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Adaptive Cruise Control brakes when I don't want it to.

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Old 06-20-2014, 07:55 AM   #1
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Default Adaptive Cruise Control brakes when I don't want it to.

I find I can't use my cruise control coming down a mountain pass, because I want to use engine braking. If I have ACC on then it applies brakes whenever the grade is more than about 2% down, it's like riding my brakes down the hill. So going down hill, I must disengage the ACC and use the paddle shifters to limit my speed. Too bad, with all the sensors on-board, down-shifting is not adaptive when the car is tilted down, throttle has been released and speed is going up.
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Old 06-20-2014, 08:03 AM   #2
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I think you can still use the paddle with the ACC engaged. You can also use the D -> S to drop down a gear or 2 with ACC on.
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Old 06-20-2014, 11:09 AM   #3
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Actually that's one of the things that impressed me the most with my CVT with regular cruise control, how it downshifts smoothly and seamlessly when going downhill to use just the right amount of engine braking to maintain the set speed. Even on a 6% grade, I don't think it uses the brakes yet.

When not using the cruise control, it also downshifts when you initially press the brake pedal, but then you can release the brakes and it will continue to hold the lower gear for engine braking until you get back on the gas or the road levels off. So there is clearly a feature in the car where it is detecting the downhill slope and using engine braking automatically. I believe it should also be present in the 8-speed auto and with Adaptive Cruise Control.
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Old 06-20-2014, 11:16 AM   #4
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Originally Posted by dicktahoe View Post
I Too bad, with all the sensors on-board, down-shifting is not adaptive when the car is tilted down, throttle has been released and speed is going up.
I agree. It should be smarter but it isn't. In this sense the ACC is no different than the dumb CC that proceeded it. Both will hold your speed at the set parameter going down but it uses the brakes to do it. I live on a mountain and use lots of engine braking in my daily dive. I also like to use the paddles to regulate speed on the highway or in other daily driving. It bugs me when I'm on the highway and the ACC taps the brakes to make some room between me and the car I'm following when a simple downshift could have done the same.

Also, I've noticed that the new ZF tranny will not allow a downshift from M2 to M1 until the rpms are at or below ~3,250. This is quite a bit lower than on the old 6 spd where that shift could be made at around 4,500 rpms. This has caused me to have to brake a little more often in the new car to reach that rev for downshifting to 1st.

Last edited by Silent Drone; 06-20-2014 at 11:17 AM. Reason: Typo
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Old 06-20-2014, 12:36 PM   #5
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I think you can still use the paddle with the ACC engaged. You can also use the D -> S to drop down a gear or 2 with ACC on.
If the OP is correct that ACC applies the brakes in the situation described, downshifting may help, but you still may have the brakes dragging and speeding up wear. Why not just rip down the incline as fast as the happy car (and clearly the Good Lord) want it to? he, he
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Old 06-20-2014, 02:10 PM   #6
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If Why not just rip down the incline as fast as the happy car (and clearly the Good Lord) want it to? he, he
This has been know to occur. Not that I know anything about such behavior personally.
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Old 06-20-2014, 02:11 PM   #7
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Why not just rip down the incline as fast as the happy car (and clearly the Good Lord) want it to?
Click the image to open in full size.

Maybe teh 5-0?
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Old 06-20-2014, 05:29 PM   #8
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Oops, oops, Officer, Sir! You're not gonna believe this, but I was stomping on them brakes but the darn ACC kept making her go faster. Sorry, sir!
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Old 06-20-2014, 05:41 PM   #9
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In all seriousness - I actually want an instrument panel indication when the ACC is braking vs just slowing down. I'd like to know for sure when the electronic driver is making the human driver look like a brake-flashing maroon...
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Old 06-20-2014, 07:15 PM   #10
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Originally Posted by irenesbob View Post
If the OP is correct that ACC applies the brakes in the situation described, downshifting may help, but you still may have the brakes dragging and speeding up wear. Why not just rip down the incline as fast as the happy car (and clearly the Good Lord) want it to? he, he
The Audi has very low internal friction for the engine as well as the AWD system - experience from their endurance racing. As a result, you may need to drop 4 gears from D8 to M4 to maintain a constant speed going down hill using engine braking alone. Brake pads are much cheaper to replace than any engine parts.

One thing I don't like about the ACC - it brakes way too early when there is a slow car in front while accelerating up hill - the car behind you think you are an idiot
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Old 06-20-2014, 08:45 PM   #11
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do the brake lights come on if the ACC is slowing the car if someone in front is moving slower?
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Old 06-20-2014, 09:57 PM   #12
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do the brake lights come on if the ACC is slowing the car if someone in front is moving slower?
Yes, any time the ACC applies the brakes, your brake lights come on.

