Wires at battery terminals/alternator signal wire

[dissonant]

Hey everyone, I recently have had my car battery light come on. I changed the battery, but my battery terminals were super corroded and
I had to clean them before installing a new battery. In the process of doing so, one of the wires connected to the positive battery post came off. It was kind of a plastic piece that connected to teeth on the front of the battery post. I attached the new battery but not that wire that came off but the battery light was still on. I drove it home and checked the voltage at the battery posts and it was less than 12 volts with the car running. I'm thinking that the alternator is not charging the battery, but I was wondering if its because of a problem with the alternator or if its a problem with the alternator not receiving the signal to allow charging. Can someone tell me what that one wire that came off the front of the positive terminal is for and if its important? Is there anyway to tell if the alternator is receiving the necessary signal required to allow it to run or a way to send it the signal? I don't want to rebuild my alternator if it turns out to be something related to the battery terminal wire not being connected.

Also, if it turns out the alternator is the culprit, i'm thinking of rebuilding since it seems most people recommend doing it yourself. i have to say that first off, i had power steering fluid leak all over it for a while before I got the power steering cables replaced. could that damage any other parts in there that a cleaning can't fix? I was planning on getting the voltage regulator/brush kit and a set of bearings only. or because of the power steering fluid leaking on it, should I just buy a rebuilt alternator?

Thanks for your time and help!

[larrym3711]

Welcome to the e39 sub/forum! If your battery terminals were corroded initially, the problem might not have been the battery, especially since the new battery is not charging either. Step one is to get both the large AND the small wire attached to the positive battery terminal...no corrosion allowed! Ditto for the negative battery connection. Then start your car and check for the voltage with the engine running at least 1000 rpm. If you don't see at least 13.5 volts (14 volts is better), then you likely fried the regulator with the power steering fluid you indicated that saturated the alternator.

larrym3711
2001 540iA Sport

[dissonant]

Hi Larry,

Yes I agree that it wasn't the battery. There is currently 2 of the 3 wires attached to the positive battery post, the large main wire and a medium sized wire that is screwed into the top of the main battery post. What I don't have connected is the small wire that has a plastic yellow connector that was connected to the front of the battery post. I would reconnect it, but I'm not sure how to get a good connection. What is the purpose of this wire and is it preventing my alternator from charging the battery? If so, then any suggestions on reconnecting it to the front of the battery post?

I would post a picture, but I don't know how to attach pics... I did post a picture to my google drive, but using the insert image image toolbar icon didn't seem to work.

https://docs.google.com/file/d/0B5qq4WWXruA3MDc1XzRObW1DTEk/edit?usp=sharing

[larrym3711]

Not sure of purpose of the (smallest) third wire on the positive post, but if it is not connected now (as in broken off), I would get a fairly large terminal lug (like as used with the middle size wire). Solder (or crimp) it in to that small wire... Then connect that new lug to the same post as the current middle size wire. Thus, you have two wires connected to the middle size connection post.

Can some other poster tell us the purpose of that smallest wire (I suspect it is a sensor wire of some sort and NOT a major current carrying wire)

larrym3711
2001 540iA Sport

[dissonant]

I looked at the connector and its actually 2 wires... I could crimp on new connectors and connect it to the side lug, but i'd like to get more information on what those 2 wires are and why they're connected to the battery this way. I'm thinking its the signal that disconnects the battery when the air bags are deployed and that it should not matter if its connected to the battery post for alternator battery charging purposes. I'm thinking it should not be connected to the battery post where the medium sized wire connects.

[dissonant]

Anyone other than Larry have any advice or knowledge about the signal wire to the alternator or the 2 small signal wires on the battery terminal post?

Thanks!