ASD/ASR - one and the same?

[fenns]

This question has been nagging away for a while so here goes:

I know that ASD was available on various models (my specific interest is the W124) and operates below 45 kph or thereabouts. ASR operates when accelerating on loose/wet surfaces but are they separate mechanisms or if you have ASD do you have ASR by default?

.....and are they actually any use?

 

[Will]

No, they are not one and the same AFAIK.

ASD is not a usual traction control system as such, it's purely an electronically-controlled, hydraulically locking, limited slip differential. It does not apply any braking force or limit/reduce the throttle and thus engine output or power.

It uses the hydraulic pump on the engine that some models equipped with SLS have, and uses the pressurised hydraulic fluid to adjust the locking on the differential according to the level of wheel slip measured by the speed sensors that are on each wheel (also used for the ABS).

I don't think the ASR doesn't relies upon such a system with a special differential system, it probably operates in much the same way as many modern cars equipped with 'traction control' do, possibly braking the affected wheels using the ABS pump?

Will

 

[Subyland]

ASD is an electronic limited slip diff. When detecting that one wheel is spinning the multi-plate clutch allowing the differential will progressively be close using a HP hydraulic pump (hence the reservoir). The prime objective is to improve traction during start off on slippery condition.
ASR is a anti-wheel spinning and works by activating the brakes of the wheel which is spinning and reducing the power. By braking one wheel the torque is automatically transfered onto the other wheel; therefore combining ASR with ASD has little interest.

In the case of ASR the electronic system receives information from specific sensors for each wheel and from another sensor indicating the position of the accelerator pedal. If the sensors detect a spin then the system automatically reduces the engine power until such time as the spin has stopped. At the same time a separate electric pump, accumulator and controlled valves isolate the spinning wheel brakes from the main braking circuit and control the spin using the wheel's brake. When the spin has stopped, all is reverted back to normal.

 

[fenns]

Will, Subyland - thank you. That's a little clearer. So, would it be correct to say that ASR is an early form of traction control whereas ASD is MB's 'own brand' of LSD and as such more useful when actually moving (albeit at low speed) than the former?

 

[Subyland]

That pretty much it. ASR was launched in 1986 so it was ahead of its time. ASD is MB's version of a LSD, but without the usual drawbacks as the electronic control allows the system to delay the unlocking of the diff (2s and 7s). But if both wheels are spinning then ASD isn't much help whereas ASR will control the spin.

 

[fenns]

Subyland - that's much clearer. Much obliged

 

[Will]

MB did do LSDs though without ASD control, for example in the early 190E 2.3-16.

As Subyland has mentioned, you can't defy the laws of physics though with either system.

Will