Note that the A6 does have hill descent control, but it only activates it on more severe grades. For a 2% grade, the car probably thinks it's fine to ride the brakes instead of increasing engine noise. As previously mentioned, you need to get up to 3000+RPM before you get any appreciable engine braking.

Once you have a steeper grade, you will notice that pressing the brakes for more than a second will trigger upshifts, so a combination of engine braking and brake usage slows you down.
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Old 06-21-2014, 08:16 AM   #13
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Originally Posted by Richard Solomon View Post
In all seriousness - I actually want an instrument panel indication when the ACC is braking vs just slowing down. I'd like to know for sure when the electronic driver is making the human driver look like a brake-flashing maroon...
Interesting there is a way to determine when the brakes are being applied. You watch the MPG display. As long as this display had not gotten to display at the green battery symbol, you're OK. After that as you feel the car slowing--the brakes are being applied.
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Old 06-21-2014, 08:24 AM   #14
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"The Audi has very low internal friction for the engine as well as the AWD system - experience from their endurance racing. As a result, you may need to drop 4 gears from D8 to M4 to maintain a constant speed going down hill using engine braking alone. Brake pads are much cheaper to replace than any engine parts."

Dropping 4-5 gears is necessary--coming down a 6% grade I need to go down to M3 to keep my speed near the 50mph speed limit. I don't know that there will be a need "to replace engine parts" because of using "engine braking". However, I have come up next to a car where brakes have been used all the way down this 6% grade and I can smell the severely overheated brakes.
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Old 06-21-2014, 09:21 AM   #15
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As previously mentioned, you need to get up to 3000+RPM before you get any appreciable engine braking.
My daily drive includes a decent and return climb that is ~1,000 vertical feet at grades of 15-17% through residential with posted limit of 30 mph. It snows here in Utah so Quattro is my friend. I know these grades well because I also bike them from time to time, like I did this morning. See this http://threekings.nslcity.org/king3.html

Anyway, when in M2 on the decent the car will hold at around 38 mph, just a little too fast IMO and certainly too fast if there are slower cars in front. When there are slower cars in front I drop it into M1 and actually apply a little gas to keep the car right on 30 mph at around 5500 rpms. I'd rather use a little gas with engine resistance than ride the brakes all the way down like most people do. By the way, when I am in M2 and just coasting with the engine doing the braking (ie, around 4200-4500 rpms) the mpg display shows that the car is not consuming any fuel. In other words, even though the engine is revving the car is smart enough to know I am not applying the accelerator so it's not burning gas.

I use the tranny/engine to scrub speed in most circumstance not just on hills.
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Old 06-21-2014, 04:28 PM   #16
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Originally Posted by dicktahoe View Post
"Dropping 4-5 gears is necessary--coming down a 6% grade I need to go down to M3 to keep my speed near the 50mph speed limit. I don't know that there will be a need "to replace engine parts" because of using "engine braking". However, I have come up next to a car where brakes have been used all the way down this 6% grade and I can smell the severely overheated brakes.
Coming from manual transmission, I've always use the engine braking/gear ratio to slow down the car, however, the paddle control on the A6 doesn't give you the same control because of the low internal friction and the close ratio of the 8 speed gear box. The ECU also try to match your speed instead of slowing down when you downshift by increasing the throttle and rpm when going downhill.

If you brake while going down hill - I don't mean riding the brake, the transmission will shift from D8 --> D2 based on your speed, if you shift to M2 or M3, it will hold the speed as long as the rpm is above 3k or so.
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Old 06-22-2014, 08:40 AM   #17
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Originally Posted by dicktahoe View Post
Interesting there is a way to determine when the brakes are being applied. You watch the MPG display. As long as this display had not gotten to display at the green battery symbol, you're OK. After that as you feel the car slowing--the brakes are being applied.
This is not always true. When the throttle is closed, of course the MPG will increase. But the A6 also has a feature where the alternator is stepped up to increase the charging rate of the battery during times when the throttle is closed, to pick up energy that is usually lost. Sometimes you can feel the extra drag from the alternator, but it is not necessarily caused by the car braking.

On the other hand when you are braking, the throttle is also closed, so the alternator will also be stepped up and the MPG will go to green battery icon. So you should just be aware about this feature, and that the green battery can also come on with throttle closed, engine braking, but no wheel braking.
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Old 06-22-2014, 08:50 AM   #18
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To put it simply, not necessarily related to braking, the energy bar indicator will usually go green when coasting.
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Old 06-22-2014, 09:10 AM   #19
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To put it simply, not necessarily related to braking, the energy bar indicator will usually go green when coasting.
Yes, and IIRC, TFM says that in the V8T (S6) the bar being green indicates V4 operating mode.
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Old 06-23-2014, 05:24 AM   #20
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I've also found that tapping the transmission into Sport mode just before a hill will tend to automatically downshift and hold that gear for the descent. Don't know if this would affect the ACC setting.
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Old 06-23-2014, 05:24 AM
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