adaptive cruise control Faults with a brand New IS 300h

[Surendra]

I recently bought a BRAND NEW IS300h Premier from Lexus Leicester. As I started driving, I suddenly discovered that the speed limits it showed were TOTALLY wrong and bizarre. It showed the SL (Speed Limit) as 90 on a 30-mph road, certain areas it was 120 mph or even 140 at times. Then when I tried the Adaptive Cruise Control (ACC) it went OK and suddenly a heard a warning beep and I had to suddenly put the brake on to avoid a collision with the car in front.

The car was booked in and initially they said that a part needs to be ordered and after a few days I was informed that there is nothing wrong and that the ACC stops working below 30mph.

I had a LS600 which worked perfectly and the car would stop if the car in front stopped. My daughter has a Lexus NX and it works fine in that car too

 

[Ian J. Parsley]

Surendra et al,

Welcome to the forum and congratulations on your new car. Please bear with me as I try to go through what I think is happening here.

In fact, I think it would probably be worth having a separate thread on adaptive cruise control in general. I think the system is excellent but you do have to understand what it is (and isn't). Excuse the long posting but I think it is worth the explanation.

Firstly, regular cruise control essentially means the car takes control of the accelerator. This means you set the speed and the car will operate the accelerator to stay at that speed, come what may. Of course, this isn't much use if there is an obstacle in front (the car takes no account of that), nor on a significant downhill stretch (because that needs the brake to be employed to maintain speed, but regular cruise control affects only the accelerator). This applies from 40kmh up. Cool.

Secondly, early adaptive cruise control systems added a radar to take account of traffic in front. With these, if the car detects an "obstacle" moving in the same direction, it assumes it is a vehicle and will move off the accelerator to bring speed into line with that vehicle. This still only applied from 40kmh up. Great.

Thirdly, what you have (I think - it's what I have anyway) is advanced adaptive cruise control with pre-crash alert. This is adaptive cruise control which also takes control of the brake pedal. Because of this, it will do a few things: a) it will maintain speed even on downhill stretches; b) it will speed up only when you straighten the car up (so as to enable you to maintain speed following a vehicle on twisting stretches); and c) it will function at below 40kmh *if there is a vehicle in front*. Magic.

An important note here. The system only recognises what is in front as a "vehicle" if it is moving forwards. Otherwise it regards what is in front as an "obstacle" and expects the driver to take action. So when you approach a vehicle which wasn't previously picked up stopped in front of you (e.g. at traffic lights), your car recognises it as an "obstacle" because it is not moving - same way it recognises a road sign or your garage wall - and you are expected to take action. If you don't, then the pre-crash system will eventually enforce an emergency stop. 

The system has to be like that or your car would stop all the time - every time it approached a hedge, tracked a car coming the other way on a bend, or anything like that.

Now then, I'll take a guess at what is happening in your case. 

Firstly, you can only operate the system *if you are travelling above 40kmh or you have a vehicle moving forward in front*. Either of those will do, but you do need one or the other. In practice, this means you'll need to negotiate things like large roundabouts yourself.

Secondly, the speed in the dash in front of you is *not* the speed limit. It is the speed you have set the adaptive cruise control to. You can set this whenever the system is working ("radar active"), i.e. whenever you are travelling above 40kmh or you have a vehicle in front, using the lever (up or down in increments of 5mph). In theory you can do this even when stopped behind another vehicle (in which case you will be told "Waiting" and will then see "Preceding vehicle movement" - when the latter happens, flick the lever up and the car will increase to the speed you have set or to the speed the vehicle in front is doing, whichever is lower.

Thirdly, you do need to stop the car when approaching stopped traffic, as it will not be picked up. Once you have stopped, you can flick your lever up ("resume") and the adaptive cruise will be operational to pick up the vehicle when it moves forward.

I hope that makes some sense - moderators may wish to consider putting this post in another general thread on adaptive cruise control, because it can be confusing.

 

[stormchaser]

Excellent write up Ian. Very helpful to me too.

 

 

Sent from my Iphone using Tapatalk

[Britprius]

With respect this does not cover the speed limit signage. I had an Is 300H as a loan car. This had the road sign recognition and worked very well. It picked up speed signs, and displayed them until a different speed sign was passed.

On a stretch of road from the village I live in to Hereford "a distance of 18 miles" the speed limit changes from between 30/40/50/60 in a variety of ways so that after a while even the best drivers get confused as to what the actual speed limit is. This system proved invaluable on this stretch of road.

John 

[rayaans]

Car will get confused with certain signs and may display the wrong ones.

The adaptive cruise control doesn't work under 30mph in the IS

The LS and NX, as well as the new RX come with full range cruise control as they are more expensive. The IS misses out on the full range cruise control

[Ian J. Parsley]

I would have to defer to Rayaan on the IS potentially lacking "full range adaptive cruise control" (I've never heard that term but I like it). 

If that is so, your car will behave more like "secondly" rather than "thirdly" above and the section on "waiting" is irrelevant to you, although you may still have the braking (i.e. your car may maintain set speed on downhill stretches - check that). 

But note that even "full range adaptive cruise control" cannot be set below 29mph and thus in effect does not work below that speed if there is no vehicle identified in front. At least, mine can't!

I still wonder about what is displayed on your dash; if it is via sign recognition, this is far from a perfected science.

[paulrnx]

I worry about threads like this. How long before someone jumps in an unfamilar car and 3 miles later wallops the car in front and claims the cruise control was at fault. Seen a few on this forum saying they use the cruise control in an NX in town which imo is taking a bit of a chance and relying on a technology that was never designed for this. Might work every time but might not and a risk not worth taking