While going downhill at approximately 80-90kph, when I release the throttle pedal I noticed my car would automatically downshift from 4th to 3rd. When I step on the throttle again it goes back into 4th. It doesnt happen at lower speeds.

At first I thought it might be an anomaly but a quick search online suggests that this is a feature of modern auto'boxes.

I find this quite interesting and thought I'd share


*My car is an Axia

5 year ago I do have waja campro auto, downhill auto downshift from 4th to 3rd end up the fuel consumption really sucks, because from 4th downshift to 3rd RPM from 2k to 3.5k really really bad fuel consumption lucky that vehicle no longer drive

Until today I not even know is that normal or part of the design

This is a good practice. For manual car also, going downhill need to use lower gear to slow down car. Continuous on brake will heat up brake fluid causing brake failure. THis is what happened to lorries and buses accident claiming brake failure. I guess the driver has no knowledge. Probably not mentioned in driving school

this feature is what they call HDC (Hill descent control).. similar emulated effect of downshifting for engine brake

Looks like you also have no knowledge. Auto car is by design and manual drivers probably just using brake.

Heavy vehicle very fast to get brake hot unlike car unless the hill is steep. If you watch F1, they are very cautious if brake too hot. Cannot stop fast. If too cold also not optimum.

HDC is not really engine braking, but use ABS braking system. This is to control at constant very slow descent normally on rough terrain

Its a safety feature done by ECU ,no worries it is normal in auto gearbox.

Okay so is part of the design but the waja effects too obviously really bad fuel consumption, then my driving style always normal hard to reach 100km/h and at normal 80km/h road not doing any close distance not much brake apply, sometimes downhill no vehicle ahead of me I try throttle maintain the speed or throttle off auto downshift begin dang , except if I throttle more during downhill but is not safe at all

End up sell it 8 year old only 60k km been drive XD

The ECU algorithm done by Mitsubishi, are you saying mitsubishi did a bad job? When you say bad, can you give the number of how bad was the FC. Anyway you have sold your car.

The waja last time I drive not Mitsubishi engine, under campro then taxi driver told me Mitsubishi engine version have no related issue that what i heard so in reality I'm not sure since I don't own the waja under Mitsubishi engine, fuel consumption ah erm every month I have to pay extra 150 for waja compare to my old altis , and the old altis have much more smoother experience on road compare to waja

The only complaint for the waja just the fuel consumption and the downshift issue the rest I'm happy with it ,

Some autoboxes have it, some don't. Those that don't, you need to manually shift to 3/S/L gear to engage it.

Waja engages it automatically on downhills, Persona 4AT doesn't, Vios doesn't. All Proton CVT does it.

For most part it is a safety feature, like when you go down from genting the road signs tell you to engage low gear - it's the same thing.

Rolling down hill with high RPM doesn't consume more fuel than idling. High RPM caused by engine braking is not the same as high RPM caused by throttle.

You Waja fuel consumption is caused by simply because it is Campro.

I used to own a Mitsu Waja and it does the same thing, downshifting when going downhill.

This post has been edited by dares: Oct 23 2021, 10:32 AM

This is correct but I wonder how people say higher consumption.

The problem is staying at D causing the car to go downhill too fast but shifting to 3 makes the car go slower than necessary.

So using lower gear then need to press gas pedal causing high rpm maybe the cause but not much power needed by engine still. I usually stay on D and apply brake once in a while though you still need to judge whther 3 or D gear is the best depending on slope.

But all the autos I have driven dun auto downshift in idle downhill unless told to (i.e. D2,D3) - unkers own ride is manual sendiri can apply on demand la... The ECU know its going downhill meh? Or measuring speed vs no throttle? Mebbe is new feature kot. Unke rprob diving 20yrs old auto haha

This post has been edited by Dezs: Oct 24 2021, 06:45 PM

becos increase rolling resistance. during slope bmw eco mode straight changr into neutral kasi free gear je. it is proven low rpm save fuel.

not a new tech. just not being implement widely

Are you sure change to neutral? I don't think so. Even pro drivers said DO NOT cruise down hill in neutral. This is dangerous practice. There is reason to it.

They said Axia but like you all my auto car do not downshift. My Axia or MyVi also .... maybe I just did not go down Genting or Cameron Highland to test

It is easy for the car to detect. It can sense the engine is dragging the car or being push by the car.

First of all, understand what is engine braking.
Once you understand it, see this youtube link from Engineering Explained.

https://www.youtube.com/watch?v=_bZlb62VVlw

There is 1 exception unless your car is older type which use carburetor, then it will consume higher fuel as it is not control by ECU.
But modern rides, usually will cut off the fuel when doing engine braking.

This post has been edited by littlefire: Oct 25 2021, 09:16 AM

speed above 120kph. rpm below 1000. which gear from zf8 is design for that u tell me.
bmw get stars for top safety (dangerous??) and fuel saving (ppl say??). there is reasong to it

inject "less" fuel i believe. cut off i dont think so lah.
u meaning down hill the exzos no smell (no fuel) at all?

Per video from Engineering Explained, the engine cuts the fuel injection when the car moves down a hill; at various times, the engine will use no fuel at all to power the engine, since the wheels themselves—connected to a transmission—make the engine spin.