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In the past, I have had weak or dead battery issues, so I bought a battery/ charging system monitor. I don't know if it's the battery, alternator, or what was creating the problem ?

I have a 2002 996/911 C2 ,(with only 24,300 miles on the clock) today I put an 'Innova' battery monitor into the cigarette lighter to monitor the charging system and battery. Upon startup, at idle, it read 13.48 volts. At 77MPH on the freeway ,the monitor registered 12.80 volts. I had parking lights on, and the a/c running (same at idle) It appeared the voltage was higher, whilst idling, than when the car was traveling on the freeway at 77mph, doing 3000 RPM. Is this normal, I would have thought at higher rpms the voltage would be higher ? Can anyone advise ? Thanks Dave.

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Thanks, never thought about the voltage regulator ? It was funny how I was getting more voltage at idle, then doing 3000 RPM ( 77 MPH) on the freeway, couldn't figure that one out ?

Is that 14 volts what it should always be when the engine is being driven normally, what are the usual lows and highs in the voltage, on a 996 ?Could there be another problem like the ground strap,or wouldn't that make a difference, Im not to savvy when it comes to electric ?

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Regarding your comments about the voltage at idle vs higher RPMs:

1. Yes, it is possible it could be the wiring harness. I believe there is a TSB and an updated harness addressing this known issue. The harness is known to exhibit increased resistance, especially at higher temperatures. You can usually see the corrosion if you have the harness off and look at the old one vs a new one side-by-side. Ie. this could have more to do with the temperature you measured idle, and then subsequently, higher RPM voltages at after the warm was warmed up on the highway.

2. As JFP stated it could definitely be the voltage regulator or something else in the alternator. When mine went out I thought it was the voltage regulator and it was tricky to diagnose because the new regulator caused the voltage to stabelize -- however, the alternator was working just fine at lower RPMs, but wasn't working consistently at higher RPMs.

In my particular case, which may or may not be the same as yours, the solution was to replace both the alternator and the wiring harness.

Edited by ferrugia
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Thanks, never thought about the voltage regulator ? It was funny how I was getting more voltage at idle, then doing 3000 RPM ( 77 MPH) on the freeway, couldn't figure that one out ?

Is that 14 volts what it should always be when the engine is being driven normally, what are the usual lows and highs in the voltage, on a 996 ?Could there be another problem like the ground strap,or wouldn't that make a difference, Im not to savvy when it comes to electric ?

With the engine running at idle, the alternator output should be in the 13.8-14.5V range.

There is a TSB on the car concerning high resistance cables (both positive or ground) which can cause all sorts of problems (hard start/no start particularly when warm, dimming lights, weird MIL indications such as PSM or ABS faults when none exsist, etc.). Many times the cable ends look awful due to built up corrosion. The cables are not outrageously expensive, but they are a bit of a pain to install because of the way they run around the engine bay.

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TSB = technical service bulletin

There is not a video or DIY to replace this wiring harness that I know of. As JFP stated, the cable needs to be replaced. It will require taking the alternator out -- which is easy to get to, but in my case was a real bear to get out. There are DIYs on getting the alternator out however, which is a good portion of the work. While the alternator is out I would have it fully load tested. Be careful having a shop like Autozone test it. In my case at least, their equipment only did a basic test which passed the alternator as OK -- however, as I stated earlier, the alternator was only half working....it performed to spec at lower RPMs but not higher RPMs, and their equipment did not catch this. My suspicion is that their equipment isn't as expensive/sophisticated as a dealers would be, for example. Good luck. This shouldn't be too expensive... ~ $375 for the alternator and ~ $50 for the harness.

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