Downhill at "Set" speed not working

It seems to me that during a ginormous road trip a couple of years ago, many times I had to either brake or use the left paddle to increase regen with the ACC on in order to maintain proper speed going downhill. I know, in theory, ACC on cars should maintain downhill speed by easing off the throttle and/or applying braking, but I don't think our 2018 Clarity touring does that.

 

Mine (2018) definitely does it just fine. Set to 40 or 30 and it'll accelerate to get to the top of a rather steep train track overpass and brake on the way down the other side. Same everywhere else I drive over hills with ACC on. Granted they're shallow hills, and there's a light at the bottom of said overpass, so I have to brake anyway. Maybe the braking it applies has a limit and the hill was too steep?

 

I have 2020 and she definitely accelerates downhill way above set speed. It is indeed in regen mode, but not enough to slow down.

 

You may have already done this, but one thing to double check is that you're actually in adaptive cruise control mode, not old-style cruise control mode. Does "Cruise Mode" appear in a box between the ACC and LKAS indications? If so, you don't want that mode -- hold the distance select button until it beeps and "Cruise Mode" disappears.

"Cruise Mode" showing means legacy non-adaptive cruise control, which won't apply brakes either if going too fast or if too close to another car. My car was delivered in that mode, and it took a few, um, interesting tests in traffic where I thought ACC was broken before I found that in the manual. It's a VERY confusing indicator and I'm not sure what Honda was thinking -- "Cruise Mode" certainly doesn't make it clear, and "ACC" still appears in that mode even though it's in no way...adaptive.

 

Interesting. I live in a flat area and haven't had a chance to try it, but this weekend I might have a chance to take a spin up into the foothills and will try to find a good downhill to try in my 2019.

The manuals for all 3 years say the following: On page 403: "Do not use ACC with Low Speed Follow under the following conditions: ... • On roads with steep downhill sections, as the set vehicle speed can be exceeded by coasting. In such cases, ACC with Low Speed Follow will not apply the brakes to maintain the set speed.". But then on page 415: "In the following cases, a buzzer sounds, ACC with Low Speed Follow is deactivated, and the message Cruise Cancelled: Slope Too Steep. Watch Downhill Speed appears in the driver information interface. • While descending a long slope and the vehicle has repeatedly applied the brakes to maintain the set speed.", implying that it might try to manage the speed briefly before giving up.

If they did actually change something, it might have been intentional for safety. Having a car silently applying the brakes continuously for miles without the driver knowing seems dangerous. Having the car accelerate downhill is a problem, but it's obvious to the driver who can take other actions like downshifting an ICE, paddles in our cars, or consciously braking. Having the brakes fade and become unavailable because the car was riding them without your knowledge is a much bigger problem. Yes, they *could* use regen only (at least until battery full, then it would have to cut out), just like they could force an ICE car to downshift the transmission (well, an automatic at least), but the complexity and testing required to do that safely could be too high.

But I will be interested to see what my car does, and to hear what the dealer says about yours!

 

My 2018 has the brake lights come on when I am going down hills in ACC: It really seems to be using the brakes. I was disappointed that regen did not seem to be enough to control speed on hills.

 

Brake lights don't necessarily mean it's using pads instead of regen -- when you step on the brake pedal lightly the lights come on even if you are mostly regenerating. I would bet that ACC feeds "brake this much" commands into the braking system around the same point that pedal input does -- because it's probably the same ACC system and used on other Hondas.

The second reference in the manual implies that it will do *some* braking to hold speed down on a short downhill, but will give up and shut off (supposedly with a warning on the screen) on a long downhill to avoid brake fade, which makes sense if that part of the system doesn't get involved in the regen/pad balance. Better to let the driver manually downshift or set regen paddles which don't use the brake pads. Note that regen alone might not always be available or sufficient for the hill depending on speed and battery charge.

 

2020, Canadian base edition: ACC fails to correctly control speed downhill


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The traction motor is theoretically capable of providing the same amount of regenerative braking force as accelerative force. The inability to hold a speed on a downhill grade is a software/programming issue, not a mechanical issue. The car should be able to hold a speed and even decelerate while going downhill without ever utilizing the friction brakes.

The reports of the ICE starting, to dump regenerative energy, while braking on a descent provides evidence that the traction motor is doing the braking, not the friction brakes, even though the brake pedal is being pressed down.

 

2018 my car will use regain but not the brakes going down a steep grade. I drive over the continental divide several times a year. I have to apply the brakes if no one is ahead of me. It brakes as much as the regain is set at But it will go faster than the set speed on ACC.

 

On downhills to get the best MPG avoid braking and even regen to take full advantage of the help gravity is providing. I end up turning off ACC during downhills for this reason because it prevents going above the set speed by using regen and/or brakes.

 

I wonder if this is something they changed in the model year: What is your build date?