4x4 ASR ETS 4ETS 4MATIC

Contined from ABS.

Automatic Locking Differential (ASD)
The "automatisches Sperrdifferenzial" is an electrohydraulically operated automatic locking differential using multi-plate clutches. It works up to 19 MPH and can be used on a single axle like in some smaller diesel models where is was optional in the US since 1991 instead of ASR which is typically used on the more powerful models. ASD is also one component in the old 4matic.

Old 4Matic (1987-1995, USA MY90-93)

was introduced at the IAA in Frankfurt in 1985. It was described as "the automatically selectable four-wheel drive (4MATIC)". It may have been inspired by the G-Wagen introduced in 1979 but was better since it has three open differentials, front, rear and even center which was not available on Gs before 1990. But instead of manual transfer case and diff locks, it uses hydraulically operated multi plate clutches. Wheelslip is detected with the ABS wheel speed sensors and the clutches are progressively engaged by the controller. It measures the steering wheel angle and calculates the expected front wheel speeds using a linear (bicycle) model (similar to the model used in ESP) and compares them to measured wheel speeds. If a discrepancy is found the center clutch is engaged and thus the front axle switched on. Normally it's a rear wheel drive and with slip, at first, the front axle is engaged with a 65% to 35% rear to front split, and the warning triangle on the instrument cluster starts to flash. With further slip the center differential is locked, with 50/50 rear to front torque split, and finally the rear diff can be locked. When braking all three clutches disengage instantaneously to not interfere with ABS. On takeoff/acceleration the front axle normally engages, proactively, regardless whether wheel slip is detected or not. It's a torque on demand system and not full time four wheel drive like newer versions. It fails on the roller hill since it lacks a front diff lock, just like the 500GE G-wagen. Owners usually like its performance. Sometimes the actuator seal inside the transfer case fails, and leaks hydraulic fluid into the transfer case; but an updated transfer case became available which should fix it. Then there is a more fundamental disadvantage: Locking differentials can exert high stress on the drivetrain, gears, CV joints, axles, bearings etc. since up to 100% of torque can suddenly be directed to a single wheel. And in a sense this contradicts 4 wheel drive, which was invented to distribute torque evenly to all wheels. This system therefore requires pretty heavy duty components in the drivetrain, which increase the costs and cause a small performance penalty due to their higher weight, including a higher unsprung to sprung mass ratio in the suspension. It's similar to PSK (Porsche Steuer Kupplung), and was quite pricey considering it's not a permanent four wheel drive. The Old 4matic was mainly used with the inline six cylinder gasoline engines, and did not expand to more powerful engines probably due to some of these considerations. In 1997 the system was replaced with the new 4ETS based 4Matic.

New 4Matic and Electronic Traction System (ETS, 4ETS)
What works for braking, optimize slip ratio to optimize traction, also benefits acceleration. Since the adhesion versus slip diagram is roughly symmetrical, it can simply be extended to the acceleration side. One can think of it as forwarded ABS:

tire adhesion vs wheel slip plot extended to acceleration

Note how small the sideways tire force becomes when the wheels are spinning under acceleration confirming that I often slide to the side when the wheels are spinning fast during hard acceleration on a slippery surface, in cars without this system. Just like in ABS, an optimal slip ratio is important to maintain the lateral tire adhesion and thus vehicle controllability during acceleration.

Acceleration slip control (ASR) or Electronic Traction Control (ETS) first appeared in 1986. Wheel speeds are sensed via the ABS wheel speed sensors and when incipient spinning is detected, on the driven wheels, in a first step, further driver demands for engine power are refused. The engine is controlled via timing retard, throttle, injection pulses or ignition. Wheel spinning can also be controlled by short pulses to individual brakes. e.g. Bosch ABS versions 5.0, 5.3, 5.7 and 8.

How can ASR/4ETS brake without a driver pressing down on the brake pedal?
It uses the existing ABS return pump, but reverses the flow with the help of two new valves in each of the diagonal brake circuits. It's simple and effective with minimal added complexity. Why didn't I think of it? :-)

System response time is fastest for braking, about double as long for throttle valve and timing intervention and about six times longer for systems acting only through throttle valve control.

Some known ASR problems
Quite a few electronic throttle-actuators fail, triggering limp-home, and it's sometimes confusing to troubleshoot, probably due to heat in the engine V, or poor wiring insulation in these model years. It seems to mostly occur in 92-95 models, e.g. 94 S600, 95 SL500, 94 E320 etc. Sometimes the symptom can be cured by checking loose connectors, wiring, rematching tires, properly inflating tires, etc.

