It’s this the only MB model you can not scanner activate the ABS ? But it is listed as it complies? Sombody at MB needs to be questioned

 

Mmr1, the vast majority of MB models aren't covered by my scanners (for ABS bleeding). I did hear back from Autophix - surprisingly quickly, actually. The reply was a little convoluted by translation, but essentially it said that most scanners don't support most MB vehicles. I suspect there's a deeply held secret sauce that prevents us plebians from being able to avoid going to a stealership. Or maybe they felt that if it was available, we'd mess up our ABS systems twiddling them. Maybe.

Denis, thanks for the hydraulic diagram. The one I really need is the internal ABS modulator diagram. I've done enough digging to get a good general idea of the concept, but it would be nice to understand what portions of the internals of the modulator are open to fluid flow when bleeding, and which aren't. And if those change with the key on or off.

The inside of our modulators look like this (from a slightly older W204, but pretty much like our X204 and newer W204 modulator):

Thos little silver tubes are actually covers (aluminum?) over solenoid shafts connected to valves. Those shafts are moved via the electromagnets in the "black part" of the modulator that's controlled by the ABS computer. Obviously, the valves aren't large (if you can cram 12 of them into such a small space, leaving room for enough material to handle the substantial hydraulic pressures involved).

My best understanding of the system (at this time, subject to change) is that the brake fluid more or less bypasses the chambers sealed by those valves. One valve is there to open a passage to a chamber that relieves pressure to the brake line for the wheel that's starting to lock up, by allowing the pressure to be reduced by including this chamber in the closed circuit. Another valve (I think it's another valve) would also shut off the passage from the master cylinder to the brake caliper, preventing additional pressure. A third valve opens if the lock-up isn't cured with the above "first step", and allows fluid to flow from the caliper line back to the master cylinder reservoir (non-pressurized), dumping brake fluid pressure from the caliper, reducing braking on that wheel. This goes on only until the wheel speed sensors tell the ABS computer that the wheel is once again turning at the correct speed, at which point those solenoids go back to their static condition. I'm sure there are likely (at least) details in the above that aren't entirely accurate, but it's a good starting point.

The theory I've read relating to the use of a pressure bleeder is that the high-pressure (20-30psi or ~2bar) fluid is forced past the valves in that ABS modulator block, pushing out old fluid, and pushing in new fluid. Maybe, but keep in mind that the operating pressure of the hydraulic fluid inside that block is MUCH higher than that (over 1,000psi). I can believe that 30psi might be enough to force the valve that controls the flow from the caliper line back to the master cylinder reservoir open, clearing that old fluid out during bleeding. But beyond that, I'm struggling to see how the pressure bleeder is going to push past any of the seals.

Maybe one or more of the solenoids is activated when the key is turned on (and nothing else is happening), but that's kind of hard to imagine because of the inherent inefficiency of that approach, and the heat it would generate. My plan is to see if there's any change in the fluid bleeding flow by turning the key on and off - if there is, it means something in the ABS modulator is changing states.

My other thought is that even if our bleeding process ends up leaving a tiny bit of old fluid inside the modulator block, "normal operation" of the ABS system will dilute this quickly, mixing that old, trapped fluid with the new, moisture-free fluid. Kind of hard to know how much dilution without testing actual flow, which ain't gonna happen. ;-) And of course, in many parts of the country you aren't likely to accidentally engage the ABS system unless you purposely drive in snowy, icy, or very wet conditions (and brake heavily, of course).

SO... if anyone out there has any info showing the pneumatic circuits inside an ABS modulator block, I'd sincerely appreciate it. With that, I may be able to work out just what WILL happen with a pressure bleeder (and what won't)...

 

The winter icy road Trick might be the easiest solution. I’m in Chicago so an empty parking lot would do it . For you where it is warm ? I don’t know. A wet road may be all you need . Or gravel .
Thanks

 

The more I think about it, the more I'm convinced that "purposely cycling the ABS" after the fluid change is going to be a good solution. When you think about how small the inside diameter of the brake lines are, and how much fluid is pushed down to the piston to activate the brakes, it's clear that "it's moving a lot". If I can co-mingle the new and old fluid inside the ABS modulator block by skidding (on ice, snow, sand or dirt) I'm betting that it won't be long before the quality of the fluid in the modulator is pretty close to that in the rest of the system.

