hihihi

Has anyone managed to get their cruise control to work going downhill?

Mine is fine on the level or going up, but downhill the car just gets faster and faster. However, I do have the radar assisted variety that will actively engage the brakes when there is another vehicle ahead.
So I'm thinking, there must be a way to get my cruise control computer to brake to prevent overspeeding when going downhill.

I drive a 2005 LS430 ACC/PCS (AdaptiveCruiseControl/PreCrashSafety).

Lynzoid

it will engine-brake to the best of it's abilities. but it won't press brakes for you

Lavrishevo

iTrader: (1)

I do not have any issues with over-speeding down hill with laser on or off. You do know you can turn the laser function off as well right? My system maintains the same speed at in all conditions unless the brake assist kicks in.

hihihi

Lucky you, 'Lavrishevo', to have a cruise control that never overspeeds. Am I right in guessing that your system automatically drops gears to keep to your chosen speed?

Mine feels like it only cuts off the fuel supply when going downhill and thus the car is free to accelerate. At least it's good at coasting.
Whether the adaptive part of the cruise control (mine uses radar) is active or turned off has no effect on downhill behaviour.

Lavrishevo

iTrader: (1)

No, I don't remember the car ever downshifting to slow down. Only to speed up going up a hill. I don't live where there are large hills with steep downhill slopes. I can't say how the car would react but I have yet to have any issues with the cruise control and over-speeding. What kind of hills are you experiencing this on?

AlexusAnja

I've found that once the vehicle goes 10mph above set speed, the trans does downshift because the rpm goes up. I have the non PCS cruise with active cruise. Try it, 10 mph over and you'll notice rpm spike and slower

hihihi

'Lavrishevo', I live on the moderately hilly Isle of Wight (off the south coast of England), and...
'AlexusAnja', I set my cruise control to 30mph along a straight bit of road that descends for about half a mile at a gradient of 7-8%. It was a quiet time of day, no traffic to worry about, and so I just let the car 'do it's thing' and observed it gradually accelerate to just over 50mph.

Also, my "Owner's Manual" says (for both adaptive- and conventional cruise control):
"Avoid vehicle speed increases when driving downhill. If the vehicle speed is too fast in relation to the cruise control set speed, cancel the cruise control then downshift the transmission to use engine braking to slow down."

So, yes, my 2005 LS430, is designed to gather speed when descending.

However, I know that my car is perfectly capable of downshifting and braking, because that's what it does to keep it's distance from a vehicle ahead.
So here is where my quest lies: can the appropriate Electronic Control Unit be updated or otherwise manipulated to activate those speed limiting measures even when there is no vehicle in front?

AlexusAnja

Yaozah... that's quite a speed jump. For the downhill i do, my cruise is at 40 and when the speed gets to about 50~52, i can see revs go.up and engine has downshifted. I would guess then that the "intelligence" is not just +10mph then but maybe a cap at 50 depending on initial speed?

2old4sport

I bought a 2014 GS350 from the local dealer in October. It had been a corporate car and had about 6,000 miles on it. It's loaded, including Radar/Adaptive Cruise Control. It's my third Lexus with this type of cruise control. Unlike any of my previous cars, the Radar/Adaptive Cruise Control applies the breaks when gravity would otherwise cause the car to go faster than 5 mph over where I have the cruise control set. If I set it a 50 mph at the top of a hill, it will hold me to 55 mph. It's wonderful! We have lots of hills, so I use it daily.
Interestingly, there is nothing in the owner's manual about this downhill breaking feature. My guess that it was put in this corporate car for testing and will be available soon in all Lexus models with Radar/Adaptive Cruise Control.