Hey everyone, I have some transmission tech questions. I'm a mechanical engineer with a background in transmission technology and I also have some background in automotive repair (one of my best friends is an expert mechanic). My knowledge is pretty limited though on some things because I'm fairly young with mainly book knowledge so that's why I'm here
Ok, so I have this older but awesome car with a GM transmission. It's a '99 BMW 530d. When it was new, it was the world's fastest diesel car and most efficient luxury car. The automatic transmission is made by GM. It's actually not too bad. The model is A5S390R (BMW#) or 5L40E (GM#).
I found a 128 page technical service document. It's rather complete as it tells what wires control each solenoid and all of the nine clutches inside. It gives instructions for a complete rebuild.
My car has around 180k miles on it. The transmission is kind of suspect but it still drives just fine so far at least. I believe that whatever clutches are controlled by the 4-5 shift solenoid slip at times because it will NOT slip in 5th gear but will in 4th with the TCC in lockup. I had a guy modify the transmission control unit to supposedly increase the clutch pressures and reduce the RPM at which the Torque Converter Clutch (TCC) goes into lockup. Those claims were somewhat true in that the TCC will go into lockup at 1200 rpm instead of ~2000. With 330 ft lbs of torque from 1300 rpm to about 4000 rpm it drives great like that. The problem is I don't like how it shifts so smooth. After his mod, it shifts much smoother (which typically means more wear) and it slips in normal drive mode when the TCC is supposed to be locked. His new program is very very clearly active when you activate it (by switching to sport to D to sport mode again). He supposedly put a "better" standard program on it than what the OEM BMW program was. I do not believe it is giving appropriate pressure to the clutch packs or the TCC. I want to boost the crap out of the clutch pressures.
In their infinite wisdom, from my understanding GM designed this transmission with a continuous slip condition on the all the gear clutches and TCC. To me, I see that as a retarded way to engineer failure. I found there is a pair of wires which goes to the pressure control solenoid so it should be easy to modify that signal (for that matter, any of the clutches!)
In order to maximize transmission life, obviously reducing or eliminating clutch slip would make sense. I know the resistance of the solenoid valve which controls the system pressure. It's pulse width modulated. It's just a crazy idea, but I found out that no signal = highest possible pressure. What if I cut the wires and place a resistor in place of it? A 4 ohm resistor should fool the TCU into thinking it's still hooked up.
It changes gears with solenoids and the pressure control solenoid changes the system pressure (minus the pressure for the TCC) so aside from shifting as hard as is mechanically possible, what could happen if I disabled the pressure control solenoid? The shock isn't likely sharp enough to damage the gears, engine or differential unless there's something I don't know enough about. The ECU should reduce engine power between shifts on its own and that won't be changed. It would only be throwing the momentum of the engine around but it normally does that with a longer time duration with more slip.
I just think it's amazing that car companies so clearly design failure into the product. That transmission should last much much longer if they didn't design that "continuous slip" into it. To me that's completely stupid. I'd like to defeat that.
What do you think? And if anyone knows of someone I could contact who might be able to answer these questions, that would be awesome!
I can't afford to replace the gearbox so any suspect problems have to wait. I also have to drive like 2000 miles in a few weeks and also can't afford to have it crap out on me with all my things in the car half way there.
Thanks,
Nick
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