Yeah, we'll never (ever, ever, ever) get 100.000% of the old fluid out by bleeding the brakes, at the ABS modulator, or even at the caliper (where you're just squirting new fluid into the bottom of the caliper chamber and letting it run out the top), but what you ARE doing is removing the vast majority of the fluid that's been compromised by moisture.

 

I’ve been watching this thread and applaud the digging you’ve all done. I’ve picked up a lot of details.
The Mercedes ABS has 12 valves because it handles both ABS & ESP functions. What is needed to bleed the fluid in the ABS control unit is to get all through valves open. Only one out of three for each wheel is normally open. Bleeding without getting them open only gets ⅓ or less of the fluid in the unit changed. So why can’t a scanner just open the valves and let us go about bleeding? Those solenoids are only designed for momentary use and can’t stand constant power without overheating.
That’s what I know. Now what I think. I think MB has worked with the supplier (Bosch?) to design a routine to cycle the valves and use the accumulator and pump to get the job done. I’ll quit here before I start telling lies. If anyone knows the rest of the details, I’m all ears.

 

With mb things are difficult for no obvious reason . But I’m sure this is with most cars. You never know what’s in the box

 

I did a brake fluid change on my G about a year ago and followed the procedure in the WIS service document. It prescribed using the pressurized bottle shown in one of the posts above. The brake system holds about 1 liter of fluid and the portion in the ABS is fractionally small. Even if there is 3% water contamination in the system, flushing and replacing the fluid without worrying about what's in the ABS accomplishes the task. Mess with the ABS fluid only if you've serviced an ABS component.

 

I'm not sure whether the ESP system utilizes any ABS valve functions that aren't in the "normal ABS functionality", but it's a good theory (and hey, without theory, this thread would collapse on itself). ;-) There's really just not a huge amount of information out there on the details we really need to work this thing out, whether because it's just never been done, or because MB purposely keeps their cards close to their chest.

I have seen multiple discussions that the power bleeder does result in at least some of the fluid in the ABS system being flushed, though I haven't seen anyone describe exactly how. In retrospect, the suggestions I've seen to turn the key on prior to bleeding (presumably to "open up" certain internal passages in the ABS system) appears to relate to older ABS systems, and may not (probably doesn't...) apply to our X204 ABS system.

And yes, MB does apparently have a routine built into the STAR diagnostic system to cycle the ABS solenoids during the fluid change process. I'm not sure if it's like other systems, where the ABS pump literally pushes the old fluid out the bleed nipples, or if the valves are just cycled while the new fluid is passing through the ABS pump / block. Clearly, there's something "different" from most other vehicles, which can be connected to scan tools that cost less than a mortgage payment to do a proper ABS bleed. Whether that's system complexity (not hard to imagine) or MB withholding information to prevent it (also not hard to imagine) is hard to say.

Right now, my brilliant plan looks like this (subject to change, open to input)...
1) Hook up a pressure bleeder loaded with a couple quarts of "the right fluid"
2) Slowly increase the pressure to determine when fluid flow begins, with and without the ignition being turned on
3) Once the "bleed point pressure and mode" has been determined, bleed all four corners "as normal"
4) Drive the car around off-road, doing repeated full-pedal "panic stops" on slippery surfaces, cycling the ABS (hopefully all of it - if there's a separate circuit for the ESP, I'm not sure how I could engage it without risking my life by drifting through high-speed corners). ;-)

That - I believe - will do what needs to be done. The more I think about the fluid flow to, through and beyond the ABS block, the more I'm convinced that the co-mingling of the old and new fluid will be more than sufficient to result in nearly 100% new fluid ending up in the ABS block. When you think about how much fluid is pushed into the tiny little tube supplying the ABS block (and rest of the braking system) by the relatively large master cylinder piston, it's clear that the fluid entering the ABS block will be mainly from the reservoir (and of course, including the content of the tube between the master cylinder and the reservoir). If the ABS cycles after this fresh burst of fluid flows through and out the ABS block (which would be the case, with a full pedal mash in effect), any opening of any ABS valve will allow any fluid in the ABS block to mix with the fresh fluid. Then, when the pedal is released, that co-mingled fluid should flow back to the reservoir.

