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THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCH996TT won't start but battery & alternator are good
Reported or suspected cause
diode pack/voltage regulator
Reported cause
No successful repair reported for this finding.
1
Identified cause
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Suspected cause
0
Reported fixes
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Discussions that report identifying a cause
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Reported cause
The problem turned out to be the diode pack/voltage regulator on the alternator, though the alternator was charging the battery during running it was draining it at a faster rate when not running.
996TT won't start but battery & alternator are good
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Leading evidence: 'diode pack/voltage regulator' (1 discussions report identifying this cause; no attributable repair resolution). Across 1 matching discussions, the most frequently mentioned possible cause is 'diode pack/voltage regulator' (1 discussions; 1 report identifying it and 0 suggest it). The most frequently discussed action is 'Check by a competent auto electrician or porsche dealer/specialist' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
Discussions that performed work without a confirmed outcome
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Performed · outcome unconfirmed
The alternator was replaced and now I have no problem even leaving the car for 5 days with no charger (before I couldn't leave it 5 hours!)
996TT won't start but battery & alternator are good
Test ' ie was seen
Performed · outcome unconfirmed
No successful repair reported for this finding.
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Discussions
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Performed · outcome unconfirmed
The alternator was 'tested' ie was seen to be charging the battery and the battery was new.
996TT won't start but battery & alternator are good
alarm
Suggestion or failed attempt
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0
Identified cause
1
Suspected cause
0
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Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
Discussions that suspect a cause
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Suspected cause · Original poster
Yes it would seem there is some sort of drain; I suspect the alarm.
996TT won't start but battery & alternator are good
still the same
Suggestion or failed attempt
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Identified cause
1
Suspected cause
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Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
Discussions that suspect a cause
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Suspected cause
If so try not locking it (ie alarm off) for a few days and see if the problem is still the same.
996TT won't start but battery & alternator are good
Disconnect battery unless you have the key in the ignition
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Suggestion
The alarm always goes off when you disconnect the battery unless you have the key in the ignition with the ignition on (car not started) when you disconnect the negative terminal.
996TT won't start but battery & alternator are good
Disconnect negative terminal
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Suggestion
The alarm always goes off when you disconnect the battery unless you have the key in the ignition with the ignition on (car not started) when you disconnect the negative terminal.
996TT won't start but battery & alternator are good
Reported and suspected causes (3)
These are separate views of the same evidence. Frequency alone does not establish a fix, and these counts must not be added together.
diode pack/voltage regulator
No successful repair reported for this finding.
1
Identified cause
0
Suspected cause
0
Reported fixes
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Discussions that report identifying a cause
1
0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.
0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.
Reported cause
The problem turned out to be the diode pack/voltage regulator on the alternator, though the alternator was charging the battery during running it was draining it at a faster rate when not running.
996TT won't start but battery & alternator are good
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9 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
911uk: 1 discussions
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Generation: 20261027T033922Z · outcome-qualification-6. Deterministic extraction; no AI model inference. How we use sources
Before I go any further I have tried to avoid wasting anyone's time by reading the very useful post in the 996 FAQ regarding the general fragility of batteries.
My 996 Turbo has suddenly been struggling to start. I've had it two months, all has been well, but when it was left for about 5 days over christmas (the longest it has been left standing between drives since I have owned it).
So I prepared myself for a minor fleecing at Halfords, but before shelling out £110 for a Bosch Silver the helpful (and clearly into nice cars!!!) guy in there came outside with his diagnostics box of tricks to have a look at my car. 2 minutes later and both my existing battery (Energizer Premium EM74-3, part code 574-104-068-UK096, 74Ah, 680A) and my alternator have been confirmed as good. I was pleasantly surprised to be told I didn't need a new battery, just that I needed to take my car on a longer drive to give the battery a good charge.
Twice now I've given the battery an overnight charge with a trickle charger, and that helps. But if I left it say 2 nights without a trickle charge, even if I used it both days I would imagine it wouldn't start.
So that's not really a long term solution.
Do you reckon I do need a new battery? If I do shell out for a new battery is it likely to last, given that I work from home so don't have a twice daily commute to give it a good charge. I could use one of the CTEK XS3600 battery chargers I've read about...but the last thing I want to be doing is connecting my "supercar" to the mains everytime I get out of it just so it will start next time!
Hello
If the battery checks out and alternator output is good, then the car must have an electrical drain on the battery whilst not in use...eithe a backup battery is being charged or some type of electrical drain is present...
It will need checking by a competent auto electrician or porsche dealer/specialist.
Mike
Thanks Mike.
I've been trickle charging overnight every 2-3 nights which is keeping it running. Yes it would seem there is some sort of drain; I suspect the alarm. When I disconnect the battery the alarm goes off...very loudly!
I have an electrical friend...are we just talking doing voltage checks either side of each electrical 'device'?
The alarm always goes off when you disconnect the battery unless you have the key in the ignition with the ignition on (car not started) when you disconnect the negative terminal. Something to do with the alarm system thinking you are stealing the battery. And yes its ear splittingly loud when your ear is next to it!
The alarm does drain the battery a little. Is your car garaged? If so try not locking it (ie alarm off) for a few days and see if the problem is still the same.
If you do a search on here for 'battery issues' you will find my thread about a similar problem which took 9 months to find. The battery went flat while the car was parked at work during the day, every day, though the car was trickle charged every night.
The alternator was 'tested' ie was seen to be charging the battery and the battery was new. The problem turned out to be the diode pack/voltage regulator on the alternator, though the alternator was charging the battery during running it was draining it at a faster rate when not running.
The way to check for this is to feel the temperature of the alternator on a cold morning with your hand, relative to the other metal bits around it, if the alternator is warmer than the engine then that is the problem. Mine was not hot but was noticably warmer than the engine or chassis. The alternator was replaced and now I have no problem even leaving the car for 5 days with no charger (before I couldn't leave it 5 hours!)
Try it first thing in the morning before you start the engine..
Good Luck
I'm wont be using the car everyday but intend in using it mainly over the weekends (other than in the summer
). The car is left parked outside , uncovered on the driveway. Does it need starting every few days to stop the battery going flat or will it be ok just starting it as and when needed?
