Hi all, I thought I'd share my experience with some recent DIY work I carried out on my 05 E53 3.0D with the 6HP26 6-speed transmission... it saved me a full transmission rebuild, and focused on the mechatronic control unit (essentially the combined transmission ECU & valve body) located in the bottom of the transmission.
Some background info
I've owned my X5 since 120,000km (75k miles), and found out that it had been chipped by the first owner to 240hp / 450lb-ft, which is right on the upper limit of torque for these transmissions. Within the first 5,000km I had updated the transmission software version to try and get rid of a harsh 2-1 downchange when coming to a standstill, which it did alleviate by about 50%.
After about 10,000km of ownership, I encountered the first worrying issue with the transmission: under full-torque upshifts from 4th to 5th, it hit a false neutral of sorts and I had to shift back down to 4th before easing off ahead of the next upchange. This issue did reoccur whenever this scenario arose, but I altered my driving style so it only ever recurred 3 more times.
I got a full transmission flush done at 140,000km (87k miles), at the same time that I replaced all the other "lifetime" fluids. It was pretty gluggy, according to the workshop, who used ZF Lifeguard 6 fluid. The 2-1 downshift thump, which was still fairly noticeable, did improve again after this fluid change, so it was about 20% as bad as when I got the car.
After this, a peculiar, intermittent issue appeared, where it felt like I was dabbing the brakes moderately hard about once a second. This would only happen every 5-6 weeks, and only under the scenario where it was in 5th gear, with the torque converter locked up, between 1500rpm and 2200rpm, only after an upshift, and only when the trans was properly up to temp. Once the trans had been allowed to completely cool (i.e. overnight), it wouldn't recur for another 5-6 weeks.
Roll forward to 170,000km (~105k miles) and there was some other general degradation of the transmission's behaviour that was starting to make me and my wallet nervous. Slower shifts, random torque converter locking & unlocking in the 1500-2200rpm zone, and a 2-1 downshift thump that had again become really noticeable, all made me realise the transmission needed some TLC.
The mechatronics DIY job
After a bit of internet DIY research, I figured that there was a good chance that the mechanicals were OK, as they are bloody tough (e.g. the friction plate & bands are carbon), and a lot of information pointed to the mechatronic unit as the cause of all sorts of weirdness. I weighed up the options of a full transmission rebuild, buying a second-hand transmission, buying a remanufactured mechatronic unit, or buying new solenoids and seals and having a go myself. I decided on the last option, and bought NZD$1300 (about USD$900) worth of parts, including:
- 10L of Total Fluidmatic MV LV
- Aftermarket pan & filter unit
- M-shift 7-solenoid solenoid kit (as opposed to the E-shift 8-solenoid kit for cars with an electronic handbrake, like the E65)
- 4x sealing sleeves
- Bridging seal
- Mechatronic sealing sleeve
I followed the excellent Mechatronics DIY guide here:
Mechatronics DIY (Transmission valve body) - Bimmerfest - BMW Forums, including meticulously wiping all of the electrical contacts on the ECU clean with contact cleaner. This guide is for an E65, so the overall shape of the mechatronic unit is slightly different, and it references the solenoid for the electronic handbrake, which can be ignored.
One thing I
shouldn't have done was separate the valve body. I tried to source the intermediate plate, but I couldn't order the correct one without having the part number printed on the plate itself, which is conveniently different to the part number on the transmission, and could be one of four different types for our cars. If you're not going to 100% commit to replacing the intermediate plate of the valve body, DON'T pull it apart. The sealing tracks may well come away in parts and may cause pressure issues once it all goes back together. I was lucky that I managed to salvage the areas where the tracks came away. The valve body was utterly spotless inside, so it didn't need pulling apart anyway.
Overall, I didn't rate this work as terribly difficult, but then again I have rebuilt engines and gearboxes before. It is essentially removing a bunch of small bolts, pulling out some solenoids, and carefully cleaning stuff. Note that having the use of a hoist will make a major difference to how sore you are afterwards.
Once all the mechanical work was done, I reset the adaptation values using INPA and then set about re-learning adaptations using the techniques on page 41 of this ZF workbook:
ZF Transmissions full workbook!! - Bimmerfest - BMW Forums
The end result
I've been very pleasantly surprised by the difference. The shift from 2-1 when coming to a standstill is now almost imperceptible. Changes now seem to be crisper and quicker, yet they're smoother and more refined. Torque Converter lock-up appears to happen earlier. The transmission also seems to be more efficient at transmitting power to the wheels, and bizarrely, it seems to make better use of the top of the engine's rev range. Was it money well spent? Absolutely. Here's hoping I can get another 170,000km out of it.