Yeah. It dove down to 4, but you could hear the car just shut down as it dropped. Again, I had cables connected to my Lexus 300 sedan and it was running (lexus) just peachy. As long as the cables were connected to my battery the 911 ran fine, (a bit high idle tho) and then literally as soon as I took off the positive off the lexus Jay put the meter on the battery and it dropped over the next 15 seconds i would say it went all the way down to 4, car shut off and there was not any juice even to run the dome light. NOTHING. so I was hoping the battery was bad, but it has been on the charger now for 5 hours or so and the charger shows at 6 amps still. I'll see where it is in the morning.

If it is still at 6, then I think it is not chargeing and I can go get another Bosche Battery. So if the battery is bad, when the car is running and you have a meter connected to the battery that wont tell you what the alternator is spitting back will it? I also actually after jumpingthe car did not see the alternator light on except for the initial flash I think. Well hopefully more info will be found out tomorrow.

According to the Porsche manual, it's not a good idea to disconnect the battery while the car is running, actually they warn against it for fear of damaging the alternator. Any good battery would have kept the car running for at least 20-30 minutes or longer if the alternator is bad. What you have is a sulfated battery that's shorting out and is like disconnecting the battery each time it shorts and runs down. I still say get a new battery before you do damage to the car.

As for the argument about recharging a dead battery, I found the following passage off an automotive site:

"Alternators are designed to keep a battery charged and power the accessories in the vehicle. They are not designed to recharge a dead battery so you must test the battery before you attempt to diagnose the rest of the charging system. Alternators seem like they are working 100% of the time but in actuality they are turning on/off at regular intervals. Attemping to recharge a dead battery will overwork the alternator and may burn it out."

Not taking sides, but it was the first one I found. The site is at http://www.ratwell.com/technical/ChargingSystem.html Hope this helps.

+1 for a new battery, and I would have your alternator checked. It may have sustained damage to one of the windings, rendering it 2/3 effective, which may not be noticeable without a full load test. I would also be concerned about the DME, since you may have been running it with a low voltage for some time. The DME is robust, but it is TTL circuitry, which can be damaged by out-of-tolerance voltage.

An automotive site? Like this one?

Are you quoting Zonas?

No specs like max power, continuous power, temperature derateing etc???

Or you could save yourself the labor of removing the alternator and just put a battery in and see if it works.

You could assume the Porsche designers could add up electrical loads and sized the alternator correctly (including battery charge load).

You could also assume the Bosch/Marchall/Paris-Rhone (whoever) designed the alternator so it wouldn't burn up under rated load.

His 79 probably doesn't have a DME. If it did I wouldn't worry too much about power input in the range of 0-14 volts. On the other hand no battery or an open battery with large spikes coming from the alternator get out your soldering iron.

Didn't want to get sucked into this one, but is the factory manual ok?


http://forums.pelicanparts.com/uploa...1165907332.jpg

Factory manual for what?

Damaged I'll buy, I don't think you could run the alternator into a short.

Define discharged, Battery voltage 12 volts? 12.5 volts? How much difference does it make to the alternator?

Looking from the alternator into the battery do you model the battery as a resistance in series with a voltage source?

Below a certain set point is the field coil full on?

There are so many things that can consume our lives with worry and concern...or be utterly irrelevant. I remember once hearing Click and Clack say that if you back a car out of a parking spot and then put it in gear (or drive) while still rolling backward and let the clutch out (or gas it), you're causing unnecessary wear. You should first apply the brakes, come to a complete stop and _then_ go forward.

Am I concerned? Only if I'm backing up at 60 or so.

I am going to contact Delco or Bosch, under the guise of a journalist doing research, and ask them about this tempest in a teapot. I'll post what I learn, either way.

bad battery is a relative term???? come on. put a battery on a battery tester, which puts a load on it, if voltage and current cant be maintained, its bad, not relatively bad, unless u want to use it power your remote. ive even seen LOW batteries test good, they just needed a charge. a batt that can be recharged is just a low batt...not bad.

Sorry for the digression, but....

Actually, this very thing is known to cause failures in VW 020 & 02a transmissions. Evidence as seen in my very own transmission:

Reverse idler:
http://forums.pelicanparts.com/uploa...1165934683.jpg

Back to our regular scheduled programming...

s

OK, Here is a brief update. I went out this morning and the charger was at 2. So it seemed to have taken the charge. Next up is when I get home we will test with the meter again after the car has sat for 8 hours or so.,

What do you all think I will find?

WHERE is the voltage regulator? I have a few items on my driver side fender wall. Is it there? thanks!

Don't know what model 911 you have, but on many midyears cars, it's an integral part of the alternator and is on the back of it (i.e. toward the front of the car) inside the engine shroud. It can be changed out, but it ain't easily accessible...

What I believe is that the battery has taken a surface charge, soon as you put load on it again, it will short and it once again discharge. Instead of beating this dead horse, why haven't you just swap batteries with your Lexus and see what happens? It'll be the fastest way of proving whether the battery is bad or not.

1979 911 SC.

well I did not want to swap batteries in case I had some issue shorting out my battery. I thought bes to charge it fully withthe battery charger and go from there. When i get home today we'll see whats up. The battery appeared to have taken a full charge now. So we'll be able to start it and let it run and so forth without any jumpers. Once I do start it, if I check at the battery what the meter is saying can that tel me if the alt is bad? Like what would a brand new battery show on the mater if the car was running, but the alternator was bad.? would the battery just show like 11 or 11.5 volts? or would it show a continuos drain? f the alt is good but the regulator is bad and teh battery is good what would i see at the meter on the battery? and lastly if the alt and regulator are both fine but the battery is bad somehow, what would the meter show at idle?

Your car may perform normally for awhile with an alternator that has a bad winding. My advice is to have the charging system checked by a professional, in spite of all the brilliant advice you've had here.

If he wanted professional help why would he be asking questions here?

Is it worth taking it out and testing it?

If you have a short in the car, or the alternator is not charging, the battery will just discharge. Either put the Lexus battery in the Porsche, or the Porsche battery in the Lexus and see what happens. Voltage should be 12-13 volts with the car off and 13-14 volts with the car running. By putting the same battery back into the car will just lead to another discharged battery with no conclusions gained and perhaps damaging the car. Your choice. Good luck Allan