yardie876

Here's a nice video that teaches you everything there is to know about cruise control on the 4RX.

Jiggz858

DRCC (Dynamic Radar Cruise Control) is really wonderful. However, the use of engine braking to quickly decelerate is quite annoying not to mention the rough down shifting of the transmission. If you have not experience this you can by doing the following: While in DRCC, quickly decrease set speed using the cruise control stalk down 10 MPH, i.e. if you are doing 70 MPH slow down to 60 MPH. Watch how the system reacts. In my case you will suddenly hear the transmission downshifts roughly, feeling a thud, and then the engine revving up to 4K RPM.

It seems this type of reaction by the engine is designed for the Hybrid where it uses engine braking to regenerate power. However, mine is not an hybrid but a regular RX350. I really would like Lexus to address this as it's very concerning with the rough downshifting and high revving of the engine.

There is also another instance where you can experience this engine braking for deceleration. While on DRCC, step on the accelerator to speed up at least an additional 10 MPH from the set speed. As soon as you reach the desired speed, let go of the accelerator and you will experience the same engine braking as stated above. And if the road is down slope, it's even worse.

GSFRX350

Yes mine does exactly the same thing, was only talking about it to my wife yesterday, the downshifts are not Lexus like refined at all - however it does not do it ALL the time, I do a lot of freeway driving with the cruise and many, many downshifts are buttery smooth (don't know it's happened unless you're watching the tacho). Trying to work out what's happening, I'm thinking it may have something to do with when the car is using the brakes to slow as well, I've noticed that the rough downshifts happen on steeper hills when the brakes may be applied - maybe the braking/downshifts together cause some "confusion" in the electronics

Jiggz858

I believe there are two reasons for this nuisance. One, like I mentioned has something to do with power regeneration and the other might have something to do using engine braking to quickly come down to the "new" set reduced speed. On my other Toyota, when I reduce speed, a conventional cruise control (Non-dynamic), it takes a little while to come down to the "newly" set reduced speed. And this is because the system doesn't do anything except to reduce engine rpm or power and letting the vehicle coast down to the new set reduced speed. In fact, the system will not even downshift the transmission but simply reduce engine rpm until the it meets the set speed and then pick it up from there. How long does this take? Well, it all depends how slow is the newly set reduced speed is from the previous setting. But it sure is a little longer than what the RX350 does, which is almost instantaneous by using engine braking and downshifting of the transmission, albeit sounding and feeling rough. Personally, I prefer that with DRCC, it just coast the engine down without downshifting the transmission for reducing set DRCC speed despite taking this a second or three longer than it takes.

Kansas

When you don't want DRCC to apply the brakes: 1) pull the cruise control lever towards you to cancel. 2) coast down to approximately the speed you want. 3) set a new speed by pulling the lever downward. 4) adjust the new set speed with up or down movements of the lever if necessary..

Sometimes I want DRCC to do the braking. Sometimes I don't. I use whichever technique is appropriate to the situation. I've been using DRCC for so many years that I almost always nail the new set speed I want the first time using the Cancel/Coast/Set method.

Nail34

Let's say you have the cruise control set at 70 mph and you reach a downhill portion of the freeway, when the car reaches about 73/74 mph it will downshift into 5th gear and the rpms will jump to almost 4000, very annoying. Before that happens and you want to keep the same set speed you can gently use the brakes until you get close to the set speed and resume (push up) on cc lever. Or you can anticipate the speed increase, discontinue cc (pulling) and continue when you again reach about 70 mph by breaking or slowing down because the road is leveling.
I would prefer the cc did nothing until 70 mph was reached again and give the driver the option to slow down by braking or smoothly down shifting.

Jiggz858

I just want to make sure we are focused on the issue on this thread, ". . . rough downshifting and high engine revving" Any car owner who had experience with dynamic or conventional cruise control (CC) fully understands Set/Coast/Cancel setting of the CC and fully understands that it will auto shift the transmission up or down as appropriate. However, the rough shifting and high engine revving is something not normal. Lexus probably thinks it is but it is not, hence this thread.

For me, to avoid this nuisance I just tap the brake to disengage CC if I want to slow down. But such action is just not practical if you want to pass a vehicle, where if you are on CC you should just step on the accelerator to gain speed, do the pass and should just let go of the accelerator and the CC will resume set speed. This is how most CC operates. But not with this 4RX. What it does as soon as you complete the pass and let go of the accelerator, it downshifts roughly and engine revs up for a few seconds and then resume set speed.

I know. I know. You're gonna tell me just to disengage the CC before making the pass, complete the pass and then resume speed and set the CC again. But that is not how normal CC works.

JSracer

I don't recall my 450 acting like you mentioned, with high rev but I will make an effort to play with it more, watching the tac in sport mode.
i do know on a hill road, the cc overshot several times creating hill in the demo I had before I got mine.
The only thing I had was someone shoot across my bow, changing several lanes right in front of me. Got beeps and warning. Don't recall much else. Not the same, but in the few hundred miles I have had the car, that was the first thing that was interesting.

The was described would be annoying as heck, for sure. Often times I set the cc, and still use the gas pedal. It sort of is my minimum speed I want to go. Many times I hit the gas to pass, in my vehicles and motorcycle with the cc set. The vehicle responds as expected every time.
Rough downshifting and high revs coasting to set speed doesn't sound good at all.

Kansas

There are "Situations unsuitable for cruise control" sections in the Lexus RX owners manual.

If you are making a pass on a two lane / 2-way road, yes, of course, disengage the cruise control before passing. It matters less or not at all if you are driving on a 4-lane road where you don't need a burst of speed to pass.

Jiggz858

I'm not really sure what it takes to get my point across about the rough downshifting and high engine revving on the 4RX cruise control. I guess next time my headlights don't work I'll just have to bring with me a flashlight and forget about fixing the non-working headlights! Or in this case just use my right foot as the cruise control instead and pretend the built in DRCC is working perfectly.

I'm not sure why I'm being constantly told just "not to use the cruise control". I've been using CC for decades since the early 90's and this 4RX just doesn't feel normal at all, as discussed above.

If you are not having this abnormality on the CC, then maybe this thread is not for you and please don't tell me again just not to use it because that is not a fix.

JSracer

why would you have to disengage it to pass a car ?? squeeze some gas, pass, let the car coast back down.

Jiggz858, i get ya, for sure. i would not be happy with it like that.

JSracer

My 450, say going 10 over set speed, it revs to over 3k so I could imagine shifting on the 350. This is deceleration back to CC set speed. Never really feel it guessing because of the cvt.
Hope this helps

I did not test with regular CC but when I did the 10 over test, there was no vehicle in the radar. When there is a vehicle in the radar, it pops up on the HUD, super easy to see.

coolsaber

If the control logic is flawed, and DRCC is causing unusually harsh downshifts (which at certain speeds would result in RPMs moving up) then I`d suggest going into the dealership. Nothing really can be done if this is this is how Lexus intended its control logic to function, besides making them aware of the issue and hoping they issue a TSB. As a side note, anyone know what systems DRCC should control during a downhill event where speed would increase above set speed, constant distance vehicle in front?
I`d assume it`d be a mixture of engine downshifting with brake application?

GSFRX350

Yes it uses a mixture of downshifting and braking.

Kansas

Because if you are driving in heavy traffic on a 2-lane road, even the shortest DRCC following distance doesn't put you close enough to the vehicle ahead so that you can pass it safety as soon as there is an opening.