Automatic transmission auto-downshift, when coasting downhill
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 Oct 22 2021, 11:11 PM
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#1
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7,938 posts Joined: Mar 2014 
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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 littlefire and cablesguy liked this post
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Oct 22 2021, 11:17 PM
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#2
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QUOTE(zuozi @ Oct 22 2021, 11:10 PM) 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 Looks like you also have no knowledge. Auto car is by design and manual drivers probably just using brake.Until today I not even know is that normal or part of the design  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. |
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Oct 22 2021, 11:22 PM
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#3
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QUOTE(ceo684 @ Oct 22 2021, 11:15 PM) this feature is what they call HDC (Hill descent control).. similar emulated effect of downshifting for engine brake HDC is not really engine braking, but use ABS braking system. This is to control at constant very slow descent normally on rough terrain |
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Oct 24 2021, 05:49 PM
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#4
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QUOTE(dares @ Oct 23 2021, 10:32 AM) Some autoboxes have it, some don't. Those that don't, you need to manually shift to 3/S/L gear to engage it. This is correct but I wonder how people say higher consumption.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. 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. |
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Oct 25 2021, 08:00 AM
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#5
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QUOTE(ktek @ Oct 24 2021, 09:18 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. 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. not a new tech. just not being implement widely  |
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Oct 25 2021, 08:04 AM
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#6
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QUOTE(Dezs @ Oct 24 2021, 06:44 PM) 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 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.  |
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Oct 25 2021, 10:04 AM
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#7
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QUOTE(ktek @ Oct 25 2021, 09:38 AM) speed above 120kph. rpm below 1000. which gear from zf8 is design for that u tell me. How do you know if the car is not put in N? bmw get stars for top safety (dangerous??) and fuel saving (ppl say??). there is reasong to it Below 1k rpm is engine on idle which normally when car in P or N position.Safety rating is just rating. It is still not safe if you drive dangerously. Do you know that coasting (or going downhill in free gear N) is even illegal is some states in the US? |
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