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Troubleshooting low voltage and charging issues


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Making this a separate post for easier searching. If your voltage reading is low but you don't know what causes it, you can do the tests below to identify the culprit(s).

 

All you need is a $15 multimeter and some cables. Run your engine till warm and low voltage shows up, then let it idle and turn on the a/c and the low beam. The current draw from the alternator should now be ~50A. You can then do the following tests.

 

I drew a diagram with the corresponding parts.

 

Porsche-996-charging.jpg

- Zoom Active -

902-05.gif

- Zoom Active -

 

 

Test #1: check voltage drop between point "C" (alternator casing) and "B-" (call that V(C, B-)). Note "B+' and "B-" are the actual battery terminal posts and not the cable connectors on the terminals. This test shows total voltage lost between the alternator and the battery on the ground side. Expect 0.2v or less. If your ground strap is bad, it will show up in this test.

 

Test #2: check V(A, B+) where "A" = alternator output at the back of the alternator that you can't see (use an inspection mirror) and expect ~0.5v or less. "A" is hard to get to. I fabricated a J-shape hook using a stiff insulated wire and just literally probe it blindly from behind. Wear protective goggles here since you will be close to the drive belt, a hot engine, and the always LIVE "A". This test shows total voltage lost between the alternator and the battery on the power side.

 

Test #3: check V(A, J) and expect ~0.2v. This tests #21, which is the infamous cable that can corrode and Porsche has also revised it.

 

Test #4: check V(J, B+) and expect ~0.3v or less.

 

Test #5: check V(A, C), your alternator output and expect 13.5v or higher.

 

Your problem is gonna show up in one of the tests above.

 

See tool to access the back of the alternator 

 

 

 
Edited by Ahsai
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  • 4 months later...

Great, Ahsai! Thanks for the effort as this could be of great assistance in the future!

One small point: Those suggested minimum voltages make a lot of sense to me but, as a curiosity, are these values stemming from your experience or are they taken from actual OEM troubleshooting documentation?

Martin

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Hi Martin, thanks!

All those check points and numbers are from my experience based on both general automotive charging system knowledge, and measurements from multiple alternators, regulators, and cables at different temperatures, load, etc. specific to our cars. I also verified the expected voltage drop by computing the resistance on the cables based on their conductivity and physical dimensions.

Also usually when there's a problem, the measurement will be significantly out of range of the above numbers.

  • Upvote 1
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Appreciate the response and add'l feedback Ahsai.

As with Jean-F's, your positive contribution to renntech 996 owners' various issues does go unnoticed.

Along the same vein, may your newest AOS suck vacuum according to specs.

Cheers!

Martin

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Thanks for the kind words, Martin. Yes, my new AOS sucks very well now. Just tested it yesterday and it's still between 4-5" of water so I'm happy. My last new one lasted only 1k miles (in fact it was bad out of the box sucking 10" of water once I installed it) and I hope this one will last longer. The bummer is my old one (9yr 50k miles) was still working well. I replaced it only as a PM.

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  • 2 months later...

Ahsai has another useful thread here:

http://rennlist.com/forums/996-forum/764630-updated-alternator-starter-cable.html

"Y" connector is 996 607 019 03 .List price is $72.

Don't make the mistake of thinking the Boxster has the same part but just a 986 part number.number .The Boxster part # is

986 607 019 02.It looks different on PET

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  • 1 year later...

 

Making this a separate post for easier searching. If your voltage reading is low but you don't know what causes it, you can do the tests below to identify the culprit(s).

 

All you need is a $15 multimeter and some cables. Run your engine till warm and low voltage shows up, then let it idle and turn on the a/c and the low beam. The current draw from the alternator should now be ~50A. You can then do the following tests.

 

I drew a diagram with the corresponding parts.

 

Porsche-996-charging.jpg

- Zoom Active -

902-05.gif

- Zoom Active -

 

 

Test #1: check voltage drop between point "C" (alternator casing) and "B-" (call that V(C, B-)). Note "B+' and "B-" are the actual battery terminal posts and not the cable connectors on the terminals. This test shows total voltage lost between the alternator and the battery on the ground side. Expect 0.2v or less. If your ground strap is bad, it will show up in this test.

