My antilocks work superbly if I'm stopping from high speeds.

At somewhere below 5 or 10mph, though, I get a weird behavior, and I'm wondering if it's normal or if others have seen it and had it fixed.

If I'm running very low speed and braking to stop for example at a light or a parking space but then my front wheels roll onto ice, I lose all stopping power and the back wheels will push me all the way onto and then across the ice patch.During this loss of braking however, if I push the tranny into neutral, I immediately come to a nice stop.

Yes, I've put up with this for years, but now that my kids are old enough to drive the car, I'd like to see it fixed. My initial conversation with the dealer didn't give me any confidence.

My other car with antilocks does not do this; the antilocks seem to work all the way to zero and the rear wheels still brake, they don't push. With my Mercedes, though, if I try to stop on ice from a slow speed, the rear wheels happily push me along.

I've tried searching the forum, but couldn't find anything.

Can anyone offer any guidance before I engage with the dealer again?

Thanks,
Arne

 

I know exactly what you are talking about, and my 2002230C coupe has done the same thing since new. I think this is primarily a function of a fairly tight torque converter, and clutch plates in the automatic transmission that don't release until you are almost at a dead stop in first gear - both of whichare part of the design for good fuel economy.

What's happening at low speed/low traction is that thefront brakes will want to lock up way before the rears, because the rears are stillgetting power from the engine,and the torque from the engine is overcoming the rear brakes. The ABS essentially senses that the rear wheels are still turning, and reduces the brake pressure in the front to equalize wheel rotation speed. So, you don't stop very fast.One way to overcome this is to just stomp on the brakes hard enough to overcome the drive line torque in the rear and get them to start locking up - then the ABS will kick in on all four wheels instead of just the fronts.Rather hard to modulate your braking this way, though - you basically only have one stopping speed with this technique.

I do the same thing you do - pop it into neutral and then it stops just fine. Kids are smart - and this is a great way to get them to understand the way ABS works, and one of the minor limitations. Then, take them out on a snowy night and let them feel the difference - that's all it will take to get them to remember to pop it into neutral in this situation. My 17 year old daughter caught on very fast.

Likely the only thing the dealership can do for you is inspect your rear brakes. If they are worn to the point where rear braking force has been reduced, relative to the front/rear balance you had when new, the problem will be exagerated. Front brake pads last about 35,000 miles, the rears about 65,000. So, if you have 50,000 - 60,000 miles on rear padsit might be time for a brake job.At least on my car, no pad sensors on the rear to tell you you are getting close.

 

I have noticed that the engine doesn't slow down quick enough on ABS braking but I've alwaysnoticed that my 2002 C230 coupe stops differently than any other MB I've owned.However, the problem disappears completely when I use the "W" setting on the transmission.
In your example where the ABS engages on ice in the front, the ABS wouldn't be working on the rears. It only pulses the brake on the wheel that is stopped too quickly as it does if it is sliding.
Try "W".

 

I tried "W" in the parking lot at work this morning, and I couldn't tell a difference. The weather has warmed up, though, and the ice patch I've been using to test this is getting pretty small.

I understand the automatic transmission issue, and the amount of 'push' I feel isthe same as if I took my foot off the brake at a light and let the idle rpms push me forward. It's weird, though, that when I've got the brake pedal completely depressed that the ABS system would completely free the rear brakes at almost zero speed, just because the front ones are on ice.

I guess I would have expected an ABS system to work independently for each of the 4 wheels, rather than have the back ones freed up just because the fronts have no stopping power. Maybe the issue is that since the speed is so low 1 or 2 mph,the system senses the same the same thing for the rear wheels as for the fronts; after all, if it didn't kick them loose, they'd stop immediately and stop the car. To the system, I can imagine then that the rear wheels which are still on dry pavement look just like the front they appear to be locking up, therefore the ABS prevents it. Maybe it's that the car is going so slow when this all starts that the system can't tell that the car would stop if it simply allowed the rear wheels to stop (lock up).

I'll try W again when there's more ice to test on, and if there's some type of ABS firmware upgrade it would be nice to get this behavior changed. Personally I can keep using the "push into neutral" technique,but the parking lot at the high school often gets packed with ice, and there's so much close quarter, low speed maneuvering required that I'd like my daughter not to have to worry about it.

If the dealer comes up with anything, I'll post an update. It'll probably have to wait till the next snowstorm, though, so that they have some good ice to test it on also.

 

The ABS is not "freeing" the rear brakes in this situation - it is actually working only on the front ones and trying to equalize the rotation speed of all 4 wheels by unlocking the fronts, which lock up first because the rears are still getting power and the fronts are on ice. All ABS systems work exactly the same way, and none of them try to control lock-up individually on one wheel without comparing it to the rotational speed of the others.If they did work independentlyto stop lock-up without checking to see what the other wheels are doing, you would never stop because all it could do is prevent any lock-up, even the final one. ABS has to work at low speeds and extremely high rotational speeds, and with a wide variety friction surfaces and rates of deceleration,so the only practical way to keep bad things from happening in most situations is to compare all 4 wheels and make sure one (or more) are not stoppingwithout the others.

The 230 C Coupe has the tightest torque converter of any car I have ever owned. Take your foot off the brake at a light without touching the gas, and you really move - a lot faster thanmost cars with an automatic transmission. The idle spped is already down around700 RPM, so re-programing to reduce this is likely not practical, but wouldhelp this particular issue. It will be very interesting to see if the dealer can come up with a solution for you - I have my doubts, but good luck.

 

My suggestion of using "W" works for my car probably because it only downgears to 'second' instead of 'first gear' and certainly feels differently during normal everyday stops. It may not have the same effect on your vehicle. "W" definitely helps traction and acceleration in winter conditions because the starting gear is always 'second'.
Don't forget that ABS doesn't neccessarily shorten your stopping distance but only pulses the brake at high speed to allow the off-speed wheel to continue turning so your steering is still available. It has been argued that some ABS systems actually increase your stopping distance in certain heavy snow conditions.
I also doubt anything can be done to fix your concern.