[W126 Coupe] ASR and Emissions Testing

[Steve Nervig]

>> 

> Steve, you are correct, I misnamed the switch.  The precaution, though, is unaltered.  If you have a problem with the ASR, such as mounting a spare wheel with a slightly (or not) different tire, Jono's suggestion of pulling the relays under the cover of the ABS/ASR module would would be a more tidy method of defeating an undesirable ASR response until you were able to rectify the problem, as the tire chain switch would not be a solution - it may be a fair assumption that it also would work if there were problems with ASR sensor signals.  The symptoms are unusual intermittent braking which are annoying, at the least.  Thanks for the clarification on my terminology - I was a bit unnerved at trying to figure a solution.

> 

> Thanks, again,

> 

> Richard


Richard, your warnings are well placed. I have gone thru the emissions testing with my 1992 300CE with ASR and have had the same dilemma. 

Luckily, knowing that I do not have a real defeat switch, I have been able to tell them so, and they have relied on my adding RPMs from the drivers seat to the proper number (2700 rpm?) and they then took the emission reading. (My car has no exposed plug wires so they could not even check RPMs thru the ignition wires.) They did it this way several times in Washington and here in Arizona, also. Other states may not allow this method, and so Jono's method may be the only way to pass the test.

And as you say, there certainly may be other instances where Jono's method is needed.

The ASR is actually a very complex system, and works pretty well, although it will not really allow a full throttle launch off the line. For those interested, I will repeat an email description that I wrote many years ago on this List. Probably more than most people want to know, LOL!

Regards,
Steve
1992 300CE Sportline

ASR kinda 'splained:

MB seems a bit confused on its name, and it's probably a matter of semantics and translation from the original German. Job 42-0800 (w124) calls it Acceleration Skid Control, while the Owner's Manual calls it Acceleration Slip Control. Acceleration Slip Regulation fits the TLA. ASR stands for "Anti Schlupf Reglung" in German.

To understand the "snow chain switch", you need to understand ASR a bit. ASR is a control system that activates the brake on the spinning wheel and depending upon conditions, reduces engine power output until the slip stops. There are several stages in the ASR activation which occur in milliseconds and are transparent to the driver.

In the US, ASR was an option from '91-on, on non-4matic vehicles only. My 300CE has ASR, with the  /!\  in the speedometer, just like the 4matic. It's tied in with the ABS and has four wheel speed sensors and a connection to the acceleration control system. There are three different control ranges in the ASR control mode using both a drive moment control circuit and a brake moment control circuit. More info is also contained in Brake System job 42-0800.

The acceleration skid control (ASR) will engage at all vehicle speeds, if one or both drive wheels begin to lose traction and spin due to excessive acceleration. While engaged, the yellow function indicator /!\ in the speedo lights up.

With the ASR engaged, the brake is applied to the spinning drive wheel until it regains sufficient traction. If both drive wheels lose traction and spin, the brake is applied to both wheels and simultaneously, engine torque is limited.

As traction on the road surface increases, the allowable engine torque also increases again and the brake is no longer applied to the drive wheels.

As I mentioned, there is a brake moment control circuit and a drive moment control circuit. The three control ranges of the ASR are:

A. Control mode with one skidding drive wheel and at wheel speeds < 40 km/h. The allowable slip threshold is higher for the Drive moment control than the Brake moment control, so the brake is applied to the slipping wheel. If it still slips too much, the Drive control comes in and retards the electronic accelerator control.

B: Control mode when both drive wheels skid or at a speed < 40km/h. Drive control has priority and retards accelerator. If wheels still skid, brake control is added.

C. Control mode when cornering at speeds between 20 km/h and 120 km/h. Depending on lateral acceleration, Drive moment control comes in sooner than for mode "B".

The console mounted switch, the "snow chain" switch, when activated, increases the speed threshold of when each mode will start to engage. This "buffers" the ASR a bit and lets some slippage occur when first starting out from a stop. 

There is no traditional locking differential; the asymmetrical braking independently applied to the rear wheels provides this function, sort of. MB has a different system for an automated rear end differential, called ASD "Autosperrdifferential".

Interestingly, I find a 1990 MB brochure mentioned that MB developed all three of their electronically controlled traction systems (ASD, ASR and 4Matic) in the same year - 1986.

The ASR was available in the S-class only (not in R107, W201 nor W124) in the Oct 88 GB "Recommended Price List for Cars and Factory Fitted Extras". 

It cost ~1500 to 1700 in then-year British Pounds (including basic cost, tax and VAT).

Another brochure I have says that ASR was available in the V-8 models in 3rd quarter 1986.

ASR was available in Europe on the 1990 W124 six cylinder gas saloons, coupes and estates with automatic transmissions, but not the 4 cylinder gas, or 5 or 6 cylinder diesel cars. It was offered on both the 1991 W126 and 124 models in the US. I believe it was approximately a US$2000 option. It came standard on the all 500E and E500 W124 from 1992 thru 1994.


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