I suppose that it would theoretically be possible to test the moisture content of the reservoir fluid to confirm (or disprove...) this theory. The moisture content of the reservoir fluid after engaging the ABS a number of times after a flush should go up as the "old fluid" in the ABS block gets transported to the reservoir. But that moisture content is going to rise only very slightly (there just can't be much fluid inside the ABS block), and my moisture content tester is pretty crude (reading to 2% increments). I suppose a more workable test would be to carefully look at the color of the fluid in the reservoir before and after my "Dukes of Hazzard ABS testing". If it gets even a tiny bit darker, it would indicate that fluid from inside the ABS block HAS made it to the reservoir.

Pardon my long-winded blathering. Sometimes this helps me think things through and (if it isn't obvious) often, it's all made up as I go along... ;-)

 

You may be overthinking this. There isn't very much fluid in the ABS unit until it activates, and then only enough to release the pressure on one wheel enters. Then, the ABS pump pushes that same fluid back out into the same place it came from (toward the caliper). Under normal operation that fluid is never going to make its way back to the master, in fact, as the pads wear, it will move closer to the caliper.

All of that fluid gets flushed out during a normal bleed procedure. If there's a significant amount retained in the ABS unit, it will be mixed with the new fluid from use and flushed next time.

Check the bleeding procedure for ABS replacement. If there's no special procedure then I'd guess there's no fluid (or air) trapped in there.

 

Here's my (possibly warped) thinking on the issue. The ABS only starts doing anything if the brakes are pressed hard. That means that the fluid that normally lives in the pipes going into and out of the ABS block has been pushed way, way down toward the caliper. Then the "ABS magic" happens, and the fluid co-mingles with the fluid in the pipes right around the ABS block. Then, when the brake pedal is released, that fluid is pushed back toward (probably into) the master cylinder reservoir.

Also, I did notice that my (Super-cheap) brake fluid tester actually measures water content in 1% increments, so that's good (more possibility of measuring any change due to the "ABS block fluid" being pushed back into the reservoir. Or not...

 

It doesn't move that far, and it takes exactly the same path back. There is no separate path back to the reservoir. Each molecule of fluid only moves a short distance back and forth as the brake is applied or released. regardless of whether it's the driver or the ABS doing the applying. During the normal life of a set of pads the fluid in the master will never get to the caliper.

 

Agreed that there's only one path to and from the reservoir to the ABS block (sorry if anything I wrote implied that wasn't the case). I was referring to the path inside the ABS block that releases "caliper side" pressure back to the reservoir (and yes, that would be via the one supply line to the ABS block).

I did some actual math, and it looks like the distance the fluid travels isn't as far as I imagined, though it's also not a tiny distance.

Here's how I worked it out...

Apparently the typical disc brake piston will move around .005" when engaged. I'm not sure if that assumes pistons on both sides of the caliper (like high-end 4-piston calipers) but I'll assume it's a typical 2-piston caliper like our X204s have. The piston radius is 0.85", and there are two of them. Figuring out how much fluid it takes to do that pushing (Volume = pi x radius squared x height). That led me to 0.11 cubic inches per piston, or 0.22 cubic inches total fluid to apply the brakes (or at least to move the brake pad into contact with the rotor).

The brake lines are 3/16" (or about 4.75mm). I looked high and low and couldn't find anything definitive on the inside diameter but my SWAG would be around 2mm (0.078").

Working the same formula to figure the "height" of the .078" cylinder that holds 0.22 cubic inches of fluid works out to about 5" total. That would mean that the fluid inside the hardline running to each caliper is moving about 5".

Since that's the amount per caliper, the amount of flow through the supply pipe between the master cylinder reservoir and ABS block would be four times that distance (about 20"). Somewhat smaller if it's a larger diameter tube (not sure).

