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Join Date: Apr 2004
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Electrical Nightmare

I went on vacation and my 1975 911 sat for 30 days. After I returned I cranked it up the Gen Light was on. I have had the ALT and REG tested and they check out fine. I have checked all connections that I can get to. There is a charge getting to the Battery of about 11.72 volts that increases to about 12 volts when RPM are at about 4500, but it won't increase beyond that. It acts like a bad Regulator, but like I said, it checks out fine when tested. Please let me know if there is some other trouble shooting I can try before bringing it to a rip off artist known as the local Porsche guy.
Old 07-18-2006, 03:16 PM
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When you say it checks out fine, what does that mean?
Is the alternator putting out 13+ volts on a bench test?
I would check the voltage at the starter terminal. This would basically check the wire to the battery. Good voltage at the starter, low voltage at the battery would indicate bad connections, (corroded).
Is the alternator ground cable in place? Is it in good condition?
What kind of voltage do you read at the fuse terminals in the engine compartment? Low voltages every where, I would suspect the battery.
I'm sure there are others that have gone through this with some short cuts.
I would say a good auto electric shop could trouble shoot this. May not need your suspect local P-car guy.
Good luck.
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'76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's.
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Old 07-18-2006, 03:55 PM
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I had it checked at a porsche shop. He said it was good. I didn't ask him about the voltage. I have been checking the voltage at the battery. I will try at the starter. I did install an altornator ground yesterday. No Joy. What voltage should I be getting at the 3 fuses in the engine compartment?
Old 07-18-2006, 07:16 PM
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I would say the voltage at the starter or the fuses in the rear could be good (over 12 volts, maybe closer to 13 volts) if the grounds or positive wire going to the battery are bad. If the voltage reading at the starter and rear fuses are low, I would suspect the battery. Have you removed and cleaned the connections at the battery?
Checking at the starter or the rear fuses eliminates the ground wire at the trans axle/body and the positive wire forward. It's as close to the alternator you can get.
This is what I would do. Someone more experienced might have a better way to check this.
The problem is the alternator is in the rear and the battery in the front with three ground cables and at least two positive cables in between. One really long positive cable.
I have been known to check grounds by running a jumper cable front to rear, bypassing the car grounds. You could do the same for the positive side, carefully.
Like I said previously a good auto electric shop should be able to trouble shoot this.
Electrons don't speak German or American. I would have said English but there is a difference.
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'76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's.
'85 911 Carrera, stock, just painted, Orient Red

Last edited by 2.7RACER; 07-19-2006 at 08:32 AM..
Old 07-19-2006, 08:29 AM
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The only thing that matters is battery voltage. If you aren't seeing 12.6V at rest and 14V with the engine running above 2000 RPM, someting is wrong with one of three things, in order of suspicion:

1) Voltage Regulator
2) Alternator
3) Wiring.

What is the battery voltage when the engine is off? If it's 12.4 volts that's a battery that's at 25% of capacity. Charge the battery for THREE DAYS nonstop, don't set the car or anything you love on fire doing so. Then measure battery voltage with the engine off and running above 2000 rpm and report back here for further instructions.
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Old 07-19-2006, 09:21 AM
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Often a bad battery, with bad plates, will simply not take a charge. The light stays on.

I would also be sure that all connections are 110%. But do bench test the battery. Only this item would have changed over the course of a month.

John
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Old 07-19-2006, 11:18 AM
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I second Jdub's opinion, get the battery checked with a load tester, usually free at an autoparts store.
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Old 07-19-2006, 11:21 AM
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Had that problem, turned out was a bad connection to the starter.
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Old 07-19-2006, 03:16 PM
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ROB and Doug, You guys rock. I checked the starter conection and it was a little frayed. I spliced in new wires and bingo, problem solved. Thanks for the help. I owe you a few beers if you are ever in Colorado Springs.
Old 07-19-2006, 06:00 PM
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