08 Box Base Alternator
 

Did an Alternator test by attaching multimeter to battery w/ running engine. Read 12 volts then fell to 10.5. Battery not holding a charge as well. I assume both batt and alternator are toast. Thoughts ? V

Even if the battery was toast the alternator would still push 13.5-14v. The regulator/rectifier is shot but now a days you can’t just buy parts, just swap components. I wouldn’t change the battery until I recharged it and swapped the alternator. The battery can only accept what it’s given. If your alt is only putting out 10.5 the battery will never be more that 10.5

If your going to test the alternator, test it at the alternator not the battery.

You could simply have a bad or dirty connection at the battery, ground points, or connections at the alternator or faulty, damaged wiring. If you want to test the alternator start at the alternator, if it tests good then do a voltage drop test at each wiring connection. Work you way to the battery.

If the alternator is in fact faulty replace it. Then do a voltage drop test at the battery cable connections. If you have voltage drop then find the bad connection or wiring by doing voltage drop tests on the wiring and wiring connections from the alternator to the battery.
If no voltage drop then charge the battery and have it load tested.
By testing this way the entire charging system is tested and proven good or faulty and the faults are found and corrected.

The way your testing you could conclude that the alternator is bad when it could just be a dirty or loose wiring connection. Spendy mistake.

You have a multi meter, go to youtube and watch how to test automotive charging systems, also how to do voltage drop tests. Or google testing automotive charging systems. Also how to perform voltage drop tests.

Ok...Battery tested good ...but ....it wasn't the correct battery for the car so I dropped in a new one. Measured voltage across terminals w/ engine running and got around 10.5 volts. I assume its the alternator or voltage regulator. Will check connections and ground. Any thoughts. Thanks V

Check the voltage as you start, pre starting it should read high 11, low 12. Drop when starting abd then 13+ and settle at 12+. 10.5v I assume is the voltage when not running also. If the voltage does not move above static charge then the alternator is putting nothing out. The car does not care what battery it has.
Getting to the alternator is pretty easy and a rebuild kit is about $60 from Bosch (you can get a cheaper brush only kit, I don’t know if it’s worth cheaping out here, I’ve done it with crank sensors and was nothing but trouble), of throw a rebuilt one in. A rebuilt is usually your old one cleaned with a rebuild kit on so it will be about competency and wallet size at this point. If you are just buying batteries off the shelf I’m assuming you are looking for the fastest option, if so a rebuilt one may be the quick abd easy option with regards to availability.
https://www.pelicanparts.com/techarticles/Boxster_Tech/82-ELEC-Alternator/82-ELEC-Alternator.htm

Alternator parts number for 2008 base 986 99760302204 cross references with 99760302200 99760302202 AL0869N
A quick glance shows
https://www.maniacelectricmotors.com/alternator-rebuild-kit-for-porsch-911-carrera-0124525106-0124525107.html
But says aftermarket brush kit but does cross reference part numbers to
F00M145651
And a search with that part number brings up seller CSS on eBay who I have gotten many parts from.
I include numbers for people who may be searching later and the links may well be expired by then.

No it was 10.5v when running. I'm assuming alternator is dead. Any issue using aftermarket rebuilt alts such as ac Delco? ******************************** offers them at pretty good price. Thanks for reply. V