Then there's the question about what happens when you REALLY mash the whoa pedal - clearly more fluid is entering the system, but it would be difficult to try to calculate how much of it pushes the caliper piston further, expands the soft lines or hard lines, or squishes anything else compressible in the system. But I would guess that a hard brake application capable of triggering an ABS reaction might involve at least a little more "fluid inches" traveled. Clearly the master cylinder has sent quite a bit of extra fluid into the system between first pad contact to the rotor and a panic stop scenario.

Then, because of the unknowns in the caliper end, I looked at the master cylinder end, and came up with a rough average of the master cylinder piston dimensions at 1" bore x 1.5" stroke. That looks to line up with my caliper numbers pretty well - an excursion of about 1" should push about 0.8 cubic inches of fluid (pretty close to what I calculated for the total of the four brake calipers above).

That's quite a bit less fluid travel than I imagined, but (I believe) still enough to get a good amount of co-mingling with the old and new fluid around the ABS block. Fluid dynamics being what they are, I'm not really going to try to figure out how the typical drive cycle moves these two co-mingling fluids in the immediate proximity of the ABS block and supply and output lines, but I figure it's like passing gas in church - it's gonna move around. ;-)

I'll be tied up virtually all day tomorrow, so won't be able to add to this (and a cheer goes up from the crowd). ;-)

 

OK, You may be right, but all the ABS systems I've dealt with have an internal reservoir and the pressure is released to there and not back to the reservoir. To get back to the reservoir it would have to push the piston in the master back and the pedal up until the transfer port is open. That would be hard on the knees...

 

A lot of good technical information here . So what do I do? Pump it ? Forget it? Get a bottle of whiskey and find a snow covered parking lot and have fun !! 🧑🏻‍🎄🎉🎉🥃🍷🤣🤣🤣

 

Last option - and enjoy

 

Bleed each wheel, starting with the closest to the master, until clean fluid comes out. Or, take it to the dealer...

 

???
So, for the last 40 years, I've been told the opposite, or did I miss a smiley face. ? 🙂

It's been ingrained that one should start at the brake furthest from the master cylinder and work your way toward the driver (US vehicle) ... RR first, LR, RF, LF.

 

Since it's all individual lines it probably doesn't matter.

 

I read through this thread. Is the goal to replace the brake fluid?

If so then just bleed each brake line. Unless I missed something critical in reading?

In the manual im looking at it is the same procedure as the W204. I have been replacing the brake fluid every 2yrs. I recently replaced the rear passenger brake line due to rust, simply hooked up my pressure bleeder while doing it. I did not touch the abs. https://mbworld.org/forums/c-class-w...ake-lines.html

Here is AR42.10-P-0010CW Bleed Brake System: https://charm.li/Mercedes%20Benz/201...rake%20System/

Sorry if I missed something and not understanding.

I buy the Mercedes dot4+ brake fluid on ebay. One tin is plenty. I received it quick and the tin looked brand new not dented.

 

The unanswered question is, is simply bleeding the brakes sufficient to accomplish 100% brake fluid replacement, or is there fluid trapped in the ABS controller that requires special procedures to replace?

 

Everything I have been coming across is just saying to bleed the brake lines. Even with removing/installing the hydraulic unit. https://charm.li/Mercedes%20Benz/201...raulic%20Unit/


Bleed the brakes every 2yrs and all should be good.








And heres the bleeding operation: https://charm.li/Mercedes%20Benz/201...g%20Operation/

Even when I removed part of the rear brake line and opened the system I followed the 5.2 instructions below, did not pump the brakes. I had my pressure bleeder hooked up to the brake fluid reservoir without any pressure, I figured that way if any fluid starts leaking out when I do cut into the brake line the reservoiur wont go dry since it can pull from the pressure bleeder. It worked out well, once I was done replacing the brake line I just opened the bleeder on the caliper and waited until no more bubbles came out. I hardly used any brake fluid.

 

That works for me.

 

Sorry, oldtimer's disease. Start at the one farthest from the master cylinder and work your way back.