Brake Assisted Traction Control Functionality ASC+T

[nickjd]

Hey everyone, after a couple hours searching for an answer I thought I'd just ask. It's about the ASC+T feature.

For reference, I have a euro spec '99 530d touring (aka diesel man wagon ;) ). I'm from Ohio but I've been studying and now working in Europe (just got a job near the arctic circle in Sweden).

Well, I tested out the "brake assist slip control feature" (for lack of a more accurate name). I put one wheel on a sheet of ice (from melting snow re-freezing in a big clear solid ice puddle) and the other on pavement. The car was on a slope such that a normal car with no LSD would have a lot of trouble.

I put it in drive and without touching the gas pedal or brake it would actually slide backwards because one wheel would have essentially zero traction (this was the goal for the test).

You could actually hear a pump clicking once the wheel started slipping (similar to the ABS pump). The car does not have DSC.

Upon touching the accelerator, the car would literally pulse forward slowly uphill. You could feel the pulsing and hear the clicking in the car from the brake and pump activating.

I noticed that upon pushing the ASC button on and off and trying the same sloped one wheel on ice test several times, it seemed to be disabling the traction control in some conditions even with the ASC on. It's like it didn't always use traction control after testing it several times.

If I'm stuck in snow or whatever, this feature would clearly help. It easily pushed the car up a hill (albeit slow) when one wheel had essentially no traction. The car was incapable of going up hill with ASC deactivated. It would just spin the tire without even touching the gas pedal, not "click" and not go uphill. It seemed that after some combination of changing gears (meaning sport manual, D, R) would disable the brake assist traction control.

The problem is, I was able to show that it did not always use the traction control when I needed it. Since the feature is more or less running in the background, I mostly have no way of knowing if or when it's working. If I was stuck, I want to know if I have it working or not. I got stuck once and I'm certain the feature was not active. It's almost like it won't work if the car was already stuck and you were trying to get it unstuck (as I simulated by going uphill on the ice and putting it in R and then D again). It seemed to have deactivated it. I'm not sure if I am drawing the correct conclusion based on my observations. Maybe there is another factor I am not seeing.

Does anyone know about what conditions will activate this system? As far as I know, it doesn't have any problems or faults in the system. It will go uphill in deep snow faster with ASC on than it can with it off which is a pretty good compromise for not having a LSD.

Any ideas?

[jlb85]

Do you have two seperate buttons? On most "single button" systems the first disable loosens and/or disables traction control (throttle-managed wheel spinning), keeping asc (yaw control) active. A second press or hold would disable asc and traction. Typical systems need the vehicle to be at zero speed and hold down the asc button for 8-10 seconds for it to go into full off mode.


Hey everyone, after a couple hours searching for an answer I thought I'd just ask. It's about the ASC+T feature.

For reference, I have a euro spec '99 530d touring (aka diesel man wagon ;) ). I'm from Ohio but I've been studying and now working in Europe (just got a job near the arctic circle in Sweden).

Well, I tested out the "brake assist slip control feature" (for lack of a more accurate name). I put one wheel on a sheet of ice (from melting snow re-freezing in a big clear solid ice puddle) and the other on pavement. The car was on a slope such that a normal car with no LSD would have a lot of trouble.

I put it in drive and without touching the gas pedal or brake it would actually slide backwards because one wheel would have essentially zero traction (this was the goal for the test).

You could actually hear a pump clicking once the wheel started slipping (similar to the ABS pump). The car does not have DSC.

The ABS valves and motor are acting to keep the car from rolling backwards.



Upon touching the accelerator, the car would literally pulse forward slowly uphill. You could feel the pulsing and hear the clicking in the car from the brake and pump activating.

No limited diff meams it is using brakes to limit wheel slip. ABS pump and valves acting to slow or stop the spinning wheel and transfer the torque to the wheel with grip. System is monitoring wheel speed delta and adjusting to keep car moving and somewheat straight. Do this too much and you will burn up the rear pads.




I noticed that upon pushing the ASC button on and off and trying the same sloped one wheel on ice test several times, it seemed to be disabling the traction control in some conditions even with the ASC on. It's like it didn't always use traction control after testing it several times.