 

Test #2: check V(A, B+) where "A" = alternator output at the back of the alternator that you can't see (use an inspection mirror) and expect ~0.5v or less. "A" is hard to get to. I fabricated a J-shape hook using a stiff insulated wire and just literally probe it blindly from behind. Wear protective goggles here since you will be close to the drive belt, a hot engine, and the always LIVE "A". This test shows total voltage lost between the alternator and the battery on the power side.

 

Test #3: check V(A, J) and expect ~0.2v. This tests #21, which is the infamous cable that can corrode and Porsche has also revised it.

 

Test #4: check V(J, B+) and expect ~0.3v or less.

 

Test #5: check V(A, C), your alternator output and expect 13.5v or higher.

 

Your problem is gonna show up in one of the tests above.

 

Hi Ahsai,

 

I am having a problem with my charging system. When I first start the car the voltage is between 13.5V-13.8V, then drops-off to nominal battery voltage of 12.5V, then will decrease in value the longer I am using power, below 12V. I ran your tests, and did not find a grounding problem. However, when I measure Test # 5 at the back of the alternator (is it the lug on the right?) I essentially measure battery voltage, no charging.

 

Would this issue be an indication of a bad regulator? I did swap another battery and it behaved the same way.

 

Thanks!

 

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If your measurement was taken at the back of the alternator (with some kind of a J shape hook), then yes it sounds like your alternator is the problem. Most likely it's the regulator but it also could be diodes failimg inside the alternator.

If your measurement was taken from the engine bay positive jump start terminal right above the a/c compressor, the result is not conclusive.

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If your measurement was taken at the back of the alternator (with some kind of a J shape hook), then yes it sounds like your alternator is the problem. Most likely it's the regulator but it also could be diodes failimg inside the alternator.

If your measurement was taken from the engine bay positive jump start terminal right above the a/c compressor, the result is not conclusive.

 

I took the measurement on the right lug behind the alternator where the black power cable is connected to the lug. Is the right lug the correct one? I see there is a lug opposite to the one I tested. I did use a J hook to make contact.

 

Thanks!

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Yes, that's the right one, the one the cable is connected to directly.

You can replace the regulator first if you don't mind the labor.

If you opt to replace the alternator, get this one AL0815N. It's new and it's on sale on amazon for $250 most of the time (currently it's not).

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Yes, that's the right one, the one the cable is connected to directly.

You can replace the regulator first if you don't mind the labor.

If you opt to replace the alternator, get this one AL0815N. It's new and it's on sale on amazon for $250 most of the time (currently it's not).

 

The fitment says it is for a 2005. I have a 2003. Will it fit/work?

 

Thanks!

Edited by mffarrell
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https://www.amazon.com/Bosch-AL0815N-New-Alternator/dp/B000IYAAYE/ref=sr_1_1?ie=UTF8&qid=1467663147&sr=8-1&keywords=Bosch+al0815n

It's for 997's so it will say not fit a 996 but disregard that. It has a higher amperage capacity, a little longer but it will fit 996's. The best part is it's brand new and relatively cheap so you don't need to take the risks of a reman unit.

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https://www.amazon.com/Bosch-AL0815N-New-Alternator/dp/B000IYAAYE/ref=sr_1_1?ie=UTF8&qid=1467663147&sr=8-1&keywords=Bosch+al0815n

It's for 997's so it will say not fit a 996 but disregard that. It has a higher amperage capacity, a little longer but it will fit 996's. The best part is it's brand new and relatively cheap so you don't need to take the risks of a reman unit.

 

https://www.amazon.com/Bosch-AL0815N-New-Alternator/dp/B000IYAAYE/ref=sr_1_1?ie=UTF8&qid=1467663147&sr=8-1&keywords=Bosch+al0815n

It's for 997's so it will say not fit a 996 but disregard that. It has a higher amperage capacity, a little longer but it will fit 996's. The best part is it's brand new and relatively cheap so you don't need to take the risks of a reman unit.

Thanks for your help. I will order one today.

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Cool. Let us know how it goes.

I installed the new Bosch AL0815N (997) alternator this morning. It was pretty straight forward, not a big deal. The new alternator definitely solved my charging problem. I purchased from Parts Geeks for $263. Perfect fit and function!

Thanks again for the tip!

Edited by mffarrell
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