So what happens when (battery disconnected... alt fried?)
 

So I decided to go drive Stinky on Monday... ended up taking a turn a little quick, the battery slid out of the box and the positive cable disconnected... then she sputtered and died. Won't turn over now.

I figure it probably toasted the alternator.

What really happens to the alternator in an instance like this? Just some blown diodes? Easy to fix? (I have a degree in electronics, and I'm not half bad at taking things apart and soldering stuff)

Bump. Anyone?

Usually (depending how the voltage regulation is set up), if the battery is removed, the charging voltage increases to the point that the regulator is damaged, but not always. Some of the voltage regulation circuits are designed with this in mind, and will prevent the damage. Either way, generally the alt itself is fine, it's just the regulator and rectifier that need to be replaced. Whether or not they can be replaced is a different story. It is also entirely possible that the alt. is fine. Charge the battery, and then drive the car around to see if it maintains the battery.

I'm no pro so until a better explain comes along.

Voltage will rise and the diodes may blow. High voltage could also damage other electricals. I have numbers around for the surge v somewhere. I was very very high. This all happens in a brief instant.

Some of the rectifier diodes in the alternator might be fried.

The voltage spike is caused by a feedback loop between the ouput and field windings. Normally, with the battery in the circuit as a sink, the regulator provides voltage to the field windings. More power to the field windings provides a greater output voltage. The battery acts as a dampner in this operation, and prevents the voltage from getting too high before the regulator can cut it back. If the battery is removed, the system voltage jumps up, as the current that was charging it is no longer needed, and thus the field current is too high. This jump causes the field voltage to increase as well, and a positive feedback cycle gets set up that results in a voltage spike.

So it could be some blown diodes, or possibly a blown voltage regulator?

I'll probably try to remove the alt tomorrow or this weekend and check the diodes. If they're not blown, I'll see about the voltage regulator. I don't think it will be too complicated for me, I just wanted to know the possibilities.

Well, none of the fuses in the front fusebox blew... haven't checked the rear. I do have an aftermarket ignition, which I pray is not fried! If it's just some blown diodes. That would be an easy fix!

Thanks for the feeback guys! I'll try to report here what I find.

"So I decided to go drive Stinky on Monday... ended up taking a turn a little quick, the battery slid out of the box and the positive cable disconnected... then she sputtered and died. Won't turn over now."


If the alternator or VR is toast, wouldn't the engine still turn over? Or am I missing something here?

That's a good question.

id911T - thanx for your description of a feedback loop; posts like this contribute to positive learning experiences every day for me on this forum.

Based on this description, would it be possible for the feedback loop to have fried the solenoid on the starter? That would explain why the engine won't crank.

Feel free to shoot down (or confirm!). Either way, it'll be another positive learning experience. :)

I have to say that I'm curious too as to why the car wouldn't start after. I've never heard of alt. spikes damaging anything other than the alt. itself, or maybe popping a few lightbulbs. Hopefully when Stinky rides again we'll find out.

I'll try to extract the alternator tomorrow, and see what it looks like. I'll check the wiring and check the VR. If something is fried, I'll get pics and will document what goes on.

It's been awhile, but AFAIK, the wires that make up the feed to the fuse panel and the starter circuit are not fused. Therefore, the "spike" may have simply burned a wire that was weak. You may find that this unintended fuse link saved your electrics. If I'm correct, the only problem will be finding the "open."

I'd start with the battery connector, since it got stretched.

I don't know P but on my previous classic (so you know it has separate biggo VR), a friend of mine and I take off the battery terminal while the engine is running just to satisfy our curiosity. Nothing went wrong after that.

Why car wouldn't start now? umm.. Maybe your battery hit the fusebox/relay hard and damage or loosen somethings?

Check your battery for voltage, you may have damaged one of the posts internally.

Tim in Sac

afaik... The spike is well into the hundreds of volts for a very short time so the amps will be low. I would surmise that a CD box or pc boards would be vulnerable along with any computer boards flying on a wing and a prayer during the incident.

this one's pretty good in case you fried the CD box

http://forums.pelicanparts.com/porsche-911-technical-forum/238353-can-sudden-loss-ground-destroy-cdi-unit.html

As Ronin pointed out, when this happened to me, the CDI unit failed. Fired right up after replacing. I probably have one around that you're welcome to swap into your car when you confirm that you've got no spark.

PM me off-thread

The very first thing I ever bought from Pelican Parts was the battery tie-down. A good investment for you too. ;)