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I had to weld nuts to the outside of rounded-out allen type bolts when removing 15-year-old rear brake calipers and carriers this weekend. As I was laying under the car welding (probably running about 55 amps, MIG) with the my ground somewhere on the chassis I thought to myself "I should surely disconnect the battery." But I was wedged under a car with a welding helmet on and was already halfway done so I didn't bother. Frankly I've done it before and never caused any damage that I know of.

Anyway, you can see where this is going. Two days later, I'm driving, enjoying my new brakes when my brake pad wear sensor comes on intermittently. I assumed, of course, that I must not have plugged the sensor's connector back in completely. After a short stop the pad wear light was off and on came my battery light. After coming home and using a multimeter it's obvious that the alternator is inop.

Any chance this is a coincidence? Do you think I've messed up the ECU or perhaps the wear sensor light was a function of low voltage?

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You need to sort out the alternator, once that is functioning correctly then check out the sensor issue - my car is very susceptible to showing spurious issues when there is low voltage and they all disappear when the voltage is fine. As for the welding - well, best practice is to disconnect the battery and some even go as far as to disconnect the alternator as well. Did it cause the damage? Well, maybe, but hard to tell now for sure.

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    Attaching the ground close to the welding operation is best practice. Disconnecting the battery does not assure EM damage, and actually can help prevent it by presenting a "load" to the input of the battery powered and attached devices (mostly the ECU when the car is off). I agree that you need to resolve the alternator / battery voltage issue before you chase the sensor. If your ground was in the vicinity I doubt that your welding affected the alternator. – mongo Apr 4 '17 at 16:10
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WARNING: You should always disconnect the battery every time you perform electic welding on a vehicle. To fail to do so can have two possible outcomes; you can damage electrical components, you can start fires

With that unpleasantness out of the way, it is highly likely that the voltage / current applied by the welding gear has damaged the low voltage control electronics responsible for operating the alternator. Modern vehicles use a control circuit which regulates the voltage in and out of the alternator to prevent over charging. If this circuit is now unable to measure voltage / current flow, it may never bring the charging system online. For this reason I'd suggest having the alternator tested as soon as possible. You may be lucky and get away with replacing only the regulator if indeed it is available for your vehicle separately.

With regards to the spurious break wear sensor warning, this may be unrelated so check the loom. However I would warn you to be prepared for all kinds of electrical faults to appear on the vehicle. Hopefully a valuable lesson has been leared here.

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    This kindof seems like it should be a comment, first of all. Secondly, the OP said that he actually didn't disconnect the battery :(. I don't think this is an actual answer to the question. – Cullub Apr 4 '17 at 2:02
  • True, but it's good advice for anyone else reading it surely. I was hoping that it could be integrated into the eventual answer. – Steve Matthews Apr 4 '17 at 2:13
  • Oh I see. I think ending it with a question that the OP answered in his question (I know, mind boggling) is bad practice though. Maybe if you add some more info on the ECU, "idiot" lights (as my grandfather would say), and the battery we could make this one into a great answer. – Cullub Apr 4 '17 at 2:18
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    Agreed, I will attempt an edit tomorrow. This does come down to a fear / frustration I have with this site though. Cars and bad practice can injure or kill. We don't seem to have a way to highlight this for other users following a thread. Earlier today I was made aware of an apprentice who caused an electrical explosion on a car he was working on. This isn't something that will simply give you an error message. Hopefully my edit tomorrow turns this answer around. – Steve Matthews Apr 4 '17 at 2:23

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