I replaced my alternator at the beginning of the year and from cold I normally get a needle thickness under 14v. When the car warms up it drops to about 13.5V

 

NBTBRV8, did you install a remanufactured Bosch alternator, or something from autozone (or advance, etc)?
I'm going to put in the voltage regulator this weekend, we'll see how it goes.

 

I went to my local Bosch dealer and ordered the part number off the back of it, the only difference is that the Bosch supplied unit doesn't have the 996 number printed on it, otherwise it is the same unit.

 

I just finished replacing the voltage regulator, now at cold start i'm at 13.9-ish.
With the car warmed up and everything on (lights, AC and radio) i'm at 12.8 or so.
Big plus compared to 12-12.1v using the old voltage regulator with just the AC running.

I'll know exactly how much it helped tomorrow after doing my regular commute, since so far i've just taken the car out for a 10-15 minute drive.

 

so good news/bad news. after driving home in the lovely Texas heat with the ac on, the alternator was putting out 12.35v at the battery. In the morning I don't think it dropped below 13.
Also, I went out to lunch, and when I tried to start the car a couple hours later to head home, it wouldn't turn over. I had a friend help me jump it and drove it to autozone. which is 5-10 minutes away.
at autozone they checked the battery and said that it was 60% charged, which kinda surprised me since it was dead minutes earlier.
Is my battery shot?

 

once you finally get this stuff sorted.. do yourself a favor and get a ctek trickle charger off ebay ( if you don't have one ) and use it once a week/overnight. whether you think you need it or not. it will save you a lot of grief related to the inordinate voltage requirements of this car.

first i would replace that battery though.

 

I'm actually borrowing a trickle charger from a friend tomorrow, so we'll see how that goes.
Depending on how big his battery is, i might do a quick swap as well just to try to eliminate problems before shelling out cash.

 

the ctek was like 75. best 75 i ever spent. was always dealing with batteries etc. the problem with swapping though...how will you know if your battery is ok? i think the new battery will be well spent money, given you were probably running the car primarily off the battery before you swapped the alt or whatever you did. just my 02. good luck with it.

 

I've had the low voltage problem ever since I bought my 996TT two years ago. The PPI identified a weak alternator as the culprit. I replaced my alternator wtih a Bosch remanufactured unit from Pelican, and the problem was still there.

It starts at 13.8V but with the A/C on and after 30 mins it drops to about 12.1-12.3v.

A year later, I figured it could be a faulty voltage regulator on the remanufactured unit and replaced the voltage regulator - no change.

I've researched this issue extensively. The following are my theories.

1) Non-OEM battery (I'm using a brand new Varta silver battery). The DME (engine control unit) is supposed to get a feedback signal from the battery to control the power output of the alternator. How many of you have this problem and are running the factory battery?

2) Bad ground / worn ground or power cables - This is my leading theory as the root cause to this issue, as our cars are now almost 10 years old. My theory is that the reason the voltage drops as you drive the car for longer than 30 mins is because a bad cable starts to heat up. Heat increases the resistance in the cable, thus dropping the voltage (V=IR) The wiring diagram shows a few main cables as possible culprits:

a) + battery cable to fuse box

(I held this cable after driving for 30 mins and it was cool to the touch. Eliminated as the cause)
b) - battery cable to chassis
(I checked this with a volt meter, and the resistance is very low and it does not heat up, so I don't think this is the bad cable)

c) + battery cable from alternator to starter motor to (+) jumping point in engine bay. (When I opened the little plastic cap at the (+) jumping point the brass bolt would get too hot to touch after about 40 seconds from a cold start. I figured that this was my bad cable and ordered a replacement. My independent installed the cable and cleaned all the grounds. The voltage improved by on average 0.2-0.3v. My car now runs at about 12.3-12.5 v with only a/c and lights on.) The Porsche manual states that anything between 12-14v is "normal." I have no idea whether 12.5v with the a/c on is normal.


d) + cable from (+) lead to fusebox (no easy access to this cable. Unable to check)
e) - cable from alternator to chassis (no easy access to this cable)


3) Alternator / Voltage regulator - Eliminated as causes for me; though for some others, the voltage regulator replacement eliminated their problem.

 

I'm going to try a new battery tomorrow, a Bosch group 49 one that's 850CCA/1000A. I have a interstate group 91 right now that came in the car, which is apparently fairly under-sized for this car.
Part of the reason is that straight after a full charge it went from 12.7 to 12.5v in a couple hours of just sitting in my garage (no fans, etc). It doesn't drain that fast when the battery was at 12.15-12.2v
Also, just turning on the headlights with the engine off when it's at 12.45v drops the battery to 12.1v. For comparison, my wife's Audi drops from 12.5-ish to 12.35 or so, a much smaller drop.

Anyway, it's worth a shot, the battery is around $100 and much easier to mess with than the alternator/cables. If that doesn't work, i might try a full new alternator, maybe it'll help more than just the voltage regulator.

 

a new battery. i think it will help you, and again, get a trickle charger. welcome to 996tt if you are relatively new to the electrical demands of these cars, which are fully in effect, even as they sleep. another energy saver: double click the remote upon locking. it will disable the movement sensor inside the car. the car is "on" even when it's not. any trick in the book, i say.

finally, as a baseline: i get a solid 14 at startup. solid 13 with everything blazing in summer heat, and 13.8 all day long, generally.

new battery and a trickle charger, once a week. ya gotta do it.

 

Excellent analysis. I think you are on the right track with the cable checks - it makes sense. I get around it by plugging in the charger any time I'm not using the car now (which isn't often) - and once a week at minimum.

 

Just did mine after I got the charge light yesterday. Funny thing is it started working again after the car cooled off. I did't want to take a chance getting stuck so I swapped out the $50 part today. Seems to be stable around 13.5V.

 

Fully charged battery dying in 30 minutes tops, need complete alternator?

 

^ many say it isn't worth it to only change out the voltage reg since the condition of the alt remains unknown and you may find you have the same issue soon after. i would just get a new bosch ( not refurb ) and be done. what i did.