Cruise control failure, nearly caused a wreck

Are others having trouble with the automatic cruise control dropping out?

When on the freeway I try to keep the car driving no faster than 60 MPH to save battery. I tend to set the automatic cruise control to maintain a fixed speed. Driving at 60 MPH on the freeway, cars whiz by on either side.
Several times while driving the car while in ACC, the unit has dropped out. I have seen posts that others have seen similar issues. At first I thought this might be caused by cars pulling in front of us, or the system sensing a wall while going around a curve, at speed. It happens with no cars within a quarter of a mile, seemingly for no reason.
With regenerative braking of this car, effects can be very drastic, losing 5 or even 10 MPH in just a matter of seconds.
Yesterday I was cruising along and noticed a large truck coming up behind me closing at a quick pace. Just as the driver turned on his turn signal to pass the ACC decided to drop out, and my car dropped speed. Luckily the driver was alert and was able to cram on his brakes. All I could see in the rear view mirror was truck bumper and grill.
I have been tolerating this bug for some time, but feel its time to get it to the service center.

Have you had trouble with the automatic cruise control dropping out?

ACC employs a camera in the rear-view mirror assembly to detect traffic. When weather and/or lighting conditions interfere with the system’s ability to perform traffic recognition, the system triggers a fail-safe mode with an audible chime, messages on the displays, and brake lights to alert following vehicles. The behavior is documented in the manual in addition to having been discussed in other threads, as you point out.

Personally, I’ve experienced the shut off only when driving in fog or into the sun. If the conditions you drive in frequently throw the system into fail-safe mode or you suspect a fault, perhaps you should disable ACC and use the standard cruise control mode. Press and hold the Reduce Distance or Increase Distance steering wheel button. See "Changing between cruise control with/without distance control" in the manual for more complete directions.

stumbledotcom said:

ACC employs a camera in the rear-view mirror assembly to detect traffic. When weather and/or lighting conditions interfere with the system’s ability to perform traffic recognition, the system triggers a fail-safe mode with an audible chime, messages on the displays, and brake lights to alert following vehicles. The behavior is documented in the manual in addition to having been discussed in other threads, as you point out.

Personally, I’ve experienced the shut off only when driving in fog or into the sun. If the conditions you drive in frequently throw the system into fail-safe mode or you suspect a fault, perhaps you should disable ACC and use the standard cruise control mode. Press and hold the Reduce Distance or Increase Distance steering wheel button. See "Changing between cruise control with/without distance control" in the manual for more complete directions.

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I'll second this in that only times I have had consistent drop of ACC is when there is sun shining into the camera or if there is very heavy rain. I think the real issue is that when ACC cancels it should probably drift a little bit more and not just drop into full regen mode (start slowing down gracefully while warning driver to take over).

Agreed, it needs a "soft" cancel, the same thing happens when you tap the brake or shut the ACC off. The only time the system should have such an abrupt shut off is when it intervenes to prevent hitting something in front of you.
Maybe this is one of the reasons BMW neutered the Regen on the recent software update?

Driving on a tree-lined road during a sunny day can also cause the ACC to shut down as has happened to me more than once. The deep shadow verses bright lighting seem to be what causes it - I think a radical light intensity change causes it to shut off. In that same vein, a dark shadow caused by anything - a building, an overpass, etc. could trigger it depending on the sun position and intensity.

As to going into full regen when you turn the ACC off by either hitting the button, or pressing the brake (why use the brakes except in an emergency!?), I always ensure I've got my foot on the go pedal before I do that to avoid that max regen situation. I almost never turn it off by using the brakes, so I don't have to deal with two-footed driving.

Softening the regen, I think, was a reaction to too many complaints about not being able to drive the car smoothly. It took me all of maybe an hour to get into the hang of it, and I do not have an issue with it when changing in or out of my ICE, but then, that's me. It takes a little initial learning curve to utilize the one-footed driving possible with the i3...if you try to drive it like a conventional car, you WILL be jerking people around. Small price to pay for one-footed driving and the increased range possible.

Going into max regen when the ACC shuts off is a reaction to potential impending collision with something in front. I usually have my foot either on or at least hovering over the go pedal, and can prevent that max regen issue at will. If you relax and have your foot somewhere else, your reaction time will result in that sudden regen situation. The system is erring on the side of caution. It is the responsibility of those behind you to be maintaining a safe following distance. Yes, I know there are way too many that don't, and that may not be any consolation, but it is both the law and the practical thing everyone should follow.

Other cars have similar systems, like the Mercedes Distronic Plus. How do they handle a CC dropout?

jadnashuanh said:

Going into max regen when the ACC shuts off is a reaction to potential impending collision with something in front. I usually have my foot either on or at least hovering over the go pedal, and can prevent that max regen issue at will. If you relax and have your foot somewhere else, your reaction time will result in that sudden regen situation. The system is erring on the side of caution. It is the responsibility of those behind you to be maintaining a safe following distance. Yes, I know there are way too many that don't, and that may not be any consolation, but it is both the law and the practical thing everyone should follow.

