I have a 76 911 that looses voltage as it warms up. At start-up it reads just under 13 volts but after about a 1/2 hr drive it drops to under 12. Since Ive owned it ,about 3yrs I've replaced the alternator,voltage regulator,starter, trans ground strap and battery.I also did a resistance test with an ohm meter on the pos from the battery to starter and the battery ground hot and cold and it checked out fine,no resistance.It always starts even when it drops below 12 but I think it's because of the new battery.I had it checked by a"pro" shop and they couldn't find anything and the alt checked out fine but I really don't think they checked it hot.All the electrics and gauges work fine .It's a shame this summer in the NE has been perfect targa weather and except for a short run to get the fluids warm it's been sitting in the garage.I had to take the Honda to Lime Rock for Vintage Race weekend.I'm currently unemployed and trying to fix this myself any help would be appreciated.

 

for cost efficency.. I'm about 10mi from Kennedy airport if you want to come here and swap your alt for one of my extras for a test flight.

that said, it seems like you need an alt rebuild.

 

I thought if the alt is bad it's bad it would show low voltage at start-up not just when it gets hot.How far are you from Northern New Jersey? Are your extra alts in gd working cond.? Is it possible the voltage regulator is screwing up ? I did notice that it gets pretty hot is this normal ? Ron thanks for the offer,but I think the next step is to pull the alt and get it tested locally.

 

A frw years ago, I had exactly the same problem. If you drove the car around town with frequent bouts of parking between runs, the electrical system worked fine. but take it on a trip over an hour or two, and the battery would die. It took me a very long time to find it, because it only happened when the alt. was completely heat soaked. It was a crack on the PC board brush holder that caused a 4 Ohm resistance when hot, 0 Ohms when cold. This dropped the armature voltage below where it needed to be to generate meaninful power. I actually just soldered the cracked track and the alt lasted for several more years--until the plastic bearing cup at the rear disintegrated from old age and no longer supported the bearing. BTW, if anyone knows where I can get a cup, pls let me know!

So, you can take the alt. off and check every part in deep detail--the cheap way--or simply replace it. It could also be a about-to-die diode or a regulator problem, esp if it is an internal style. I learned the hard way the only way to find these problems is to test everything.

Good luck. These things can be a witch to find. And, I understand the budget thing all too well.

 

A few years ago, I had exactly the same problem. If you drove the car around town with frequent bouts of parking between runs, the electrical system worked fine. but take it on a trip over an hour or two, and the battery would die. It took me a very long time to find it, because it only happened when the alt. was completely heat soaked. It was a crack on the PC board brush holder that caused a 4 Ohm resistance when hot, 0 Ohms when cold. This dropped the armature voltage below where it needed to be to generate meaninful power. I actually just soldered the cracked track and the alt lasted for several more years--until the plastic bearing cup at the rear disintegrated from old age and no longer supported the bearing. BTW, if anyone knows where I can get a cup, pls let me know!

So, you can take the alt. off and check every part in deep detail--the cheap way--or simply replace it. It could also be a about-to-die diode or a regulator problem, esp if it is an internal style. I learned the hard way the only way to find these problems is to test everything.

Good luck. These things can be a witch to find. And, I understand the budget thing all too well.

 

I'm with Dave on this one. I was just told a 3.2 story about the rpm sensor making the owner nuts. He used a heat gun to heat indivudal system parts. Sure as $hit, no start when he heated the rpm sensor at the flywheel.

I have a very good operating Valeo with internal regulator. It was checked at a re-builder last month. If you have an external regulator you could briefly "full field" the alt at the regulator. Or we could try my old working external reg. I could find it and check if it matches yours.

I'm about 45min from the GW Bridge or 30-35min on a midnight run. If you leave before daybreak it's doable to slide between commuter traffic in one day.

 

Thanks for all the reponses.I do have a external regulator. What do you mean by full field the regulator? If I have a external regulator I can I use one with a internal regulator? Ron I would have to pull the alt and regulator and bring them to you I don't trust my car to go that far.I'll try pulling the alt and give it a close inspection.If that doesn't work I take you up on your offer. Thanks again alot of nice people make up the Porsche community.

 

the external regulator may be just sitting there if an internal regulator alternator was installed. you'd have to pull the alt to confirm what you have.

"full field" with an external regulator means putting battery voltage to the external regulator field terminal. This will cause your alternator to max out the amps. So if the alt maxes out it's your regulator that's the problem. If it doesn't max the alternator is the problem. I suggest a new external regulator if your alternator needs rebuilding. Also some guys suggest keeping the external reg alternator 'cause the internal alternator regulator is sitting in a high heat environment, which may cause reg destruction, whatever.

Why not pull the regulator and alternator and bring it to a re-builder for testing?

Most shops don't charge. I read that one guy was charged $10.

 

Just for the record it's spelled losing.

 

I'll probaly pull it out tommorow and get it tested.I would have pulled it out sooner but I had it tested by a pro shop about 8 months ago and they said the alt was fine so I ruled that out.I do have an external because I replaced it with the alt when I had it rebuild about 3yrs ago.Is it normal for the voltage reg. to get hot.Thanks again for all the advice.Sorry about the spelling, I should use the spell check.

 

I pulled the alt. and had it tested and it checked out fine.The rebuilder suggested that maybe the battery is bad and to put it on a charge .The battery is new so it's probaly not that but I'll make sure it's fully charged.He told me he had a customer with a similar problem and he put in a alt with a internal regulator and the problem went away,but that tells me the regulator was probaly bad.I don't want to spend 250-300 on a new alt and have the same problem.Is there any more tests I can do while the alt is out of the car? I'm not really understanding how to do a full field test on the reg.If the reg was bad it would be doing just the opposite and overcharging the battery.

 

New batterys can go bad early.

Silly ques, does the red "G" idiot lite come on when you only turn the key to "on"?

 

Remember you have what appears to be a heat related problem here. This may not show up on your normal short test at a shop. Not comforting, but....

 

didn't think of that Dave..

ok.. to add to the above "g" lite thread..
Does the "g" idiot lite come on when low voltage when hot.. ie: turn car off when low voltage, turn key to "on" only, check "G" lite operation.

 

The G light goes on in the start position and off when the car starts.It doesn't go on at all when the car gets hot and voltage drops.Another words it does what it suppose to do.I should have insisted on the rebuilder running the test for more then a few minutes,but when I suggested that he said it wouldn't make a difference.Dave is right it is a heat related problem,maybe I should get it tested by another rebuilder.