Cruise Control Question

[hooD]

This question is not e-body related.
How does cruise control keep the speed constant when going downhill?  I'm not talking about steep declines, just gradual steps down to where your car would gain 5 mph without applying any gas pedal.  If the brakes are being applied, as I am assuming, would this not eventually add wear to the brakes prematurely and actually reduce your mpg's?

[NoMope Greg]

First, are you sure the car will speed up?  Has it been driven down this same hill without the cruise on?  The only way to slow the car without using the brakes is to downshift the transmission and use the compression of the engine as a brake.  With older cars, the cruise wouldn't slow or maintain speed if the car was going downhill.  I have no personal experience with them in modern cars, but I know that with modern transmissions having more gears, it's possible for the computer to downshift the car without you being aware of it.

For example, some of our buses are fitted with cruise and Allison B500 six speed transmissions.  These buses are, on average, about 10 years old.  Use the cruise on these and they speed up going downhill because the trans stays in 6th unless the driver manually downshifts it.  Our newest buses are fitted with ZF AStronic 12 speed automated manual transmissions (commonly called "manumatic" in the mags.)  Turn on the engine brake and the computer automatically drops a couple of gears to start slowing the bus.  I've not tried it to see if it works on cruise control on a hill (I don't drive that much any more), but I suspect that to be the case.  I'll have to ask one of the drivers who drive these buses often.

[71chally416]

I don't know that the normal CC does anything but alter the amount of throttle. I know of no automotive CC that applies the brakes and I know of no mechanism a car with CC has to do that. It's hooked only to the throttle. It just releases the throttle when it speeds up going downhill and assumes you will brake if the speed gets to fast.

[Chryco Psycho]

with the throttle closed it would take a pretty steep grade to continue to accelerate

[ViperMan]

Cruise Control does not apply brakes.  To apply brakes you would need an additional actuator somewhere to apply braking at one or all of the wheels.  That I know of, only the premium Lexas/BMW/Audi cars do this as part of the traction control system.  Most cars only have an actuator assembly on the throttle cable or throttle valve itself.  When going down hill the car simply speeds up.  I have gone down mountain with the cruise set where I've actually had to manually apply brakes, which of course deactivates the Cruise Control module.  At the bottom of the hill, I then "resume" cruise control.

P.S. - Off topic - did you know Vipers didn't come factory-equipped with cruise control??  That's gonna suck this weekend...

Jeff

[Chryco Psycho]

No CC or ABS , the Vipers are drivers cars with no computer babysitting you
too bad , I guess you will have to sell me the Viper now !!

[ViperMan]

Nah - I can get cruise control for $530 from Snake Oyl products.

I'll manage. 

Jeff

[73Chally]

CC does not apply brakes except for the new adaptive cruise control systems on the high end cars.  If the decline is long enough or steep enough, you will gradually increase speed.  The CC uses the engine braking to try and maintain speed as best as possible on declines.

[hooD]

   Thanks to everyone for clearing that question up for me.  I googled http://wiki.answers.com/Q/How_does_cruise_control_work  and you guys were dead on.