[NiftySVX]

Much good information has been given and I am not disputing any. Input shaft speed could be used to calculate load in a round about way because there is going to be a higher difference between input shaft speed and crankshaft speed, and thus, wheel speed when the engine is running hard against the resistance of the drivetrain, as is obvious to anyone who has ever driven an automatic transmission vehicle. However, engine load is traditionally calculated as a function of the mass air flow meter. But, our cars do not have an input shaft speed sensor, only VSS1 and VSS2 which are on the rear output and the differential case. Input shaft speed=turbine speed. Turbine speed=engine speed (minus the slipping of the converter obviously) and thus does not vary with road speed, as the speed of the input shaft is before there has been any gear involved. The main purpose for this sensor that I have seen in practice, as a technician, is to monitor the lockup clutch, the torque converter, and even to detect slip in the clutches between the input shaft and the output shaft. Interestingly, if our 4eat were equipped with such a sensor I think we would see our cars illuminate the MIL often due to an excessive difference in speed between the input shaft and the output due to slipping internals.

The service manual is actually quite accurate as to how the ACT 4 system works, aside from being fairly vauge about the map specifics. It is pretty logical, and has served well since introduced in the XT in 1987.5. I often explain it to people who ask in this manner:

Think of it as if the tcu were a person. If you asked the TCU when it would think transfer apply should be high it would tell you the same thing you would think. It would say, well, I bet there is a high likelyhood of wheel spin when the wheel speed is 0 and the tps is 100%, with the trans in 1st. You would agree, because that would be a car that is at wide open throttle at a stand still. Conversely, you would ask the TCU what it thinks it should be doing when you've got a speed of 40mph, range D speed 4, throttle 10%. The computer would tell you it's not going to need a high duty because the car is cruising and is not likely to experience wheel slip. Each time though, the TCU is gonna ask, hey, how fast are my rear wheels going? (VSS1), and what about my front wheels? (VSS2) because if they are turning at too much of a different speed I am gonna go ahead and lock that clutch down tighter than a (insert your favorite euphemism) and forget the rest of that stuff for now, because none of my predictions matter if the wheels are slipping, because that is what we are trying to prevent in the first place.

That is a simple, accurate way to explain what the TCU is doing and I have explained it in this way for years to a number of different people, including Subaru sales staff, my mother, my accountant (he has a subaru) and several exes (that may have been why they are exes, but i digress...)

The important thing to remember here is that it is not complicated. I know that we all agree on this matter, yet we are having some kind of communication error within a thread where a poor soul is just trying to make his transmission drive the car.