ELECTRICS - what happened?

[Craig 930 RS]

Driving along, Porsche 930 with twin plug MSD ignition .... and sputter, miss....voltmeter reads under 8 volts.

Gets worse, more missing as if spark occurs only every 3 or so firing chances.
New battery added today and fires and runs - (former battery was 6 yrs old anyway so got a new Odyssey as a matter of course)

Alternator 100%? Was the battery providing all the energy and the alternator doing zip?

Not certain, haven't had an alternator go out on a car in years!

[911nut]

Batteries get more resistive as they age. If that's the case, the alternator wouldn't be able to charge it no matter what.

[Jim2]

Was the 8 volt measurement taken with the engine turned off? or measured with the engine running?

[Craig 930 RS]

Engine running: I hooked up the new battery and the voltage while engine running is a tad over13v.

I'll need to measure while running at 2000 rpm

Does a voltmeter dash gauge measure voltage coming from the alternator or battery?

[Jim2]

Some what the higher of the two. Look at the voltage with engine off (battery) and compare to when it's running (alternator). The output from the alternator exceeds battery voltage, about 14.8 or more iirc. This simple test will not deterine if the alternator is capable of full output load but it will quickly display if your alternator is kaput.

[h20cooled7]

Depends where your tapped in at, if you want to get technical,but the alternator provides voltage throught the whole car. Just dont tap into regulated voltage, a good source for reading alt voltage is like a cigarette lighter circuit,something that doesnt get used that often,well Im not a smoker though.

[Ken911]

my voltage regulator on my alternator crapped out a couple months back the alternator was only a year old. measure the voltage with it running should be around 14 then turn on the lights and the ac. see if it drops to 12 volts or so, if it does your voltage regulator is going. I pulled my alternator out was going to spend the several hundred for another one. but no one had one in stock so on a whim i took it to a alternator repair shop. He tested it and said nothing was wrong with it and wouldnt charge me anything. I was only having the problem when the engine was heated up good. when i told him that he said the voltage regulator might be going and failing when hot which he couldnt duplicate. So i bought a genuine porsche voltage regulator and the problem went away. The old one looked a little burned when i took it apart.

[Craig 930 RS]

Results across battery:

Measured with nce quality VM; 13.3 volts with car off, 13.7 volts with car at 2,000 rpm.

Drive it to see what the gauge voltage does?

[Craig 930 RS]

Well, crap.

Went for a 45 min drive and 30 min into it the voltage started dropping...from 12.5 BAM down to 10....then a big drop to 8 and engine cutting out...

[h20cooled7]

sounds like you are running off the battery, car will run off the battery till you deplete the voltage to keep the engine running.possible alternator or regulator, make sure all main grounds are good also, trans to chassis and battery ground to chassis.