6l90 TCC apply ramp strategy. ECCC
Vehicle in question: 2011 Silverado 2500, 6.0 gas, 6l90 trans
For years I have been trying to solve a hesitation/chuggle/choppy acceleration issue with this truck (in fairness I go through spats of obsessing over it to not thinking about it for 6-12 months). Maybe this is old news for most of you, but let me post a little bit of what I have learned over the years of playing around with this... hopefully for the benefit of someone out there.
The sensation that has bothered me so much all comes down to the TCC lockup strategy used in this truck. Which is an ECCC lockup strategy (Electronically Controlled Capacity Clutch), where the toque converter does not completely lock up at all. Instead is it set to a constant slip of 20 rpm (in this particular vehicle). And that is done to reduce harsh drive line sensations and temporarily broaden the gear ratios (the ability to control slip is also used in the actual shift process allowing the use of inertia to smooth out clutch to clutch transitions). However, the system is not really able to "hold" the slip to 20 rpm, instead it uses pulse width modulation (pulsing the fluid pressure on the TCC) to attempt to maintain 20 rpm. As the slip deviates from 20, for example when the throttle is increases, the TCM increases the pressure and/or pulsing to bring the slip back to 20.
The issue that I have is when the TCC slips well past 20, which is common under load, the rate at which the pressure is increased to bring the slip back to 20 creates a jerking affect and often resonates like a spring, jerking all the way through a moderate acceleration. This is most noticeable from about 1200-1800 rpm. To resolve the issue, I have reduce the desired slip to 10 rpm in 3-6th gear and increased the + side of the apply ramp table by about 10% across the board and up to 30% in the slip rpm of 2-12. Currently the vehicle is much better to drive.
It seems there is a different strategy for almost every different vehicle out there. Some people seem to think that you want to keep the torque converted unlocked for most of the gears only locking at cruise and higher speed and others seem to think locked. I do notice in the full throttle TCC tables of my truck that the lock speed is 6mph for all gears (meaning at full throttle the torque converter is locked on whatever the desired slip tables are programmed for). Corvettes seem to use a 0 rpm desired slip for most gears, and Camero's used 0 rpm for the 1st gear or two, then more slip on the higher gears. I guess what I am suggesting here is that with so much variation in strategies in vehicles, it has to be less about longevity and reliability and more about the feel / performance.
Now here is the real question.... In my quest to reduce the "spring" like action of my torque converter bouncing back to the desired slip I have spent a lot of time pondering the curve of the TCC apply pressure ramp. There are 3 options for this curve: Linear curve - basically a straight line increase of pressure with slip. Exponential curve - where pressure rate of increases is more and more with more slip. and Cubic where the rate of increase in pressure is less and less with more slip. It would appear looking at the stock tables, that the factory curve was cubic - where they start off aggressive with 2-12 rpm of slip and although the pressures are still increasing with more slip, they are not increasing at as fast a rate as the slip continues. I would be very interested to see if this curve is the same with other chevy vehicles ? or any other vehicles for that matter.
The other item that I am not 100% comfortable with is the Regulator Offset, which has been described and the min pressure in the TCC regulator circuit. The stock setting on this is 26.69 psi. What i am unclear about is why such an odd number and how does that interact with the desired slip tables.? That number was picked for a reason...... ? So if the slip tables are changed, the min pressure should probably change with it? (or visa versa )
I am also still trying to find out what the TCC adapt is...?