4ETS
4ETS is an extension of ASR/ETS from two driven wheels to four driven wheels. In the M-Class it's a full time four wheel drive system using three open differentials. Torque is automatically directed to the wheel or wheels with grip. Porsche calls this system Automatic Brake Differential (ABD), used in the Carrera 4, for example. It's also called Electronic Brake Differential (EBD) or Elektronische Differenzialsperre (EDS).

A simple experiment
Drive over some rocks lifting a wheel. Three open differentials direct the torque via the path of least resistance, the lifted wheel/s spin, you are stuck. Now what? If you don't have 4ETS nor mechanical lockers, an experienced off-roader might get the locking pliers out, and stick them on the brake hose of the wheel on the rock, with traction. This wheel can then no longer be braked. Then drive over the rocks again, feathering the brakes just enough to keep the lifted wheels from spinning. YOU DID IT. You used an old off-roading trick, acting just like 4ETS - and can now drive over the obstacle without getting stuck.

M-Class on roller hill

A 4ETS example We drive over this rock with the left front wheel. Each wheel receives about 25% of the total torque. The front of the car rises and the front right lifts as well and off the ground and starts spinning. If it's a large rock, the diagonally opposed wheel, the left rear in this case, may experience less traction as well. Energy again flows in the path of least resistance, and is lost towards the spinning wheels and the left front, driving over the rock, loses torque, as well as the right rear, which is firmly on the ground. 4ETS senses the wheelspin and briefly, typically lasting only a few seconds, applies the brakes to the spinning right front and maybe also the left rear, slowing the spinning. The braking fools the open differentials into sending equal amounts to the wheels with grip, which can then maintain the torque, about 25% of the total at each wheel. If three wheels lose traction, pic (2) on the left, a rare occurence, which would get most other 4x4s stuck altogether, up to three wheels are slowed by braking. The M-Class is one of the very few off-roaders which can drive up the rollerhill, which simulates this rare situation. I saw the test demo in Oakland during the MB Powertrip, and a RX300 failed, while the ML drove up without much difficulty. Pretty amazing. Up to 100% of the torque can be transfered to any axle according to the manufacturer, briefly, and is then distributed more evenly, split 50/50 front/rear in early M-Classes and G-Class, 48/52 in later M-Classes, and split 40/60 in the C-, E- and S-Class, roughly 25% to each wheel, whether it's braked or driving. The system adapts automagically - and without locking pliers :-)

Spinning wheels are slowed by braking and the wheel speed differences maintained by ETS are:

• 5 km/h - 3 MPH - in high range

• 2 km/h - 1.2 MPH - in low range

In some off-road situations, this may be too much, e.g. when spinning wheels can dig the vehicle in. Wheel spin can be reduced. It takes less than 200 milliseconds to build up the pressure in the calipers. The wheels turn just about 3 inches during this time when crawling in low range, and thus can't "dig". Note that in 2001 Mercedes introduced 4ETS+ which uses pressure preload further reducing spin, in low range.

How does it work on tall soft sand dunes?
S U P E R. Put the foot down early and power all the way to the top - 4ETS doesn't even flicker. One reason why the MLs do so well on the Dakar. Tires deaired, ESP OFF or ON.

Is 4ETS reactive instead of proactive?
Not really. 4ETS is proactive where it counts, at high speed and high performance and mechanically directs torque with three differentials to wherever it's needed. It's so good that it's increasingly replacing older reactive torque-on-demand systems. Only at low speeds may 4ETS become reactive, when, like ABS, it detects an increase in the slip ratio to pulse the brakes. Solution: A wheel travel sensor detects the load on the wheel, and brakes the wheel even before it lifts off or slips, proactively.

Who invented 4ETS?
Mercedes developed 4ETS with Magna Steyr in Graz and Continental Teves. Rumor has it that another development effort was underway, employing a Torsen Torque-on-demand differential, but was not used in the end. The new system had proved itself in tests: more control, superior driver feel and performance handling. A similar system was under development for Mercedes passenger cars (4matic). To test it for the M-Class a G-wagen was rigged up with it in 1993. Some minimal amount of torque is lost to heat in the brakes, estimated at less than 1% of total energy, since 4ETS usually intervenes at slow speeds, and only for a few seconds. This even surprised the inventors during the initial development, who had expected to see hot brakes! They rigged up lamps to see when and how long the brakes were activated at each individual wheel. They were only pulsed briefly to their surprise.