If I'm stuck in snow or whatever, this feature would clearly help. It easily pushed the car up a hill (albeit slow) when one wheel had essentially no traction. The car was incapable of going up hill with ASC deactivated. It would just spin the tire without even touching the gas pedal, not "click" and not go uphill. It seemed that after some combination of changing gears (meaning sport manual, D, R) would disable the brake assist traction control.

The problem is, I was able to show that it did not always use the traction control when I needed it. Since the feature is more or less running in the background, I mostly have no way of knowing if or when it's working. If I was stuck, I want to know if I have it working or not. I got stuck once and I'm certain the feature was not active. It's almost like it won't work if the car was already stuck and you were trying to get it unstuck (as I simulated by going uphill on the ice and putting it in R and then D again). It seemed to have deactivated it. I'm not sure if I am drawing the correct conclusion based on my observations. Maybe there is another factor I am not seeing.


Some systems can tell if the car is stuck and will relax the traction control thresholds, and will also disable based on a temperature model to prevent brakes from burning.





Does anyone know about what conditions will activate this system? As far as I know, it doesn't have any problems or faults in the system. It will go uphill in deep snow faster with ASC on than it can with it off which is a pretty good compromise for not having a LSD.

Any ideas?

[nickjd]

Thanks for the reply. It only has one button (ASC). Holding it down does not do anything like it would if it had DSC. It does not have yaw control because it doesn't have DSC. I wasn't wondering about how it works, but how it decides to activate it.

I think the car is too old to have temperature sensors in the brakes. I've done a lot of work on the car and I know it doesn't have them. Do you have any ideas how it decides when to use the brakes to control slip?

My thought was that it SHOULD be on all the time as long as the ASC is turned on.

[thejlevie]

The system activates only when it sees that one driven wheel is turning faster than the other driven wheel.

[spoonerDee]

Thanks for the reply. It only has one button (ASC). Holding it down does not do anything like it would if it had DSC. It does not have yaw control because it doesn't have DSC. I wasn't wondering about how it works, but how it decides to activate it.

I think the car is too old to have temperature sensors in the brakes. I've done a lot of work on the car and I know it doesn't have them. Do you have any ideas how it decides when to use the brakes to control slip?

My thought was that it SHOULD be on all the time as long as the ASC is turned on.

The temp sensor isn't in the brakes. It's the outside temp sensor that is located in the front bumper and has been on most every BMW since the 90's if not earlier.

[nickjd]

The temp sensor isn't in the brakes. It's the outside temp sensor that is located in the front bumper and has been on most every BMW since the 90's if not earlier.

Yeah, it's on the bottom plastic cover on the right side roughly in front of the front right wheel. But what would the outside temperature have to do with the traction control? Maybe it could change some parameters but I highly doubt it would deactivate the system based on temperature. My test was at the same temp and time so temperature shouldn't have been a factor.

[thejlevie]

I don't believe ambient temperature is an input to the ASC system.

[jlb85]

The temperature of the brakes is an internal model to the abs brain, has nothing to do with ambient (or very little), and uses no temp sensors. It computes time to disable based on wheel speed, speed delta (difference) and brake line pressure (if the abs module has a pressure sensor, some don't) on a conservative "estimate" assuming stock tire and brake pad specs. During abs development engineers will be taking thermocouple data and this info is used to build a reference model they apply to the final calibration in the abs ecu.

If the button says ASC then it is only traction and should be on or off.

[nickjd]

Thanks for the reply. That's interesting.

I did a little more searching and found this explanation:
1. below 25 mph, only the spinning wheel is strongly braked
2. 25-62 mph, the spinning wheel is lightly braked
3. above 62 mph, engine power is cut by throttle closing

I don't think it works exactly like that because it cuts the engine rather quickly. I suppose that must mean both wheels are spinning. I've seen other explanations that it will brake both wheels to prevent them from accelerating in the case of both hydroplaning simultaneously.

I still can't find any information which indicates if it decides to deactivate itself when stuck like I was able to duplicate with one wheel on ice.

[bayerische]

A lot of speculation, detail, and quandary for a borderline useless feature.

[nickjd]

A lot of speculation, detail, and quandary for a borderline useless feature.

I'd beg to differ. When it worked, it actually easily pushed the car uphill with one wheel having absolutely no traction. I just want to figure out why it deactivates itself in the case of when I simulated being stuck.

