For some background, I just purchased the Z4 about two weeks ago. I bought it because it had an issue with the SMG pump, and I got a good deal on the car. After fixing the pump everything seemed ok, After repairing the pump, I scanned using ISTA, and I had no error codes.
Then I drove the car around 250 miles and the battery light came on. I figured it wasn't the battery, because the previous owner had the battery replaced by the dealer about four months ago. Anyway, I measured the battery voltage at no load (engine off) and I read 12.2v. I also measured the voltage at load with the engine running, and I measured 12.0v, so the alternator was not charging. So, I took the alternator off and too it to a local rebuilder. He first bench tested it, but then realized the regulator needs an input signal from the DME to produce voltage. Then, he put an old regulator on the vehicle (from a Bosch unit before the DME integration), and tested it again. This time, it produced voltage within spec. So, his recommendation was to look at the wiring, and the fuses to ensure there weren't any blown fuses, or broken wires.
I checked the wires and didn't see any problems, and I also checked the fuses in the fuse box, and the computer box, and all were OK. Next, I unpluged the data line to the alternator, as the fail safe, as I understand, is for the alternator to default to 14.2v with no data. Still, no voltage difference with the engine on, and with the engine off.
Any ideas of anything else to check? I'm thinking it must be the regulator, but I have to wonder if that is a normal failure for an alternator with 78,000 miles.
Hi, Was there a solution to this issue as my Z4 has the same problem.
I believe he changed the voltage regulator on his alternator.
I have an ‘04 and it wasn’t difficult to change the regulator on my alternator as a “refresh”. I kept the old regulator as a spare as mine was still functional although the voltage output wasn’t as strong as the new replacement. If the replacement regulator fails for some reason in the future I have a spare ready to replace as an immediate fix... or long term if wanted.
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