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996TT Voltage Issues -Help Needed!


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My car's two year old battery failed about a month ago, and I've been chasing electrical system issues ever since.  The 996 Essential Companion book says you should see 13.8V across the battery terminals with the engine running.  I was seeing low 13Vs or high 12Vs, so I started the search for the missing volts.  Obviously, the battery was replaced with a new AGM battery with the correct Ah rating.  Next step was to replace the voltage regulator.  No significant increase in system voltage at the battery.  So next, I installed a remanufactured Bosch alternator.  This seemed to help.  Upon start-up, I would see about 13.5V across the battery.  But after the engine has run awhile and really heats up, the voltage drops down to about 12.7V (the picture of the voltage reading is at idle after about 45 minutes, engine at idle with the A/C running).

 

So not sure what to do next, I took the car to the dealer for diagnosis.  They claim everything is normal (see attached excerpt from service receipt). 

 

I've read I should check various connections to look for corrosion.  So yesterday, I dropped the rear engine tray and checked the engine ground strap - looks good, and opened up the box where the alternator/starter cable drops down to the connection mounted.  Again, no corrosion, but did notice a good kink in the cable (not sure if that matters).  The only thing I've read is that sometimes the alternator/starter cable gets over-baked near the starter and that causes voltage issue - supposedly an issue on the 997's; don't know about the 996's.  My concern is that I don't think the battery is getting properly charged while the car is running.  I've had a Innova voltage gauge in while the car is running, and the voltage is always .6V - 1.0V below what the gauge shows when I use it in my MB SUV.

 

So my question is whether or not my car's running voltage is "operating as designed" as the dealer claims, or whether further voltage tracing is warranted?  I'm very curious as to what voltage others are seeing across the battery with the engine running?

 

All help and suggestions appreciated.

connection at transmission.jpg

ground strap no corrosion.jpg

service report.jpg

voltage across battery at idle.jpg

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1 hour ago, spooltime said:

My car's two year old battery failed about a month ago, and I've been chasing electrical system issues ever since.  The 996 Essential Companion book says you should see 13.8V across the battery terminals with the engine running.  I was seeing low 13Vs or high 12Vs, so I started the search for the missing volts.  Obviously, the battery was replaced with a new AGM battery with the correct Ah rating.  Next step was to replace the voltage regulator.  No significant increase in system voltage at the battery.  So next, I installed a remanufactured Bosch alternator.  This seemed to help.  Upon start-up, I would see about 13.5V across the battery.  But after the engine has run awhile and really heats up, the voltage drops down to about 12.7V (the picture of the voltage reading is at idle after about 45 minutes, engine at idle with the A/C running).

 

So not sure what to do next, I took the car to the dealer for diagnosis.  They claim everything is normal (see attached excerpt from service receipt). 

 

I've read I should check various connections to look for corrosion.  So yesterday, I dropped the rear engine tray and checked the engine ground strap - looks good, and opened up the box where the alternator/starter cable drops down to the connection mounted.  Again, no corrosion, but did notice a good kink in the cable (not sure if that matters).  The only thing I've read is that sometimes the alternator/starter cable gets over-baked near the starter and that causes voltage issue - supposedly an issue on the 997's; don't know about the 996's.  My concern is that I don't think the battery is getting properly charged while the car is running.  I've had a Innova voltage gauge in while the car is running, and the voltage is always .6V - 1.0V below what the gauge shows when I use it in my MB SUV.

 

So my question is whether or not my car's running voltage is "operating as designed" as the dealer claims, or whether further voltage tracing is warranted?  I'm very curious as to what voltage others are seeing across the battery with the engine running?

 

All help and suggestions appreciated.

connection at transmission.jpg

ground strap no corrosion.jpg

service report.jpg

voltage across battery at idle.jpg

 

Only problem is that the cables corrode internally(where it cannot be seen) on these cars, leading to high resistance and lowered voltages.  I would run a voltage drop test across both primary cables; if you see more then 0.5 V drop, you need cables.  I would also load test both the battery and alternator, which may have a weak diode that would only show up under load.

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Thanks for the response.  When you say "run a voltage drop test across both primary cables", can you please be more specific (auto electric beginner here)?  Do you mean the cable from the alternator  to the starter, and the cable from the junction on the transmission up to the power distribution box?  If so, I need it spelled out for me.  Which terminals on the alternator cable should be used to test the voltage drop (alternator to starter, battery positive in engine compartment to starter, starter to junction on transmission, or alternator to junction on transmission - there's three primary connections on this cable and I want to make sure I use the right ones)?  And same question for the other primary cable? 

 

And just to confirm, since both the battery and alternator are new within the past 30 days (alt is a Bosch reman), you would still load test both?  Thanks.

 

And finally, do you think there is an issue with the voltages I've reported, or is my car "operating as designed"?

Edited by spooltime
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Your stated voltage measurment is weak.

