Electrical: Please help me understand

[Xavier PJB]

Based on the thread, http://www.early911sregistry.org/for...040#post760040 , I'd like to understand what was my electrical problem (but my brain ) which drove me to too quick conclusion, however was really willing to help.

The engine is a 3.2 running PMO carbs, apart from the carbs the engine is 100%stock with DME still in place and partially plugged etc...
Was 400 kms from home, mid morning after a track session and a quick break at the track car park, the car was not fired up, no starter, but lights.
Tried several times, no starter.
I decided to ask one of the Porsche team's mecanic to give a hand to diagnose and fix. He tried to fire the engine up : no starter. Check the battery level, and was below 12v. Grabbed a extra battery and put in // with cables, and the engine fired up. Unplugged the cables and remove the extra battery - engine is running and got to iddle as regular.
Light on, 1500 rpm, the voltmeter indicated less than 12V. The mecanic diagnosed that the alternator was out of order. He told I could do track sessions but the electrical accessories would die as the batteyr level is going down until death of it.
I decided to get to the track, not stopping the engine of course. Did one 30 minutes session without any issues.
Out of the track I bought a new battery and a pair of cables, refueled too (I had to stop the engine). Restarted the engine with the pluging the new battery in // with the freshly new cables. It started instantly. Remove the cables and the battery. Went onto the highway and drove nearly 400 kms, but not using any electrical accessories, which was not really safe btw).

The day after, I strated the car with the same procedure, and check again the tension at the battery at 1500rpm - voltmeter shows the same as yesterday (less than 12volts)
Drove the car to my mecanic, did several things (which I do not know what) : same diagnosis - alternator is dead.
The alternator was changed by a brand new and it fixed the issue - the battery was still the same at the initial one - we did not changed it. After a short drive, all electrical accessories were up and the battery level was about 14V.
Did the procedure with idle at 1500 rpm, and the meter showed about 15 v.

So my conclusion was : you do not need a functioning alternator to avoid the engine running, but the electric accessories would not work.
Based on the thread at the begining of my post, It seems am wrong.

Then, I started to think (it happens sometines ) : the 3,2 engine as a regulator built in the alternator, would a faulfty regulator could have caused my issue but not keeping the engine to run off ?

I'd like to understand why, please enlight me.
Xavier

[Olivier]

The answer is relatively simple IMHO: Your alternator wasn't completly shot. It delivered enough power to spark your plugs, but not enough to do its job correctly (= loading the battery, give current to all the electrical bits...)

For your information, on "gas" engine, you need a spark to burn your gas. So, that said, you definately need a power source. Diesel is a bit different, as the diesel combustion is a bit different, once the engine is hot, it doesn't need any help to explode (auto- inflamation).

So, Your alternator was "near dead" and not exactly "dead". Or you may also had a faulty voltage regulator, this things can cause a lot of mess...

Olivier.

[davep]

I agree with Olivier that the alternator was not completely dead. I had an alternator bearing fail in a Chevy one night, and the alternator dropped out of it's bracket a little. With the alternator light on, I drove it home in the dark, driving the last distance by moonlight. There was no sound from the starter when I tried to restart just after I shut it off. It did restart the next morning though as great credit goes to the battery for recovery. So, you cannot really drive 400 km on the battery alone with the requirements of the fuel pump, ignition, and engine management. I'm not sure how low the battery voltage must go before an engine management system will fail.
Starting the engine requires a great deal of current from the battery, and if the battery has low voltage (nominal voltage is 12.8, but freshly charged may be over 13.5) then the battery cannot supply the current at a sufficient voltage to start the engine.
A bad regulator can cause all sorts of problems, and when it is built into the alternator, then it is hard to diagnose..
Of course, the idiot light in the dash is an essential part of the circuit, and it must function properly for the charging system to work. Some people have tried to replace all the instrument lights with LED's only to find that the ALT idiot light CANNOT be replaced with an LED.

[Xavier PJB]

Thanks guys, make sens.

Cheers
Xavier