[EdirtySicks]

It is not speed related. It should not deactivate itself unless you push the button.

First, when a wheel slips your rear brake on that tire is pulsated.
If you still are breaking traction then a throttle plate flaps close and cuts engine power.

Edit:
From BMW

Automatic Stability Control plus Traction ASC+T, Dynamic Stability Control DSC

Function

ASC+T and DSC incorporate an array of running-gear control systems that help maintain the car's stability even when driving conditions are poor.

ASC+T optimises both traction and dynamic stability, particularly when you pull away from rest and accelerate. DSC detects unstable states such as understeer and oversteer and within physical limits it helps keep the vehicle on course and under control.

ASC+T and DSC, therefore, are effective contributors factoring into ride comfort and active road safety.

ASC+T incorporates the following functions:

Anti-lock brake system ABS
Cornering Brake Control CBC
Electronic Brake Force Distribution EBV
Automatic Differential Brake ADB or, in the case of cars with all-wheel drive, ADB-X
Engine drag-torque control MSR
DSC incorporates the following functions:

Automatic Stability Control plus Traction ASC+T
Dynamic Traction Control DTC
Dynamic Brake Control DBC.

Extremely responsive sensors monitor the speeds of rotation of the road wheels and in the case of DSC, also the steering lock angle, lateral acceleration, braking pressure and the car's movement round its vertical axis.

With ASC+T If wheel rotating speed differences occur, the system identifies this as a risk of wheelspin, reduces the engine's power output and if necessary applies the brakes at the rear wheels (with DSC: at all four wheels)

In addition, DSC compares the cars actual dynamic situation as detected from the various sensor signals with the ideal dynamic situation at any given moment. If deviations occur, for instance in the form of oversteer or understeer, DSC stabilises the car in a fraction of a second by reducing engine output and applying the brakes at one or more wheels as necessary. In this way the danger of skidding is avoided before it can reach a potentially hazardous level.

The driver may find it difficult initially to grow accustomed to the action of the system However, it ensures the best possible forward traction and dynamic stability at all times.

Some noise may be generated when the brakes are being applied.

[jlb85]

Thanks for the reply. That's interesting.

I did a little more searching and found this explanation:
1. below 25 mph, only the spinning wheel is strongly braked
2. 25-62 mph, the spinning wheel is lightly braked
3. above 62 mph, engine power is cut by throttle closing

I don't think it works exactly like that because it cuts the engine rather quickly. I suppose that must mean both wheels are spinning. I've seen other explanations that it will brake both wheels to prevent them from accelerating in the case of both hydroplaning simultaneously.

I still can't find any information which indicates if it decides to deactivate itself when stuck like I was able to duplicate with one wheel on ice.

Yeah that sounds fishy to me too. There is much more involved than just a certain wheel speed, and for a fact (as you have tested) the engine throttles to reduce power via the secondary throttle body (at least on my e36) at any speed. It will compare rear wheel speed to front wheel speed to determine "slip" for traction control intervention, and side to side wheel speed delta for brake-activated limited slip diff.

As far as timing out, it may be difficult to determine specifically. Do you find it timing out too soon for you?

[nickjd]

Hey EdirtySicks, thanks for the info. It appears that it's more advanced than I previously thought. Are you sure the old ASC+T incorporates all that? That actually sounds fairly impressive compared to what I thought it did (only limiting wheel spin). I think the pump and solenoids all function properly because I found it really does steer the car on ice and snow where it otherwise would just understeer or slide (if you have the brakes sufficiently applied to activate the ABS). I was impressed it steered the car with 1100 lbs of stuff in/on/behind the car on ice before I got studded tires.

Edit: I guess the CBC is the only thing I didn't know about. Everything else is basically standard.


Jlb85, I do think it times out or something. I think it would have run all day if I just let it sit there idling in gear with one wheel spinning on the slope. But once I put it in R then D again it wouldn't work. If I were stuck, I'd probably be going between R and D to rock it back and forth. A few years ago I got it stuck in Norway where one wheel was on gravel and one was on hard packed snow (I had proper snow tires). It didn't budge. I accidentally drove into that position and it wouldn't work. Luckily two cabins of Norwegians came out and had a car-get-unstucking party lol.

