BMW Z4 Alternator Issues: Power Loss & Battery Problems

Hey there! I have some experience working with cars and had a similar issue with my 2011 BMW 335i last year. The symptoms you're describing sound exactly like what I experienced, weak starts, dim lights, and battery problems. After trying to diagnose it myself, I took it to my trusted mechanic who confirmed it was indeed a bad alternator (generator). The repair cost me around 960 Euro, including parts and labor. They also checked the battery and electrical system to ensure everything else was okay. Mine was fixed within a day, and it's been running perfectly since then. The 3-series and Z4 share some components, but before I can give you more specific advice, could you share: Has the battery been replaced recently? Are you getting any specific error codes? Does the issue get worse when using electrical accessories? Let me know these details and I might be able to help further!

Thanks for sharing your experience! My Z4 has 153961 KM on the clock, and this issue started right after my latest service. Yeah, the symptoms match exactly what you described with your 335i. The battery is fairly new, replaced it about 6 months ago, and I'm getting a "Battery Charging Malfunction" warning on the dashboard. The problem definitely gets worse when I turn on the AC or headlights, they actually dim quite noticeably when I'm idling. That repair cost sounds about right based on what I've been researching. Good to know it was a relatively quick fix for you. I'll probably bite the bullet and take it to the shop tomorrow. Just wanted to make sure I wasn't missing something obvious before spending that kind of money on repairs. Thanks for the help!

Hey again! Yeah, those symptoms with your Z4 really confirm what I suspected, it's definitely the alternator. Especially since you mentioned it gets worse with the AC and headlights, and you've got a relatively new battery. That was exactly what happened with my 335i. I actually remember now that my issues started after a service too, weird coincidence! The timing makes me wonder if something got knocked loose during the service, though in my case it turned out the alternator was just at the end of its life. Just a heads up, when I got mine fixed, my mechanic mentioned it's worth getting the serpentine belt checked while they're in there. Mine was showing some wear, so I had them replace that too. It only added about 50 bucks to the total bill, and it saved me from potentially having another problem down the road. Since you've confirmed all the same symptoms I had, I'd say you're making the right call taking it to the shop. From my experience, it's not something that gets better on its own, and you definitely don't want to end up stranded somewhere. Let us know how it goes, I'm curious if it turns out to be the same fix as mine!

Thanks for that detailed follow-up! The serpentine belt tip is really helpful, I'll definitely have them check that while they're working on it. You're right about not wanting to get stranded; I've already had a few close calls where the car barely started. I'll schedule the appointment first thing tomorrow. Better to get it sorted now than risk more problems or end up stuck somewhere. It's a relief to hear from someone who dealt with the exact same issue and got it fixed without any major complications. At least I know what I'm walking into now, both problem-wise and cost-wise. I'll definitely update here after the repair to let you know if it was indeed the alternator. Thanks again for taking the time to share your experience and advice, it's been super helpful in confirming my suspicions!