Alternator madness, please help!
 

I swear I've read every alternator thread out there, but still I need help...

My 1970 E blew it's alternator. The alternator light came on and there was no charge voltage, I got 11.8V at 2K RPM at the batteries. I had the alternator rebuilt, and I was told that "the diodes were all blown off the positive plate." With the car off I have 12.33V at the batteries, and that is with a number of starts on them trying to test various things (I will recharge them before starting again...)

So I installed my rebuilt externally regulated SEV Marchal alternator, and all was good for 2 days, then the light came back on, and I had no charge again. So I pulled the alternator, took it back to the guy that rebuilt it, and he said that it had done the same thing again, "blew the diodes off the positive plate," so he rebuilt it again (for free).

I knew this was a symptom of some problem so I did my research, and was told to check all the grounds again, and the batteries. The batteries seem to be fine, they recharge fine on my trickle, and advance tested them, though I don't know if it was a load test, it looked more like they just put a voltage meter on them.

So I concentrated on the grounds. Discovery 1) I had no alternator->block ground strap, and 2) my ground through the harness was intermittent at the 14 pin connector. I figured this must have been the cause of both alternator failures, so made a nice stout ground strap, and fixed the loose pin in the 14 pin connector.

I installed the 2nd rebuilt alternator, and still, the alternator light is on, and I have no charge voltage. This is where I started running out of ideas...

I checked the following...

* Continuity from the battery positive clamp to the blue wire on the alternator, 7.5 ohms, and full continuity with the key on.
* Continuity from the red (output) post to the battery positive clamp
* solid ground from the alternator
* cleaned all the grounds I could find
* tried bypassing the voltage regulator by hooking the blue wire to the black wire at the regulator, no change.

Questions:

Is there anything else I can check before I pull the alternator again and take it to be tested, again?

Can I test the voltage regulator in some way? My regulator is mechanical, and original to the car ( i believe )

What are the purposes of the capacitors that are hooked to both the blue and black wires on the voltage regulator?

thanks,

Britt

What about the transmission ground strap?

Seems fine, solidly attached, no fraying, other than a visual test, how else would I test it?

--britt

Check the ohms resistance between your different points starting at the battery and moving to the rear of the car.

Not at all trying to be a wise guy...but you *do* realize...11.8V is a completely dead battery ?

12.6V= 100% charge
12.4V= 75% charge
12.2V= 50% charge
12.0V= 25% charge
< 12V = dead

To clarify the batteries have 12.33V right now, and thats with a number of starts on them to test the alternator, measure current etc...

The 11.8V was the voltage running.

So Wil, does that mean that my batteries are dead if they are showing < 12V while running without the alternator? Or is that normal for a running car w/o a working alternator?

Britt, That voltage is really low while running. This means the battery has no capacity to even run the minor things like ignition while running. you have probably blown the diodes by trying to charge a dead battery with too high internal resistance. This drives the current load really high and overloads the diodes.

You can do a load test. Measure the voltage while cranking the car over. See how low it drops.

Try charging the batteries separately if they hold a charge they might be OK. It is also possible that you need new ones if the batteries discharge over a period of a few days/weeks.

The batteries are in parallel right? Can you disconnect one and measure the voltage of each individually?

To blow the diodes they would have to get hot and the only thing I could think of is over current.

When I installed the each alternator the batteries were fully charged from my 2A charger, charged one at a time, out of the car. Something like 12.7V on each when done.

While cranking with both batteries in the car I got down to 7.75V or so. Is that a big drop?

I'm charging the batteries again, we'll see how they do, they're about 5-6 years old now (Interstate brand).

Can I measure the internal resistance of the battery? I tried using my meter across the poles but got no response from the meter (digital meter, just read 1 like it does if I'm not touching the probes to anything), is that normal?

Rick, after doing the load test, and removing the batteries to charge them, I got 12.09V on one battery and 12.30V on the other

http://autorepair.about.com/library/weekly/aa101604c.htm

Says yes, thats a big drop... though I was load testing half charged batteries...

B

Ouch! 7.75V is pretty low.

I think your batteries are toast! Time for the recycling bin at your FLAP.

fully charged the batteries last night, both were 12.6V this morning. Tonight, however I had one 10.5v battery. I busted out my new $5 hydrometer and confirmed a dead cell. The other better had 3 marginal cells. I think it's new battery time.

Now do I buy 2 $105 Interstates or 2 $155 gel cells... Hmmm

911 alternators, whether they're new, rebuilt, Australian, Bosch, Marchel or whatever are some of the most retarded pieces of automotive crap ever made.

Valeo must be French for piece of crap.

my marchel lasted 39 years... I've heard the complaints about putting the alt so close to the motor, but is there something specifically lousy about the alternators Porsche uses? Inquiry mind wants to know...

I guess you solved your own mystery - a shorted battery seemed to have taken the alternator out. Not really something to blame on the alternator in my books. I believe modern cars have something like a 60 Amp fuse in line to protect the alternator. But on our cars it is connected straight to the battery.

Ingo

Replacing the batteries did not fix my alternator light. I've pulled the alt. and regulator, again, to have bench tested, again.

And the alternator is dead, again. They haven't disassembled it, but they suspect bad diodes based on the whine it was making when they spun it up.

So we rebuild again! Cause replacing is _MORE_ expensive. And while we rebuild, I'll check the f*ing ground cables a few more times...

B

Well I may have not killed it myself this time. The alternator shop discovered that the brackets that hold the wires going to the diodes had worn through the insulation, which caused everything to ground out.

replacing those wires (and tossing the brackets) restored my alternator to a working state.

I shall install tonight!

Man, that is a hell of a fight.

BTW... the capacitors perform the task of lessening the ripple voltage from the alternator. The diodes just rectify it (get the full sine wave to a half sine wave), the capacitors flatten it out a bit or it would swing from 0V to 13V in a half-sine wave.