Why is no torque and heat lost in the brakes?
Because 4ETS generally intervenes at low vehicle speeds, and only in brief smooth pulses.

4ETS allows each wheel to act independently. And even if only one front wheel has traction the vehicle can still proceed. M-Class 4ETS works without engine power intervention, unlike ASR. There is no clattering in curves due to locked diffs and premature wear on tires, gears and axles due to drivetrain binding, which occurs with locked diffs. It's a beautifully simple and elegant mechanical full time four wheel drive system. An important advantage is that the driver does not have to decide when to lock one or more differentials. Because when you come around the curve and see the ice patch it may be way too late already. Or you may not see the black ice at all. Locking diffs at the wrong moment or even in the wrong sequence can have serious consequences and thus manufacturers try to avoid manual locks altogether, particularly on the front axle, where steering can become impossible.

Another advantage is 4ETS's tight integration with ABS. In older systems, for example the old 4Matic system, or any systems employing viscous clutches, or differentials using ABS poses severe difficulties. Usually the clutches have to be unclutched rapidly when braking to avoid interference with ABS and vehicle controllability problems. Viscous clutches have to be rather lose for this reason (or little bias in the case of Torsens), usually restricting them to low performance drivetrains, for example minivans.

• 4ETS quad-lockersŪ advantages summarized

• low maintenance, high reliability four-wheel drive

• three open differentials, front, center and rear

• full-time mechanical four wheel drive

• good driver feel, control, performance and handling

• each wheel is allowed to act independently, no bump-steer

• no axle binding resulting in sluggish steering response or vibration in tight steering situations

• full integration with braking, traction and stability systems suitable for performance vehicle dynamics

• light weight of under 200 lbs

• wear items are inexpensive and easily accessible

• up to 100% of torque can be sent to either axle, briefly, then

• even torque distribution to all wheels split 50/50 front rear in early MLs and G-wagen; 48/52 later MLs

• beautifully simple and effective with a proven track record

• disadvantages

• four wheel drive is often unnecessary and carries a cost, weight and fuel consumption penalty

• some wheel slip, preferred by some drivers, controllable by manual button; or use lockers

4ETS testing

4ETS/4ESP controller

4ETS arctic testing

MK20 valves

4ETS ice slope test

large MK20 picture
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Continental AG

What do the Mercedes engineers say?
"This systems allows every wheel to act independently, but it does not allow a wheel to slip," says Dr. Gerhard Fritz, vice president for development at Mercedes-Benz and in charge of the M-, G- and V-Class. "It controls every wheel and the average velocity of every wheel."

"4ETS compares favorably to locking differentials in another important way. In tight steering situations, there is no axle "binding," which the driver would notice as sluggish steering response and vibration."

"For us it was clear that it would take an independent axle to combine the off-road abilities and the luxury-car ride," Fritz says. "We've compared the M-Class with the G-Wagen on the off-road test track. The M-Class runs faster."

Fritz scoffs at the notion that an independent suspension is not tough enough to handle the rigors of serious off-road driving: "Conventional wisdom says a rigid axle is more stable off-road, but that is not the case," Fritz says. "If you hit a big bump, or boulder, you can damage the axle. Once the axle is bent, it's over. With an independent suspension, you might bend a control arm but it would keep going."

"The only limit to the M-Class 4wd system is the friction of the road," Fritz says.

M-Class 4ETS kicks in up to about 36 MPH (60 km/h) and if engagement conditions are maintained beyond 60 km/h during acceleration, control is effective to up to 48 MPH (80 km/h). In other models, then called 4Matic, and also with 4ETS+ starting in 2002, it can engage up to 24 MPH (40 km/h) and remain engaged also up to 48 MPH (80 km/h).

The system tracks the brake temperature at each wheel. A wear sensor indicates when brake pads need replacement.

Lateron the Mercedes C-Class 4Matic, E-Class 4Matic, S-Class 4Matic, G, GL, R, Jeep Grand Cherokee, BMW X5, Landrover, Porsche Cayenne and VW Touareg among others adopted similar drive systems.

Continue to ESP.

More
M-Class transfer case
ETS Plus
dynamics + suspension
Continental Teves press release 1999 Mercedes M-Class
M-Class on arctic ice test track
M-Class off-road
Automotive News: "ENGINEERS LEFT CONVENTION ON THE SHELF..."

created in 1998 and updated from time to time

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