I believe the voltage drop and the ABS light are not related. It could be a coincidence and you are having two issues.
The ABS light will come on if any wheel (ABS) sensor died or was disconeected/damaged during tire replacements.
Voltage drop: you may want to check the condition and tension of the alternator/AC belt: this belt requires a lot of tension (if I remeber correctly, 9 units on Porsche tension gauge) to work properly.
I would start with the belt.
Thanks for the reply. I sure appreciate it. I did discover that the front two break sensors are missing. (Those are the ones that show when the pads are below a certain level.) I will have to check to see how to replace those. It is interesting that you mention that about tire replacements, because that is exactly when the problem occurred.
The problem with the voltage drop turned out to be the alternator, which was rebuilt and is now working great.
Thanks for your reply.
There is something to concider:
I believe most of owners do not use the pad sensors. As I understand, those sensors are designed to provide continuity (12V) as long as the pads are not worn out (i.e. the pads keep the switch pressed). On my car the sensors are missing, but the sensors' plugs are there and the wires on them are conected, so the continuity is provided. I saw those sensor on sell somewhere on the Internet if you still want to install them.
I've been wondering about that. Thanks for your thoughts on the matter. I guess my feeling was that it never hurts to have something that will indicate when a problem occurs - if that sensor is working reliably. But if there are false readings that come out of the sensor, then I guess it makes sense (pun intended) not to use them. <G>
I have seen them for sale very inexpensively on eBay. (I think they are around $10 apiece.)
Same thing happened on my 95; the belt from the crank that runs across the AC compressor and Alternator broke and the Red !, Anti-Lock Brake, and Battery lights all came on immediately. Your post gave me a scare; but when I replaced the belt, $25 from Carquest (did the power steering bet as well for an additional $13) everything went back to normal. Maybe in you scenario your tension on the belts is off a bit (I just eyeballed mine with out the tool). If you found anything else out from your situation others might appreciate it as there is little information readily available.
As far as I can tell, my situation was the same as yours. Everything seems to be working OK. (Other than the things on my list to fix that were already there, that is. <G>)
I'm obviously no expert on this, but this clearly seems like a ground issue where there is an exposed wire or bad ground that is causing this problem. I think any competent electrical diagnostician could probably find the problem. (I ain't one, unfortunately.) <G>
I managed to get hold of a second hand ABS relay to test my theory and it seems to have worked. Since I fitted it two days ago everything has worked normally. I thought your problem sounded similar and that it might be worth trying to.