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That makes sense, except the car also has the ultrasonic sensors which can do the same function. So they could soften the release and still maintain a degree of collision avoidance.

epirali said:

That makes sense, except the car also has the ultrasonic sensors which can do the same function. So they could soften the release and still maintain a degree of collision avoidance.

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The ultrasonic sensors are for parking maneuvers. They operate best at short range so most systems turn off at about 15mph. Other implementations of adaptive cruise control employ radar-based sensors. The i3 has only the camera system.

While I will grant that the full-regen effect is disconcerting. It's probably safer than a softer reaction if only because the sensation gets your attention that much faster.

epirali said:

That makes sense, except the car also has the ultrasonic sensors which can do the same function. So they could soften the release and still maintain a degree of collision avoidance.

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The camera can see much further away than the US sensors, and those are not used for this. Per the Owner's Manual, the max range on the US is about 2M, or a bit over 6'; way too close if there's an obstruction that at anything other than crawling speed.

jadnashuanh said:

epirali said:

That makes sense, except the car also has the ultrasonic sensors which can do the same function. So they could soften the release and still maintain a degree of collision avoidance.

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The camera can see much further away than the US sensors, and those are not used for this. Per the Owner's Manual, the max range on the US is about 2M, or a bit over 6'; way too close if there's an obstruction that at anything other than crawling speed.

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Good point. Still I prefer not to have a sudden halt just because something *might* have appeared in the way within a fraction of the second. Understand safety, but its also not very safe to suddenly slow down going 65 mph.

I am in favor of a "soft" disable for the ACC where regen doesn't kick in. We don't need regen to alert the driver as a tone and the display could be used instead.

The combined effects of sudden dropout AND full regen can be unexpected to the following driver, and though it is true that driver should be maintaining a safe following distance we all know very well that not everyone does this and I'd rather avoid chancing fate that way.

Still love ACC though. I keep my foot resting on the pedal.

spinball said:

Still love ACC though. I keep my foot resting on the pedal.

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+1. Even the best ACC can't predict everything, we still need to maintain control. It's not like the car is driverless.

I33t said:

Other cars have similar systems, like the Mercedes Distronic Plus. How do they handle a CC dropout?

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I know this is an old post but the ACC hasn't been updated obviously so my comment is still relevant.

I've driven about 50,000km in my 2014 M-B SUV with adaptive cruise control - Distronic - so I can compare the ACC on our new i3. The M-B has NEVER dropped out. It follows accurately through intersections, T-junctions and bends. It has NEVER missed seeing another vehicle. I routinely use it for almost all driving even in the suburbs because it's completely reliable and dead simple to use - one movement of one knob turns it on, sets and engages it. A rotating mechanical knob remembers my preferred distance setting - I set it once three years ago.

I assumed the ACC on the i3 we just bought would be easily as good as the four year older technology on the SUV but it's not. In a 400km round trip it dropped out 4 times coming over a hill because the poor thing got the sun in it's eyes heading west in the afternoon. It dropped out three times for no apparent reason. It cannot follow a car and maintain distance across an intersection or through a T-junction with any reliability. Returning in the evening, it completely failed to see a large SUV in our path - the car symbol failed to appear in the display - as we approached quite rapidly. It also failed to see a motor bike. The switching-on process is a PITA. One button to turn it on, another to set the speed and still another to engage it. Another knob needs to be set to the distance setting I want every time I turn it on.

Apart from that - by far the i3's worst feature - I love driving the car except for the rest of the software which is a teenage nerd's dream and an ergonomic disaster. Being used to software in cars built for adults has obviously spoiled me.

Libelle said:

I've driven about 50,000km in my 2014 M-B SUV with adaptive cruise control - Distronic - so I can compare the ACC on our new i3.

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Apparently Distronic Plus uses 3 radar transceivers whereas the i3's ACC uses only a single relatively low resolution camera. It is likely that the Distronic Plus hardware is considerably more expensive than the i3's camera and maybe more importantly, uses considerably more energy. Using less expensive, more energy-efficient hardware in exchange for worse performance under certain conditions might have been a compromise that BMW felt was worth making.

Since this thread started, BMW adjusted the timing and rate of regen once the ACC drops out. There is easily plenty of time to keep the car moving at speed after it drops out if you're paying attention. Prior to that s/w update, yes, the regen was stronger (someone said the programming parameter went from 27 to 23 or something like that - no idea how that relates to the actual values of regeneration) and, the timing is longer and the ramp-up is smoother rather than being fairly abrupt.

Given the use of a camera verses radar or lidar to ascertain traffic ahead, it does a quite decent job. Personally, on some of the winding, narrow, tree-lined roads here in NE, on a bright and sunny day, the dark shadows and bright spaces can cause it to drop out. Probably the worst culprit, though, is a low sun angle. I don't drive it all that much at night when ACC is useful, so can't comment on its operation under those conditions.