I have been wondering what keeps the car from gaining speed when you let go of the gaspedal going downhill at hgw speeds.
Ex:When going downhill and the cruise ctrl is set at 70mph and the car reaches 72-73 something breaks the car so that it wont go any faster.
What happens? Is it the transmission, engine or brakes that slow down the car?
The transmission. The intent of the 04's was for the automatic to give the driver the more sporting feel of a manual. Originally my car would blip the throttle on downshifts. With each re-flash of the transmission the car feels more like a "normal" Cadillac than a sporty car.
From what I can tell the PCM engages the torque converter clutch to add engine drag to keep the speed steady going downhill. At first it was weird but it actually works pretty good. It will go on and off as needed to keep the speed steady since the throttle is closed already. If the hill is too steep the car will finally start to speed up since neither the TCC nor closed throttle is slowing it down.
I was reading more about this and came across this note:
"Driver Shift Control (DSC) lets the driver switch from automatic to a clutchless high-performance manual transmission. Once the driver moves the gearshift lever into DSC mode, a quick tap is all that is required for smooth, crisp upshifts or downshifts within a selected range. It also has coast clutches to provide engine braking in all gears"
Looks like it's done with both the TCC and "coast clutches".
I believe the less than great gas mileage is attributable to the lack of free wheeling. No data or knowledge to support that. It was always just my feeling when I tried for fun to get as much gas mileage as I could out of my v8 AWD SRX and found the vehicle fighting me on coasting.
Well, this is another part of it. It could contribute to your pain.
"Both transmissions also feature shift stabilization, which minimizes “gear hunting” in hilly or mountainous regions, along with downgrade detection with brake assist and electronically controlled engine braking."
My V6 averages 21-22 but getting over 23 will require divine intervention unless I drive 45 MPH.
Frankly, I think they do this with a stick against the rear tires.....
Usually what keeps you from gaining speed down hills is "engine braking".
Basically that means that the gears "hold" when the power is going backwards so to speak.
This is a function of transmission design and programming.
Older transmissions used sprags clutch controls to provide engine braking.
Some transmissions don't provide for engine braking unless you put the shifter in a manual selection position, in other words, not overdrive.
GM doesn't apply brakes to maintain cruise speeds, it is just how the trans and engine are designed.
This can vary from transmission to transmission.
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