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A voltage problem that has been plaguing my 2002 996 C2 , 911, for ages,  is when I first start the car, according to a cigarette lighter type electrical gauge device, everything is ok showing 'about' 13.3 to 13.8 volts, and a green light on the gauge.  It remains that way ( 13.3 to 13.8 volts) for about 5 miles or about 10 minute or so, being driven or just idling.

 

Then after that (a few miles or 10 mins) it starts dropping to as low as 12.8 to 12.2 volts,and in the red ? This always happens after the motor etc heats up,hence it appears to be a heat issue I think ? Last year I removed the alternator took it to a local alternator specialist that replaced the voltage regulator with an updated version ? And when bench tested it appeared fine.

 

However once back on the car, it happened all over again ? Brand new battery, all connections appear ok,  it's only AFTER the car gets up to a normal operating temperature, after about 10 mins or so.  It baffles the hell out of me, and I always carry my jump start box , just in case. Any ideas as to what it is ? I was thinking of just buying another alternator and taking a chance on thats what it is ?

 

Speaking of that, Pep Boys, Auto zone , O'Reilly's etc, sell them for about $155.00 with a lifetime guarantee ! The dealer wants an outrageous price, silly money,  and a foreign car part place wants about $375.00 for a re-built Bosch. Any thoughts on the cheaper ones , and what my problem/issue could be ? This is a great car save for this alternator, electrical issue ??? Thanks a lot Dave. 

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Oh man, still struggling with this? Don't let a simple problem ruin your Porsche experience! Since you can reproduce the problem easily, all you need to do is measure the voltage drop at a few points to id the culprit with 100% confidence, as opposed to guessing and pondering.

With your symptoms, the most likely culprits are the alternator, the starter cable, and the ground cable. Alternator passing a test when cold means nothing as it can still fail when hot. As temp goes up, the resistance of corroded cables also go up.

I've replied to you in details before so I won't repeat myself here http://www.renntech.org/forums/topic/44471-voltage-regulator-affected-by-heat-hence-low-voltage/?p=240055

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Just a short in the dark... since the voltage is good when you start the car, but drops after 10 miles or so could it be bad radiator fans that are drawing too much power.  Maybe pull the fan fuses if that's possible and see what happens.

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If you get a new alternator, make sure it has a clutch on the pulley so it can overrun.  The cheaper ones are fixed.  The clutch prevents unnecessary surges / wear that can occur with sudden engine RPM changes.  

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If you get a new alternator, make sure it has a clutch on the pulley so it can overrun.  The cheaper ones are fixed.  The clutch prevents unnecessary surges / wear that can occur with sudden engine RPM changes.  

 

Ok Thanks , I was wondering about that, as if I do in fact need a new alternator,  the dealer wants silly money/buko bucks $$$, a German parts place here in Daytona wants about $350 or 360, for a rebuilt Bosch,  but Autozone, NAPA, Pep Boys, and O'Reilys have them for about $155 to $180 or so with a lifetime guarantee, sounds good , why pay more, but I'll have to check on that "clutch "  you mentioned. Not sure if they have that, would that make a serious issue if it didn't have one ? 

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Follow Ahsai's advice.  Most likely a faulty regulator which is a $35 fix. Save your money and donate to Renntech!

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G'day britdave,

I had a similar problem with my 98 996. The main difference was that I was unable to re-start my car after it had warmed up. As you take your jump start box, it may be the same problem?

I ended up replacing the Y cable that goes to the starter/alternator/jump start point in the engine bay - Problem solved!

Looks like Ahsai steps will enable you to conclusively determine exactly where the problem lies.

Good luck in hunting it down

Cheers

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If you get a new alternator, make sure it has a clutch on the pulley so it can overrun.  The cheaper ones are fixed.  The clutch prevents unnecessary surges / wear that can occur with sudden engine RPM changes.  

 

Ok Thanks , I was wondering about that, as if I do in fact need a new alternator,  the dealer wants silly money/buko bucks $$$, a German parts place here in Daytona wants about $350 or 360, for a rebuilt Bosch,  but Autozone, NAPA, Pep Boys, and O'Reilys have them for about $155 to $180 or so with a lifetime guarantee, sounds good , why pay more, but I'll have to check on that "clutch "  you mentioned. Not sure if they have that, would that make a serious issue if it didn't have one ? 

 

I don't know what problems you could have w/o the clutch, but I prefer driving my car to working on it, so I try and fix everything right the first time.  The only ones I could find with the correct pulley were OEM / Bosch. 

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Funny, but at one of the cheaper places ( costing $155- $180.00 )  Either Pep boys, Autozone, NAPA, Or O'Reily's , I forgot which of them ?

 

Two, parts guys (out of four)  asked me if I had an automatic transmission, or a 6 speed stick, they said there were two different part numbers ? The other two, showed only one ? Further investigation here, and on Pelican Parts (another $350.00 + alternator) discussed the need for the pulley/clutch on the 6 speed stick, but NOT on the 911/996 with an automatic tranny, Hmmm ? Most of the cheaper ones also had a 'lifetime warranty' Hate to waste $200.00 bucks for nothing, anyone got an opinion on that ? Cheers Dave. 

Edited by britdave

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A reason is that an engine with automatic transmission does not react so violently to the motor brake as an engine with manual gearbox, therefor it is not necessary to use a freewheel pulley. An alternator with freewheel pulley will use plastic rollers instead of metal in order to reduce the mass forces also. You may use an alternator with freewheel pulley always together with plastic rollers on an engine with a conventional alternator pulley and metal rollers, but not vice versa.    

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