Sorry if I'm being a bit thick but ...

If I have cruise control on and have the speed set (for example) to 40mph and then go down a steep hill, how does the cruise control keep it to 40mph when engine breaking alone wouldn't be enough (i.e. just easing off the accelerator) ? Without cruise control on I would brake or change gear in order to maintain 40mph down a steep hill.

Does the cruise control actually apply the brakes in oder to slow the car down in the situation above - and if so, do the brake lights show?

 

as far as I know, the brakes will not be applied to slow your vehicle down when in cruise control. typically the car can downshift which will slow the vehicle down into (if not close to) the given speed that you have set it at. thus, since the brakes are not used, no brake lights will be lit because no braking is triggered by the brake pedal. if i were you and going down a hill, i would put it in neutral to not only save gas but to keep from worrying about having to hit the brake which would then turn off your cruise control automatically.

 

oh, forgot part of your question pertaining to engine braking not working good enough. that is true. it can only do so much because remember, your vehicle is not the lightest little bugger in the world, its got some junk in the trunk if ya know what i am sayin. i've gone down some pretty decent hills in my area and downshifted to maintain the speed i wanted, but i hate to see the rpm's fly up which will blow off my hard earned money put into that big ol' gas tank. thats why i use neutral and just slightly apply the brakes.

 

Thanks for the reply. My Mazda6 is a 6 gear manual gearbox. I'm not convinced that Cruise Control changes gear - but I may be wrong.

And Advanced Drivers in the UK (trained and examined by the Institute of Advanced Drivers) would not recommend shifting into neutral under any circumstance other than stationary because it would reduce control.

 

i goofed around with my cruise control today.... you are right. the gears do not shift down when declining down a hill. it rather just stays in the present gear and picks up speed until the brake is applied.

 

Brake control is not used with cruise control. The PCM which controls cruise will not apply brakes to slow you down. If engine braking cannot counteract speed increase down a hill then cruise will shut off. Going up a hill the PCM can select to shift out of Overdrive to maintain speed. If you shift into neutral then cruise control will also be cancelled.

 

As a big rig / 10 wheeler / 53' footer truck driver , I strongly recommend that you do put it in a lower gear before going down a steep hill (if steepness requires it) and apply brakes on lightly to slow you down gradually and more important : consistently . You do not want to apply brake pressure harshly once you have accumulated speed downhill , additionally you should not brake on and off repeatedly : doing this will create excessive heat and you will feed oxygen to rotors in between pumps of pedal that will increase heat and cause failure of pad adherence due to glaze effect and boiling brake oil and result in a probable fire !!!

The key is to slow down before going down and maintain constant and controled speed with light constant pressure on pedal (when needed only) : in return will produce less generated heat . If you control your speed by reducing it ahead ; you will not end up in the red line rpm range and blow that engine of yours

The way cruise control works is that it uses engine compression to slow and maintain speed (will happen on averaged raised roadways) and the ranged (margin of play) is minimal ... ex: set at 55 , will go +/- 1 or 2 miles to maintain its speed . If you are going down a steep hill : it will not maintain speed as mentioned but will increase due to excessive gravity and you will probably run in the red line .... same as going up a very steep hill : if you do not have enough momentum you will slow down enough that your cruise control will come off and you will have to downshift to a lower gears to stay in the appropriate rpm range to have enough torque to giddy'up !

Going down a steep hill , YOU want to be in control , not cruise ! Slow down before and maintain through .

The same goes for a curve (think of a 53' footer tipping over ) you slow down BEFORE the curve , and in the curve you keep it on the pull (slight gaz pedal pressure) : this will keep the suspension tight and in control instead of soft and tippy .

What started as a wannabe simple explanation turned almost in trucking 101 lol !!! I like to be well understood heheh

 

I know this discussion is a little old but so be it.
The given answer made me react to it.

Save gas?
Hi revs doesn't always mean hi gas consumption.
Just try it, even (or specially) if you don't have cruise control (on).
Go down hill, shift down one gear and don't(!) hit the gaspedal.
You're running in neutral, that all the gas being wasted.

Next thing:
How do you intend to slow down going downhill in neutral?
By braking? And what if going down hill is a long road?
Your brake fluid starts to boil, litterally!
By heat transportation from the brake discs to the brake-pads to the brake cilinders to the brake fluid pushing beyond the cilinder.

If this is happing you will end up having no brakes at all.
I repeat: No Brakes At All.
Until this point is reached: Your brakes work less and less.
(So by going downhill you end up braking harder and harder untill you have no brakes at all)
By doing this you wear off your brake pads and brake disc (it takes long for the discs but they do wear of).
The brake-pad surface will change due to the heat and never be the same!
This means less brake power in future due the changed brake-pad surface.

Your engine can stand the revs!
Do you ever worry hi revs when you accelerate you engine?
Just watch the revs by going downhill, keep them in the same (safe) range you would accelerating.

Never go down hill in neutral!!!
-----------------------------
Stay in control! If you have to react instantly, do you know which gear to choose?
And if you don't have the right revs for the gear you have choosen, your engine will decrease your
drivetrain so heavilly that it even could spin your car. You don't want that to happen downhill with an instant reaction.

Shift down and put some more gas in your tank afterwards,
but know by driving this way that your whole brakesystem (discs/pads/oil) is always in perfect condition.

Stay in control. And drive savely!

 

For safety sake, applying brakes kicks cruise control off. Cruise control doesn't shift gears, but on automatics, you may go into "freewheel mode" downhill (i.e. no compression braking). So long as you're not redlining the engine, don't worry. You don't waste gas because the throttle is closed to idle setting. Better still, compression deceleration creates a huge manifold vacuum while at the same time gives the engine a cooling fresh air purge. This helps keep plugs, valves, O2 sensors, cat., EGR valve, IAC etc. etc. active, clean and perky, hence is actually beneficial to the engine.

 

Cruise control does not "shift your transmission", but if you have an automatic transmission it will certainly shift with the cruise control on just as it would if you were operating the throttle manually. I usually downshift (or turn off overdrive) on a steep incline that will push me above the speed that I want to run. That is the safest and best method for maintaining speed on a steep decline. It won't hurt your engine nor waste much fuel.