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w2hmh 04-04-2014 09:46 AM

1991 350SD ASD Operation
I have an unbent 1991 350SD (170,000 miles) that is exhibiting a thumping noise from the right rear of the car. I haven't actually timed it, but it happens when idling or driving and occurs every minute or so quite continuously. I believe it has something to do with the ASD unit in the car. I grasped the ASD hydraulic unit and can feel a thump in the lines and this unit that is the source of the noise. My ASD fault light is not lit and appears functional. It lights when the car is started and then goes out and stays out. I have also tried testing the ASD by driving on a snow covered road and stepping hard on the throttle and the light lit up and the system seemed to work. I have also bled the system per the Mercedes manual. Is this thumping sound normal operation or is there something else going on here. Any help would be appreciated.

mclare 04-04-2014 09:51 AM

Mine never did that. It did other things, but not that.

Do you have the manuals? I have a complete set of parts including two ASR Hydraulic pumps if you need them.


compu_85 04-04-2014 10:02 AM

ASR is quite different from ASD :) ASD just locks the differential, the system has 3 speed sensors just like the "normal" W126's ABS.

Can you take a picture of the unit you're seeing the thump from?

Because it occurs when you are stopped I would think it's something with the ASD as well. Has it always done this?

I have a friend who has a 300D 2.5 with ASD, I'll ask him if he hears the unit cycling on his car.


compu_85 04-04-2014 10:03 AM

PS: I'm in the process of getting the service into books on ASD. If I'm able to get the info I'll share with the rest of the class.

w2hmh 04-04-2014 12:55 PM

Thanks.....more info is good..this car definitely has the ASD option....the unit is located under the car in the right rear of the car where the ASD valve is located and the thump can be felt in the unit and in the lines going to and from current hypothesis is that for some reason the ASD controller senses that the car is being powered on again and again...I am guessing this as the manual that I have indicates that the ASD control module will pulse the ASD valve briefly on powerup and that a mild thump from the right rear may be heard.
My current approach is to monitor the control voltage to the ASD valve to see if it is being pulsed (I guess that it is)....then on to the ASD control unit and perhaps the OVP relay....intermittent contacts, etc?????

Dr.Grillz 04-04-2014 02:49 PM

People often mistaken ASR for ASD. Totally different systems, one deals with brakes (ASR) while the other is a hydraulic limited slip system.

My 190E AMG when I first bought it had a non-functioning ASD unit that had a thumping sound coming from the front left fender. The ASD pump/unit is actually located there (at least in the W201/W124's). The assembly unit has an accumulator that looks a lot like the SLS accumulators seen on wagons, 560SEL's and 16v 190E's. However, this accumulator has "ASD" stamped on it. They are available only from the dealership, I'm not sure whether there ae aftermarket alternatives but I wouldn't risk it. Simply replaced the sphere/accumulator and refill and you should have your problem solved.

w2hmh 04-04-2014 04:30 PM

Thanks...might be worth a shot....

mclare 04-04-2014 06:15 PM

Well is his problem in the ASD or ASR. I know they are two different systems, but the way he described it to me, he actually meant ASR. If he took it out on the ice and tried to skid it, the ASR would take over and not let him do that. Also, the ASR has the warning light in the center of the speedo as mine did, and I assumed he was referring to that.

Dr.Grillz 04-04-2014 09:17 PM

ASR and ASD both utilized that little window in the speedometer that would light up when activated.

A little info on the two from a 1991 article that provides good info. I have driven two cars with ASD (my 190E AMG and a 2.5-16) and can say that ASD is a great option to have and beneficial on snowy roads.


At Mercedes, "A" stands for acceleration, whether on dry pavement or icy road, whether on a straightaway or a curve.

The reason you can move with confidence when others can't is traction control, the next logical progression in the electronics that brought us anti-lock brakes to stop a vehicle in a straight line regardless of road surface by controlling wheel spin.

If a few computer sensors can stop a car without a wheel spinning, add a few more to control those same wheels when one or more hits ice or snow while moving.

Mercedes offers two forms of traction control for the 1991 model year: automatic locking differential (ASD) and automatic slip control (ASR).

The initials don't follow the wording because something is lost in the translation from German, say the folks from Mercedes.

Suffice it to say that both are designed to work on ice or snow. ASD stops the wheels from slipping when you start from the curb, light or stop sign and ASR keeps the wheels from slipping to provide directional stability and traction while you're moving, such as going into or out of a curve, swinging out to pass or crossing a viaduct or bridge.

ASD is simpler and is offered as a $1,050 option for 1991 on the 300D 2.5 turbo, 350SD turbo and 350SDL turbo diesel models, and in the gas driven 300SL, 300E 2.6, 190E 2.3 and 190E 2.6.

ASR is more sophisticated and is a $1,975 option on the 300E, 300CE, 300TE, 300SE, 300SEL, 420SEL, 560SEL, 560SEC, 300SL (automatic only) and 500SL.

ASD automatically equalizes the torque, or pulling power, to each wheel to provide traction.

ASR sensors measure wheel speed so that if a rear wheel slips, the anti-lock brakes are applied to the wheel or wheels to equalize torque. ASR sensors also reduce engine output so the power provided matches available traction.

Because snow and ice were missing, Mercedes demonstrated ASD and ASR with a parking lot "paved" with soap-caked rubber mats.

The task was to propel a 300D 2.5 turbo from a standing start on a surface made to approximate a street after a couple-inch snowfall -- first without ASD, then with it.

The soapy mat was positioned so the right tires would travel over it and the left were on dry pavement. We floored the pedal and the reaction was predictable, the wheels spun, the soap suds splattered fender and rocker panel and the vehicle wavered sideways while making little or no forward progress.

Next, ASD was activated and the same maneuver performed. The sensors determined that the right rear wheel was spinning out of control and the ASD activated so that torque was directed to the slipping wheel -- all in milliseconds -- so the car pulled away in a straight line without the least quiver.

Then came a 560SEL with ASR, this time on a course with alternating soapy mats so that one wheel, then the other, would hit the obstacle. From there it was a soap and water soaked pylon-lined course that turned sharply to the left so we would have to maneuver as if to avoid a car that suddenly pulled into our lane.

With ASR disengaged, the tires couldn't bite the mats. Attempts at acceleration brought only wildly spinning wheels.

With ASR activated, the sensors instantaneously picked up on the wheel spin when accelerating from a standing start. Those sensors applied the anti-lock brakes, and you could feel the pulsing in the pedal. At the same time, it felt as though we were attached by chain to a granite block; the engine went into a lumbering mode to help equalize power output to the traction available. Wheel spin was brought under control and the car pulled away in a straight line, with the driver in command.

Each system has built-in diagnostic capabilities to continuously monitor whether it's working.

"If the safety logic detects an abnormality, a -- light goes on as a warning," said Peter Patrone, Mercedes product planning manager.

ASD and ASR have been offered in Mercedes cars sold in Europe the last two years.

"We weren't going to introduce such systems without some real-world proven performance first," Mr. Patrone said.

w2hmh 04-05-2014 06:15 AM

My car definitely has ASD
1) The data card lists option 211 "ASD-Auto Locking Differential
2) There are hydraulic lines that go to each axle half on the differential
.....just ordered the accumulator on a hunch...sounds like a worthwhile shot....I know these things can go bad as I had issues with a 3000TD with leveling suspension that had a bad accumulator...I will post results after I receive and install the accumulator...

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