What is this switch for? '92 500E
 

http://fototime.com/211A99FD139C5DB/standard.jpg

It's the second one from the right and has a tire tread symbol on it. The car didn't have an owners manual to look it up with. Looking at pictures of other E-class on Ebay I noticed all the 500E's have it but I only saw one 400E with it.

That's the ASR switch. It's not an ASR defeat switch but I believe you are supposed to punch that button when you have snow chains on the car. Someone will have an owner's manual handy. It's my understanding that all 124 500s have ASR but it was optional on the 400s/E420s, which is the reason why I have the latter. I will not own a car with ASR - nothing but trouble, and usually inexplicable trouble.

The switch reduces the impact of the ASR system to 30% of normal (but as deanyel says, doesn't completely defeat it). As he says, it's intended to help get you unstuck if you have tire chains, but I've used it many times with dedicated winter tires without chains, too.

Personally, I wouldn't be without ASR (since I need to drive in snow this time of year). The 400E/500E produces enough torque that you get a lot of spin if you encounter slippery roads.

I'd also suggest you call 1-800-FOR MERC and order an owner's manual.

Actually that switch, often referred to as the snow chain switch, allows the wheels to slip up to 60 km/h (~35 mph) or so. In other words, it prevents ASR from kicking in until that speed. Once you hit approx 60 km/h or approx 35 mph, ASR is on once again.

To check if it works, engage the switch and drive, the light should go off around 35 mph...

Just to clarify -- it does not actually completely defeat the ASR system under speeds of 35 mph. ASR will still engage at lower speeds with the switch ON (but the ASR system will allow more than usual wheel slip before it engages).

You can probably get an owner's manual from your dealer.

This is probably more than you wanted to know, but...

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, for instance, 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.

Great info, Dakota! Thanks.

Just a nit-picky point -- ASR is "acceleration slip regulation" since the system is traction control only, not anti-skid...

I agree, Steve. MB seems a bit confused, and it's probably a matter of symantics and translation from the original German. Job 42-0800 calls it Acceleration Skid Control, while the Owner's Manual calls it Accelration Slip Control, and as you say, Acceleration Slip Regulation fits the TLA. I should try to find out what the ASR stands for in German.

In German it is
"Anti Schlupf Reglung"
( Anti Slip Control would be the closest word for word translation)

It lets you do 1/2 donuts in wet grass :D

That, and what the guys above told you. ;)

What they all said.

Above 100 mph it ejects the front passenger:D

What I know is that this button makes the car starts on the 3rd gear not the 2nd as normal so the car will take off heavly.

Trashed ASR.. could this be the culprit
 

I found a decent 500E locally for a price I like so I drove it. Hm.. FUN! BUT, one thing kept me from buying today. When starting off (in each case I was pulling out of a turn) there was a weird shudder that I thought might have been the trans but... it also almost seemed like an engine stumble too. But off idle rolling there was not shudder or hesitation and the trans seemed to shift better than the 300E.

I told them to service the car and I'd come back on drive it again. They are doing that over the next two days. Does this sound like something that ASR could be doing and if so, maybe punching that button might reduce/eliminate this and verify that it's the ASR?

Just wondering. Because for the price, I could probably have the tranny rebuilt and still be at market price. Body is not perfect and it was filthy... so.. once it's cleaned up it might show it self to be a rat too. *shrug*

So how does the ASD "Autosperrdifferential" fit into this, if at all?