that's WAY too cheap. So I looked up the AL0815n and the price had dropped to $380, so I bought that (again) and somehow saved $20.. I just hope this resolves it! I'm going to buy a digital MM, I'm sure mine is fine, but I don't know how to interpret the 30v in AC mode...

Not sure I understand,I've used meters similar to yours. If you put your mm on the DC 50 where, does the needle go ..? How about if you put mm on the AC 50v setting, where does the needle go to..? Do these both with car running and off and report back, maybe I can shed some light on it...

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My post above covers that. DC all good at 13.8v (same reading at battery side and engine side). Flipping to AC -50 and reading from + plastic box in engine, it's where I have the probe pointing to in the image. 30v (or however it converts at the 50V setting.

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Yup, looks like ac 30v. Probably no need for another MM. Might want to flip to the 250v ac scale and test a house outlet (115-120v) to verify MM is reading ac correctly...

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Sooo.... stick the probes in the outlet? I did that once with a fork when i was 4...

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Actually, yes, that's exactly how one would measure voltage at the plug. However, given your uncertainty overall with electricity and using the multimeter, I would actually advise you NOT to do this. I don't think the forum should be responsible for you electrocuting yourself.

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Ha good point, but I have used it in past to check fuses are off before placing lighting fixtures. I risked it and AC V=120. So meter appears to work. So alternator is totally messed up I assume.

Yes the alternator has a bad diode. Does your battery drain in a few days? That would confirm a bad alt.

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Nope, everything runs as expected. Battery always charged. Opperating volts at 13.5-14v. Issue is what I originally posted about, when it hits 6500 (and now 5000) rpms, volt meter spikes to 16v, dash lights brighten and then batter light and abs light come on, AC and fans switch off, and stays that way till engine RPMS decrease. I replaced volt regulator, but new one performs worse, engine idles at 15v and runs at 15.5v. Pelican parts said it might be faulty. I put back in original regulator and operating volts back to 13.8/14. But sttill issue of revving engine and volts spike to 16v. Alternator was changed 4 years ago according to carfax, so I don't know if there is another issue or not...?

I have found when diodes are bad on a alternator, the alternator will be warm to touch when engine is cold.

There are SO MANY THINGS in a car that would not do well with an AC electrical feed. It's good you found it.

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@theprf Got any tricks on how to get a lead onto the alternator output terminal?

I'm not familiar with the engine bay arrangement for the NA cars, sorry. If the alt output terminal isn't accessible use the jump point instead.

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Unfortunately that isn�t a reliable way to do it in this cars because of the Y-cable that is prone to failure. It might make you think it�s an alternator issue when it�s actually the cable.

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remove alt, add piece of 16gauge with a ring terminal, reinstall and use the wire as a test lead

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Yeah, I was hoping someone had finally come up with a brilliant way to do it without removing the alternator. 🥲

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if we were looking at low voltage on the dash voltmeter, then yes, the jump terminal measurement point might mislead one to think it was an alternator problem, but in this car it is too high a voltage so the Y cable must be fine and we can use the jump pint as a proxy.

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That�s true. Good point.

Brand new 997 Alternator coming Friday, so hopefully we'll see no more AC, and not more V spike at high rpm! And maybe, that whining sound I hear around 5000rpm from rear will be related and kill three birds with one alternator!

Odds are high a diode pack and a set of brushes and you'd have a ready spare... THey are not a complex device.. Tearing down the old one can be educational, you can see what failed usually..

Ok so new 997 alternator in and it all seems to be back in working order and no more V spike at high rpm! There is still a low level of AC V when measuring at the engine bay. I can't figure out what this means or if it is normal (previously the needle was over the Red 6). The Alternator is brand new from Bosch.

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The voltage is 30 Vac in both measurements. This first time you were on the 50 volt range. This time you are on the 250 volt range. Good job fixing your car.

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Good job. That reading seems strange, but if it were me, unless someone else has a very specific reason to be concerned, I wouldn't over-analyze it. If the DC voltage is sitting nicely around 13.5-14.8V (or so) all the time, then you should be good. Wash your hands and pat yourself on the back.

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Ah good catch. I had switched to 250 to test it in the house plug and got flipped around. Any idea what it means that it's still registering AC Vs? Anyone else able to hook up at the engine jump point and see? I did + box and hood latch.

That's because the alternator doesn't put out a true DC current, it puts out an alternating AC current that is rectified by cutting off the negative parts of the sine wave... A true DC current like the battery puts out will be a straight line on a scope, where as the DC current from the rectified alternating AC current is a half sine wave on a scope.. When testing an alternator for a problem other than the voltage regulator you would use a scope to test for problems.. On a scope the true DC ( like from the battery) is a straight line...from the alternator you will have one of these below... You probably have a pattern like "D" in the above, and fixed it with a new alternator...

Back in the 70's 80's 90's Dealerships "required" technicians to rebuild alternators, starters, carbs ,ect...if under warranty..I rebuilt hundreds of them . Usually just needed a set of ball bearings, a brush set, and a set of diodes...In order to "replace" an alternator we had to make an estimate of the repair costs that exceeded the cost of a replacement part,then submit it to the manufacture for approval.. Since the Manufacture set the labor time it didn't pay a lot to rebuild an alternator, but it payed even less to just replace it so we rebuilt them always...and the parts dept. always had the necessary parts in stock. Nowadays you would have a hard time just finding the parts to rebuild them...lol

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Thanks for that explanation! So sounds like measuring AC Vs is okay, where as prior posts said any AC was a sure way foward to certain electrical components deaths - I'm curious if others do the same meausrement if they all show the same AC volts...? And yes, now I have a spare alternator which might just have one minor issue - I don't think I could fix it and sell it, not sure what to do with it besides keep it for spare..../?

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If your meter is set on the opposite current scale being produced, sometimes a less expensive meter will show an erroneous voltage when it shouldn't. Higher quality meters can accurately read AC voltage in a DC system. My Fluke can. jmo