Alternator troubleshooting procedure?

[I Love BMW's]

I had my son's 135i (N55) towed to the dealership since it wouldn't start and AAA was unable to jump start it. The next day, the dealer told me they jump started it and checked the alternator and it was only putting out 12.9 volts and it should be at least 13.5 volts. I questioned the starter and they said it was working fine.

The dealer replaced the alternator and then called me to say that the new one was putting out 13.5 volts, but that the starter was working sporadically and needs replaced. Needless to say, I was pissed since it was the starter that I brought the car in to be replaced in the first place.

Does anyone have access to the BMW troubleshooting procedures and range on acceptable output from the alternator? I strongly believe that the alternator was fine and only the starter needed replaced. Would like to have something "official" for when I speak to the manager tomorrow.

[NorthernDancer]

12.9 is too low
13.5 at least is correct
14.2 or .3 is often seen.
Many videos here on how to test the battery yourself. Buy yourself a DVM.
Often no start situations are loose or dirty connections especially ground cable left lower side of engine. Also, loose positive connection near jump start terminal in engine compartment.
Are there not any good BMW indy garages in your area?
Membership in BMWCCA for $48 is really, really worth it.

[JimD]

12.9 means the alternator was not working. That is the easiest way to know if it is working. A fully charged battery should be about 13.2 volts. If you read a voltage above that, the alternator is working. Putting the headlights on would confirm it. With the alternator working, the voltage should stay at or above 13.2. Without it, voltage will dip significantly.

Harbor Freight sells a little battery load tester which also tests the alternator. It displays the voltage in color coded ranges to make it simple. About $30. I think you can get the same thing off Amazon.

[I Love BMW's]

Thanks for the replies. Unfortunately the car wouldn't start and left my wife stranded while I was out of town for the week - otherwise I would have had it towed home and done my own troubleshooting and repairs.

Update to the situation. I feel somewhat vindicated since I didn't believe the alternator was the issue: The dealer replaced the starter (in addition to the already replaced alternator) and the vehicle still had intermittent starting issues. Turns out it was the cable from the battery jump point under the hood to the alternator/starter that had corroded on the inside.

The dealer installed the new cable, replaced the original starter and alternator and now everything is working fine.

[Derek Keenan]

I started my car up and it was idling for about 30 seconds the died, went to start it up again and it just cranked, got a ton of electrical codes, circuit shortages, transmission malfunction. The next day I went to try and start it and it still just cranked, battery read 12.0 volts, hooked my other car up to the positive and negative terminals under the hood of my bmw with jumper cables and tried jumping the car, still no start just cranks. Had a buddy with a known working battery reading 12.6 volts and installed it in my car, first try car started up perfectly fine then died 5 seconds after, so I tried jumping the car to see if the car would stay on and it just cranked. I'm thinking the alternator is bad and I ran off my battery for a bit. But I'm not sure why the car won't start with jumper cables

[NorthernDancer]

It's a loose or corroded battery cable somewhere. One like this is the usual culprit.
https://www.ecstuning.com/b-genuine-...e/12427551714/