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Down hill and cruise control...

12K views 21 replies 12 participants last post by  baymnn 
#1 ·
I am just wondering about the cruise control and how it functions. Previous vehicles I have owned with cruise enable and set, the vehicle tended to remain at the speed set going down hill. Yeah, it may accelerate a couple kms or so but the Jeep seems to increase and not slow down. Yesterday on a particular long down hill it accelerated to about 12 k/hr from where it was set to the point I had to hit the brake. Is this normal?
 
#2 ·
Yes, very normal. At low rpm the engine is producing very little engine braking. Your speed is increasing because you are going down a hill..... If you want to maintain speed, move your shifter to the left and select 6 or 5, enough to get your engine rpm to 2500-3000 and you should have no problem maintaining the set speed. Also, learning to drive a manual transmission would probably help teach you a few things about engine speed vs vehicle speed. Something everybody with a license should know.
 
#3 ·
Do you have regular cruise control or Adaptive Cruise Control with the ACC turn on? Regular will speed up (no braking) ACC will maintain going down hill.
 
#4 ·
However it does so by applying the brakes. I usually just hit cancel and move the stick left and bump it forward so it downshifts. Then towards the bottom of the hill (or whatever works) hit resume and bump stick back right so she finds the right gear and maintains the set speed.

I have not been using Cruise, just Adaptive Cruise, and the only fault I find is the use of the brakes rather than downshifting as the 45RLE in my 03 Liberty did.

YMMV
 
#6 ·
I just let it slow down using the brakes. I'm fine with that. Brakes are cheaper and easier to replace than transmissions.
 
#7 ·
After only one experience with fading my brakes going downhill too enthusiastically back in highschool, I would not use my brakes to regulate my speed when going downhill, especially during long descents.

Brakes do overheat, and then they don't work.

Engine braking is the correct way to control descent speed. The brakes should be used to augment engine braking, but not to replace it. Especially on heavy cars like the cherokee.
 
#9 · (Edited)
After only one experience with fading my brakes going downhill too enthusiastically back in highschool, I would not use my brakes to regulate my speed when going downhill, especially during long descents.

Brakes do overheat, and then they don't work.

Engine braking is the correct way to control descent speed. The brakes should be used to augment engine braking, but not to replace it. Especially on heavy cars like the cherokee.
At 4101lbs for the Trailhawk, I do not consider it to be a "Heavy vehicle" My 5500 lb Silverado is a lot heavier. Engine Braking is probably the best way to control/moderate downhill speed, but most people are not used to driving a 9-speed trans where the top 4 gears are all overdrives. Most are used to 4-speeds and an overdrive. Maybe 3-speed with overdrive depending on model year. It will take time for drivers to adjust. My 2012 Volt is set to use coasting to help re-generate the battery, so it will pick up speed on almost any downgrade. Took some time to get used to the idea of no engine braking. Adaptive Cruise Control may be different, I have no idea.
 
#8 ·
When I use ACC and I'm going down a hill I hit cancel and let the Jeep coast and actually speed up, I'd rather that than let the cruise apply the brakes, why not use the momentum to save fuel and avoid wearing your brake pads, then at the bottom of the hill when the speed goes back down to my set speed, I hit resume. On my Ram I basically do the same thing, the only thing is the truck downshifts to reduce speed instead of braking, like al7fi said in the earlier post.
 
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#10 · (Edited)
<<When I use ACC and I'm going down a hill I hit cancel and let the Jeep coast and actually speed up...>>

In my neck of the woods the next step would be noticing the flashing lights in your rear view mirror.

<<I would not use my brakes to regulate my speed when going downhill, especially during long descents. >>

That's where we only partially agree. On long descents, absolutely downshift and use engine braking. On rolling hills the brakes do the job, especially nice sized 4-wheel discs like the V6 Cherokee has. On top of that, the 3.2 doesn't seem to exhibit strong engine braking from my experience.

With a transmission as problematic as the Cherokee's seems to be, I think I'll avoid unnecessary downshifting. But that's just me.
 
#11 ·
Maybe I'm paranoid :). I see engine braking as both a way to conserve fuel, and to ensure that my brakes have the best shot at being ready for an emergency.
Unfortunately the 2.4 doesnt have the best engine braking either.
You really have to intervene early rather than relying on it to slow you down once you are going too fast.

I upgraded to the cherokee from a '04 VW golf gti, so to me the cherokee is a heavy vehicle :)
 
#12 ·
Sport said <<After only one experience with fading my brakes going downhill too enthusiastically back in highschool...>>

You made me remember a high school experience of my own. I had a '63 Rambler V8 and I faded the brakes so badly (4 wheel drums) that they would no longer stop the car, even from a crawl. This was after driving like an idiot on country roads and deliberately seeing how badly they would fade. IIRC, I just coasted along until they cooled down enough that I could get it stopped by the next time I got to a stop sign.
 
#13 ·
On long grades, the transmission will eventually downshift, it's just set to use the brakes first. I'm guessing this is to keep complaints about shift busy-ness to a minimum.

Also, the brake system monitors brake temperatures (indirectly, by calculating the amount of heat generated from the braking being applied as well as a model for cooling, outside temperature, etc.). If the brakes start getting too hot, the ACC system will stop using them and will downshift instead (where appropriate). This would be long before you get the "overheating brake" message on the EVIC, so there will still be margin in the brake system.

Also, brake pads and rotors today aren't anything like what was around 20+ years ago. They have much higher thermal capacity especially the pads themselves.
 
#20 ·
Thanks , Mine actually downshifted on NCC whilst dropping down a steep grade to try and keep the set speed , My 11 year old Dodge Truck used to do this as well. I also noticed the mpg's dropped right off as well , Must have shut the injectors down I think .
 
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