BMW M4 Transmission Nightmare: Grinding Gears & Limp Mode

Hey there! I had something similar with my 2019 BMW M3 (also owned an M5). I have some experience working on cars, but transmission issues can be tricky. Had those exact symptoms last year, rough shifts, warning lights, and limp mode. Turned out it was just a faulty transmission wiring harness connector. The connection was corroded and causing communication issues between the transmission control module and the car's computer. Took it to my regular workshop, and they diagnosed it pretty quickly. They cleaned and replaced the damaged connector and cables. Total bill was around 95 Euro, and the car was ready in about 2 hours. No issues since then. Before I can give more specific advice, could you share: Are you getting any specific error codes? Did these issues start suddenly or gradually? Have you had any recent transmission service done? Let me know these details, and I might be able to help further!

Thanks for the detailed response! My car has 28598 KM on it and just had its service last month. The issues started quite suddenly, was driving normally one day and boom, all these problems. Haven't gotten any specific error codes yet since the warning lights just popped up yesterday, and I wanted to check with the community first before heading to the workshop. Your connector issue sounds similar to what I'm experiencing. That's actually reassuring to hear it might be something relatively simple and not a complete transmission failure. Did your car also make those grinding noises when shifting? This is what's worrying me the most to be honest. Though since mine's still under warranty, I suppose I should probably take it to the dealership first. Just wanted to be armed with some knowledge before I go in.

Hey again! Yes, my M3 did make those awful grinding noises when shifting, scared the life out of me too! Since your symptoms are identical to what I experienced, I'm even more convinced it's likely the same wiring harness connector issue. I should mention though, in my case, I first suspected the radiator oil pressure like your mechanic suggested, but that turned out to be a red herring. The symptoms can be really similar, which makes diagnosis tricky without proper testing. Since your car is under warranty and the issues started suddenly like mine did, definitely take it to the dealership. While my repair was relatively cheap at an independent workshop, you shouldn't have to pay anything under warranty. Just make sure to mention the grinding noise and the sudden onset of problems, this will help them pinpoint the issue faster. By the way, after my experience, I learned these connector issues are somewhat common in our M cars, especially when the connections get exposed to heat cycles from the transmission. The grinding noise happens because the transmission computer isn't getting clear signals about when to shift, so it gets confused and tries to engage gears at the wrong time. Keep us posted on what the dealership finds, would be interesting to know if it's the same issue I had!

Thanks so much for sharing your experience! This really puts my mind at ease knowing it might just be a connector issue. I'll definitely head to the dealership tomorrow morning and mention everything you've pointed out about the wiring harness connector. Makes total sense about the grinding being caused by confused shifting signals, I was imagining much worse scenarios! I was getting really worried about potential transmission replacement costs, so hearing about this common issue is such a relief. I'm actually feeling pretty optimistic now that it won't be anything major. The sudden onset matches exactly what you described, and the fact that it happened to your M3 with the same symptoms is really reassuring. I'll update the thread once I get it checked out at the dealership. Really appreciate you taking the time to explain everything in detail. This is exactly why I love the BMW community, always helping each other out with real experiences!