 

You should be testing the primary cables, the large ones running from the battery to the ground and starter, these are the ones that tend to develop internal corrosion. If you are unfamiliar with this test, do a search as this has been covered several times previously.

 

We always load test both the alternator and battery when there is a problem.  While this requires a load tester, it verifies that both are capable of delivering both the correct voltage and current (amps) as required.

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  • 2 years later...

This is a belated reply to document the resolution of my car's low voltage problem.  Replacing the cable between the alternator and the starter entirely resolved the issue.  As JFP correctly noted, apparently the cable I replaced had developed internal corrosion causing an increase in resistance.  It's frustrating when a Porsche dealer tells you everything is okay when you know it isn't.  Now having the requisite voltage flowing throughout the vehicle is a beautiful thing.

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40 minutes ago, spooltime said:

This is a belated reply to document the resolution of my car's low voltage problem.  Replacing the cable between the alternator and the starter entirely resolved the issue.  As JFP correctly noted, apparently the cable I replaced had developed internal corrosion causing an increase in resistance.  It's frustrating when a Porsche dealer tells you everything is okay when you know it isn't.  Now having the requisite voltage flowing throughout the vehicle is a beautiful thing.

 

👍Glad you got it sorted.  This is exactly why we always suggest running voltage drop tests on the primary cables, they are a known issue with these cars and even Porsche released "new & improved" cables to try and address the issue.

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Thats really interesting and good to know! Can I ask exactly which one you had to replace spooltime or which one you suggest JFP ?..I'm guessing #28 on this parts cat? Thanks for the info

Screen Shot 2020-10-13 at 17.09.11.jpeg

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All of the larger cables are susceptible to this problem.  The longer the cable, like to the starter, the worse the problem because their length exacerbates the resistance issue, leading to larger voltage drops.  The only real trick to checking each one with either a multimeter or Power Probe unit (Power Probes actually have a specific setting for checking voltage drops, plus the Power Probe's long leads back to the battery make the testing process easier).

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jpflip - In my case the culprit was cable #28 on the parts diagram, which runs from the alternator to the starter.  But for good measure and to avoid future concerns, I also replaced #36, which is the engine ground strap, and because my car is blessed with a Tip / paddle shifter set-up, #24, which crosses over the transmission on Tip models.  Cables #17 (MT) and #19 (TIP) are the long cables that run up to the distribution box below the battery in front.  I didn't bother to replace this one (but have a new one on the shelf just just in case) because I didn't find a significant voltage drop on it from rear to front. 

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Excellent information thanks a lot to both of you!!!! Giving me another thing to do this winter! My car is in storage starting the first of november ;-(

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  • 1 month later...

Hi all,

 

As I’m changing the battery, I’m checking the whole charging system as well. I measured the battery to ground cable: 0,5 Ohm and the battery to distributor: 1 Ohm. Would these be normal values?
I’m going to remove them and clean the contact areas to see if those values go down as they seem excessive, with so short and thick cables.

Also tempted to simply buy new ones, the ground is just 15€, the other is more, 47€.

I‘m getting 13.6V engine running with no loads and 13.2V all loads on. It’s not bad but I wouldn’t mind having closer to 14V.

Edited by laalves
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13 hours ago, laalves said:

Hi all,

 

As I’m changing the battery, I’m checking the whole charging system as well. I measured the battery to ground cable: 0,5 Ohm and the battery to distributor: 1 Ohm. Would these be normal values?
I’m going to remove them and clean the contact areas to see if those values go down as they seem excessive, with so short and thick cables.

Also tempted to simply buy new ones, the ground is just 15€, the other is more, 47€.

I‘m getting 13.6V engine running with no loads and 13.2V all loads on. It’s not bad but I wouldn’t mind having closer to 14V.

 

The correct test for the primary cables is voltage drop rather than resistance.  No primary cable should show a drop of 0.5 V, if they do, they should be replaced regardless of what resistance testing shows.

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Thanks, I did that, under load (i.e. during starting) and the MAX I saw in the voltmeter was 18 mV of voltage drop in the ground cable and 25mV in the + cable, which is obviously negligible. So I guess those cables are good.

 

I got 14.10V cold in the jump points, just after start, whilst reading 14.00V in the battery. However, letting it run and warm up, both values went down slowly but surely and after 15min of idling I had 13.8 in the back and 13.6 in the front.

 

When I turned on A/C at LO setting, headlights and rear window defroster it went down to 13.00V in the front. If I throttled up to 4000rpm (it's a Tip, it won't go above that whilst idling) I would get 13.72V in the front under those loads.

 

I'm beginning to suspect that either the regulator or the alternator is going. Since the regulator is not expensive, I'll get a Bosch one and replace it to see what happens. 

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A known problem with the 997.1 where the cable between the alternator and a joint under the car fails. A TSB was issued on this. A common problem that presents as a bad alternator or battery but s actually the connection between the two.

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