Avalanche Trans Control Unit Issues, Slipping & Grinding

Had a similar situation with my 2004 Chevy Silverado, definitely know a bit about transmission issues after that experience. Started with the same symptoms: gear slipping, rough shifts, and the transmission control module acting up. After scanning for codes, my mechanic diagnosed a faulty control module. The automotive ECU was misreading signals and causing the transmission to behave erratically. Ended up paying about 1000 for the new control module plus programming. The repair fixed everything, transmission's been shifting perfectly since. Based on your description, sounds very similar to what I dealt with, but would need some more info to be sure: What error codes is it showing? Is the check engine light on? Does it happen more when the engine's cold or warm? Have you checked the transmission fluid level and condition? Let me know these details and I can share more specific info about what to look out for. These Chevy transmissions are pretty similar across models, so lessons learned from one often apply to others.

Thanks for the detailed feedback and questions. After reading through the diagnostic steps, I checked and my check engine light is constantly on. The transmission issues mostly occur when the engine is warm, especially during highway driving. The transmission fluid level seems fine but looks darker than it should be, last fluid change was at 95,000 KM. The main error codes I'm getting are P0700 and P0730, which points to transmission control problems according to my scanner. At 120,783 KM, I'm wondering if this is more than just the control module and if I should prepare for a bigger automatic gearbox repair. The grinding during shifts has gotten worse over the last week, even in limp mode. Would you recommend replacing just the transmission control module first, or should I get a complete transmission diagnosis done before spending money on parts?

Yeah, your situation definitely rings some bells with my Silverado experience. Those P0700 and P0730 codes, combined with the dark fluid and grinding, paint a clearer picture, and honestly, it's more complex than just the transmission control module I initially thought. While my issues were solved with a control module replacement, your grinding noises and dark fluid at 120K suggest you might be dealing with both electronic and mechanical problems. The transmission diagnosis should absolutely come first, I learned this the hard way by jumping straight to replacing parts. In my case, the transmission control issues were isolated, but the symptoms you're describing (especially the grinding in limp mode) point to potential internal wear. Given the mileage and dark fluid, you might be looking at worn clutch packs or solenoid problems alongside any control module issues. My suggestion would be to: 1. Get a complete transmission diagnosis first 2. Have the fluid analyzed, this can reveal internal wear 3. Get the transmission control unit tested separately The automotive ECU and transmission problems often go hand in hand, but that grinding noise makes me think there's more going on inside the gearbox. Better to know the full scope before spending money on individual components.

Thanks for that thorough breakdown. Makes a lot of sense to get the full diagnosis first rather than throwing parts at the problem. I'll schedule a complete transmission diagnosis at a transmission specialist next week. The grinding noise definitely has me worried about internal damage, especially since it's getting worse even in limp mode. You're probably right about the worn clutch packs, that would explain the dark fluid and slipping issues too. While I was hoping it would just be a transmission control module like in your case, sounds like I need to prepare for potentially bigger transmission repair costs. I'll have them test both the mechanical components and the automotive ECU during the diagnosis to get the complete picture. Will post an update once I get the full diagnosis results and cost estimate. Really appreciate the detailed advice on troubleshooting these transmission problems properly instead of just replacing parts and hoping for the best.