Voltage Drop at high RPM

I am experiencing strange electrical symptoms where the voltage drops under load at high rpm & load. Last week, when idling the voltage would drop from 13.5 to 10, have a disco of warning lights, then go back to 13.5. I replaced the battery, cleaned the battery earth strap and changed the voltage regulator. This seemed to fix the problem; there is now a steady voltage when idling. (14.3V at engine jump point, 14.15 at battery). HOWEVER I then took her for a quick spin (en route to essential shopping items of course), and noticed the aforementioned problem - the voltage drops under high rpm & load. Seems to drop to about 12.5/13 volts (not sure how accurate the voltmeter is in dash?). Queue the discoteque of warning lights. When I lift off and engine brake at high rpm the voltage goes back to normal which means it is not purely based on rpm but the throttle must also be open... Here is a video, apologies in advanced on the camera skills. https://youtu.be/YlaxSTO8b-w

The voltage regulator does just that, regulates the voltage from the alternator, there is also a full wave rectifier pack in the alternator with 4 diodes in it which rectifies the AC generated to DC before it gets to the Voltage Regulator, it looks like the diodes are failing under high duty to me, most multimeters have a diode test function on them, but can`t really test the diodes under high current conditions. Rectifier units are cheap, like voltage regulators, so I`d go for that to be honest. Flickering of the HVAC display or OBC would indicate presence of AC running through the electrics and also be a pointer. The voltage gauge in a 996 isn`t very accurate, it`s just a guide really :thumb:

Also might be worn slip rings causing the brushes on the reg to loose contact. Did you notice their condition when you were fitting the reg?

It may be worth checking the condition of the drive belt and tensioner too. If that starts to slip it will give you what you are experiencing, though usually accompanied by heavy steering as the power steering pump is less efficient.

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Engine running on idle and at operating temp , full electrical load so high beam , fogs on blower on max etc etc and check the battery voltage with a voltmeter .. Anything below 13 and you need to replace the alternator as 90% of the time this will be the fault . loads off and you will probably get 13.5 + . Garages don't tend to replace voltage regulators as the fault can still be with the alternator .

Thanks all for suggestions. I can see why garages replace entire alternator units rather than trying individual components I noticed that the 3 idler pulley bearings are quite noisy, so I'll replace these and belt anyway and see if that cures the voltage drop... If not time to start investingating idle voltage readings with full load, then look at slip rings and diodes...

Just to add that my GT3 is on 178K miles (many of them at, ahem, high RPM) and it still has its original alternator. I replaced the diode pack at about 120K miles due to exactly the same symptoms as the OP's. Obtained from Poland via eBay for < £40. Still in there going strong.

Very good to know! Also kudos on the high mileage, high RPM GT3 :thumbs:

When the throttle is full open at high rpm, the max current draw on the injectors and coils is significant. My own turbo happily sits at 13.8 volts on the gauge on road, but regularly drops to 13.0 after many laps if used on a track. It takes about 200 miles for the voltage to fully recover DESPITE having a new 160 amp genuine Porsche alternator and a 110AH Bosch S5 battery. All cables are good as is the earth point. If the pulleys are noisy definitely replace them quickly. One other possible problem may be the battery itself, I once came across a bizarre problem on a race bike, the bike would run strong all race long except from a hard race start. At the start, it would literally turn itself off! The battery was broken internally making intermittent contact only under a hard race launch. The other suggestions from all are also still totally valid, by the way. Hope you find the issue. :thumb:

That is interesting. I didn't know that coil/injectors had such high current draw. I have just replaced the battery though (72Ah, 720 CCA) which should be ample. Idler pulleys are not too bad. No noises with belt on & engine running. They just make a slight whir/rattle when spun by hand. Easy and cheap enough to replace; I'm getting to know this engine bay quite well :wink:

I had voltage problems on my car a couple of years back (high revs also) and changing the regulator was a cheap and effective cure for me.

Worth pointing out for future reference that another failure mode of the regulator is the battery discharging itself through a failing regulator, sometimes in as little as 12 hours. Link to regulator on eBay here: https://www.ebay.co.uk/itm/Porsche-...Alternator-Voltage-Regulator-New/310859976681

Update on the above... The rectifier pack arived but it was the wrong part (I got it from autodoc) Does anyone know the part number for the correct rectifier?

The Bosch regulator fits loads of models, including all the 996 ones. Better to fit the genuine article: https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=303075507319&_sacat=0