I need to find a place to test it again. Maybe tie it to a tree and put one wheel on ice ;) That probably wouldn't be as good of a test with my new $750 studded tires :D

Thanks everyone for the replies. I've been wondering about the "black box" of this system. Oh and if it makes any difference, my car has no throttle plates at all. It's a 3 liter diesel ;) I know the throttle plate is integral into the ASC and DSC system on the gas models.

[EdirtySicks]

Early 94 got ASC, Sometime mid 95 got the throttle flapper IIRC. And all 96+ had ASC+T, DSC didnt come till the e46 era

[nickjd]

I read that the e39 should have ASC+T as a standard feature. In '99 the 540i had DSC as standard (my mechanic's '99 540i has DSC).

Only if there was a way to prevent the engine from being cut back so much. I WANT to spin both wheels on ice and snow!

I recently had my car diagnosed at the dealer for an unrelated problem. It cost $140 to do that here but whatever. Nothing had any errors surprisingly. I guess I can be reasonably certain the braking system has no faults.

[thejlevie]

On a diesel engine, speed/power is controlled by the injection system. So your car pulls throttle by pulling injection.

[jlb85]

The car might be going into a "rocking" mode to help get it unstuck, once it sees D-R-D sequence with wheel spin, which correlates with what you are seeing. It is common in MB cars, we introduced it to American cars when I did ESP development a few years back. On cars with less winter-specific-developed ESP systems (honda, toyotas) we would get them stuck all the time in deep snow as they would always pull throttle with wheel spin.

[nickjd]

That must be is. But how would turning off the brake assistance help it rock? I'd think that would only make it worse. I guess it could also allow for a bit more wheel spin but wheel spin is always worse than rolling (especially if both wheels have a bit of torque applied and the alternative is digging a hole for the wheel).

[thenewBMWguy]

I have to agree that it sounds like the car is going into a "rocking mode" or something. If it comes to rocking, the designers probably thought it would be better to leave it up to the driver at that point. Nothing would be worse than the computer working against you by breaking and cutting engine while you are almost over the hump.

[jlb85]

This. The spinning tires help in digging the car out. The "rocking mode" lets the driver use the vehicle's inertia to push and dig through the heavy stuff, like when stuck in your driveway after a big snow or if you get atuck in a snow bank on the side of the road.

My idea was to have a tell tale on the dash and a single switch where multiple depressings would take it into various modes, including burnout mode where it would lock the fronts and dump all ressure from the rear brake lines :P

[nickjd]

Yeah, I wonder if there is a chance that the braking could make it worse. The problem is it seems that it still cuts the engine but also discontinues using the brakes to help you.

Having a burnout button would be cool. But I found by just turning ASC off, holding the brake pedal down and flooring it, the diesel man wagon will start a standing burnout at 1500 rpm ;) Having 330 ft-lbs of torque starting at 1300 rpm helps a bit :D
Amazingly, it left two black lines starting and ending at the same position. I had no idea the car could do it.

[325bob]

Wouldnt 2 burnout streaks indicate lsd?

[nickjd]

Wouldnt 2 burnout streaks indicate lsd?

I think it just means that my brakes are extremely close to the same and the road surface was pretty much the same.

I was surprised too. I did not expect it to do that. Usually when you see people do that in pickup trucks or whatever it's only one tire spinning.

[mithoangel]

It's the outside temp sensor that is located in the front bumper and has been on most every BMW since the 90's if not earlier.

[jlb85]

Yeah, I wonder if there is a chance that the braking could make it worse. The problem is it seems that it still cuts the engine but also discontinues using the brakes to help you.

Having a burnout button would be cool. But I found by just turning ASC off, holding the brake pedal down and flooring it, the diesel man wagon will start a standing burnout at 1500 rpm ;) Having 330 ft-lbs of torque starting at 1300 rpm helps a bit :D
Amazingly, it left two black lines starting and ending at the same position. I had no idea the car could do it.

You be cheating with that Diesel goodness!!

The burnout mode would stop the rear calipers from clamping, saving the rear pads and preventing a temp timeout, since tires are not warrantied from the manufacturer but brakes sometimes are ;)

I can't tell you which cars already have this due to non-disclosure, but some already do :P