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Cruise control not breaking downhill


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My old Volvo and friends other cars (VW) break when on cruise control and going downhill to maintain the speed.  My RX does do that and will keep going until gravity or I break which I believe is wrong.  If I set say 30mph and go down a hill the cars speed will increase, however if I change the CC speed on stork to 25 then the car will break

I have logged a call with Lexus about this and had a phone call with the service manager who said it cant break as that would disengage the CC.  I have advised him that the radar breaking and changing the speed of the CC does cause it to break and it is only pressing the break that will turn off the CC so why doesnt the car break on its own. 

I feel this is a software problem, has anyone else noticed this or raised this with their dealer ?

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Never seen or had the problem but yes, the system works in such a way that you set the CC speed and it keeps that speed whether going up or down or on the flat. That's how every CC system I've ever had works.

Even the radar CC will slow down if the car in front slows down and then speed up again when it speeds up.

You need to go back to the dealer and have it out with them.

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On the four cars I have had with ACC (two ISs and two RCs), the set speed on downhill stretches has always tended to be exceeded by 1-2%, and by up to a maximum of 3-4% on particularly steep ones.  Any larger excess would need to be looked at by your dealer.

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I've found the cruise control on newer Lexus vehicles allows the vehicle to speed up when going down hill, all my older Toyota/Lexus vehicles have managed to maintain a speed within 1 mph of the set speed. Happens at low or high speed. Seems the standard CC doesn't operate the brakes, only throttle, whereas dynamic CC uses brakes as well.

Your vehicle seems to be behaving as designed.

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The IS300h used to do that. Down steep hills it would speed up. 

Jag XE did that too 

Mercs dont tend to do it as they brake for you whilst going downhill. 

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2021 ES manual specifically states that it will not control you speed going downhill and it will exceed the set speed. Mine and what you're describing for yours are behaving according to the manual.

CC "On steep downhills
Vehicle speed may exceed the set
speed when driving down a steep hill.
"

ACC/DRCC "On steep downhills, or where there
are sudden changes between sharp up
and down gradients
Vehicle speed may exceed the set
speed when driving down a steep hill
"

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We have some steep hills here in North Devon and I usually set my cc at about 63mph knowing that the speedo and corresponding HUD read about 3mph fast at this speed.  However, the car does gain further speed of maybe 4 or so mph over the indicated speed down a long or steep hill but when this is occasionally exceeded, I lightly touch the brake which of course requires a reset of the cc.  So another confirmation of car not braking automatically in these circumstances.

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Lexus’ admission in its manuals that set speeds “may” be exceeded on downhill gradients could be read as a disclaimer in the event that a legal speed limit is exceeded as the possible result of a driver’s unreserved and consequently misplaced reliance on the CC system’s accuracy. 

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2 hours ago, Rabbers said:

Lexus’ admission in its manuals that set speeds “may” be exceeded on downhill gradients could be read as a disclaimer in the event that a legal speed limit is exceeded as the possible result of a driver’s unreserved and consequently misplaced reliance on the CC system’s accuracy. 

Most definitely the case. As is their constant reiteration that these things are only an aid to driving. The driver must not rely on them

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The right way to make acc working downhill is to set the speed and if you need you have to decrease the set speed without braking, as if you activate brakes automatically cruise control disengages. If you are coming from a flat road with ACC and begin to go downhill speed tends to increase just a  little, let's say from 50 to 55, so to keep a lower limit you have to diminish cruise speed to desired one. Remember that if you activate brakes ACC goes off!

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On 10/8/2021 at 7:54 PM, Richolf said:

Thanks all, I am in Adaptive CC

It doesn't matter if you have adaptive mode enabled, the vehicle will speed up on a steep decline if there isn't a vehicle in front to allow the adaptive part of the system to take over and apply the brakes.

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Cruise control only uses engine or regen braking when going downhill, it normally doesn't apply the brakes. 

On steep hills the car may exceed the set cruise control speed.

If it's a regular route you are taking, then maybe selecting more aggressive regen braking would help slow the car down more and also help charge the hybrid Battery up more. 

Of course if the hybrid Battery is fully charged I don't know if the regen braking is reduced or disabled, which may be part of your issue.

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I insist, my ACC on RX uses engine (regenerative or not) braking going downhill, and brakes also if there is a slower vehicle; the only difference is that it occurs at a speed slightly higher than the set one.

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Just an update as Lexus customer service have responded and said the car will not break when going downhill if it exceeds the set speed limit and they have no plans to introduce it.

It’s a shame as all the equipment is already installed for this and seems most other premium brands already do this.

 

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If I'm not mistaken, this used to be a feature in a lot of manufacturers, but then there were widespread complaints about brake disc consumption, especially the rears, so got dropped.

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13 minutes ago, peniole said:

If I'm not mistaken, this used to be a feature in a lot of manufacturers, but then there were widespread complaints about brake disc consumption, especially the rears, so got dropped.

Seems a strange argument as if one were not using cruise control one would still be using the brakes anyway, to slow or keep to the speed limit on a steep downhill?

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12 hours ago, NemesisUK said:

Do you see the brake lights come on?

Uhm, I can't as I am driving inside car 😁

What I notice is that to keep the set speed (+ tolerance ) engine RPM increase like a car in a lower gear; if car reaches a slower car ACC adapts speed to that braking also, like it usually does.

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6 hours ago, NemesisUK said:

Seems a strange argument as if one were not using cruise control one would still be using the brakes anyway, to slow or keep to the speed limit on a steep downhill?

Apparently, brake bias was different, with cc using rear bias for a smoother (less nose dip) braking.

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My take on this thread is that the ACC is more easily operated than described.  I have always supposed (and still do) that the ACC only applies the brakes on downhill gradients as the result of a more than slight excess over the set speed or, alternatively, when a vehicle in the road ahead enters the selected radar range. If the excess caused by the gradient is slight (see several posts above) the ACC stabilises the speed by throttling back, and only when it cannot maintain the set distance in respect of the vehicle ahead will it apply the brakes and disengage the speed setting pending manual re-engagement. I generally find it easy to detect (admittedly without fear of contradiction by any in-car system) when the ACC is applying the brakes because of the comparatively powerful and possibly sudden slowing effect thereby produced.  At high speeds, say 130kmh+, the braking effect combined with the rapidly increasing closeness of the preceding vehicle can be so startling as to cause one to override the system by unnecessarily stepping on the brake pedal as a reflex action.  Or so I find though others may not.  If proof is desired that the ACC is applying the brakes, it can be had in damp conditions when the glow of the brake lights is reflected off the road surface into your rear-view mirrors.  Another way is to judge the behaviour of the cars behind you, which more often than not react more quickly when they see brake lights.

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  • 2 weeks later...
On 10/8/2021 at 4:39 PM, Richolf said:

My old Volvo and friends other cars (VW) break when on cruise control and going downhill to maintain the speed.  My RX does do that and will keep going until gravity or I break which I believe is wrong.  If I set say 30mph and go down a hill the cars speed will increase, however if I change the CC speed on stork to 25 then the car will break

I have logged a call with Lexus about this and had a phone call with the service manager who said it cant break as that would disengage the CC.  I have advised him that the radar breaking and changing the speed of the CC does cause it to break and it is only pressing the break that will turn off the CC so why doesnt the car break on its own. 

I feel this is a software problem, has anyone else noticed this or raised this with their dealer ?

I am well versed in cruise control my lexus rx400h does not have the self control breaking which is v.common on Volvo's BMW mercedes and my jaguar xf but the newer 450h apparently has this spec mine is a 2008 Rx btw 

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