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spooltime

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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Posted (edited)

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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