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  #1  
Old 12-31-2002, 11:21 PM
tvpierce's Avatar
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No ABS in reverse?

My driveway was covered with ice this morning, and when I was backing out of the garage, the ABS did not engage. It worked fine going foreward. Is this normal?

Jeff Pierce
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Old 01-01-2003, 11:42 AM
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As far as I know this is normal. ABS also does not work in reverse.
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  #3  
Old 01-01-2003, 11:51 AM
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I would have to guess that ABS DOES work in reverse although I have never read anything one way or the other (actually I think I would have read it if it didn't). My guess is based upon the many false ABS activations I have felt in reverse. I can do it almost at will backing off my alignment ramp.

I would say that the threshold for activation is a mapped function. This means that there is a formula for when, based upon the circumstances in existance. Different thresholds are evident while cornering. Its possible the criteria would be different in reverse.

Interesting question.
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Old 01-01-2003, 12:04 PM
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Consider this possibility. At very low speeds, such as backing out of a driveway covered with ice, if all wheels lock up simultaneously the computer thinks the vehicle is stopped! I've been able to demonstrarte this many times on a variety of MB's. The two things necessary for this to occur are 1. Vehicle speed is low(<5mph). 2. The road surface is sheer ice! When we have these conditions in my neck of the woods, I make it a point to show my trainees that this is possible.
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Old 01-01-2003, 12:41 PM
PaulC
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Quote:
Originally posted by ILUVMILS
Consider this possibility. At very low speeds, such as backing out of a driveway covered with ice, if all wheels lock up simultaneously the computer thinks the vehicle is stopped! I've been able to demonstrarte this many times on a variety of MB's. The two things necessary for this to occur are 1. Vehicle speed is low(<5mph). 2. The road surface is sheer ice! When we have these conditions in my neck of the woods, I make it a point to show my trainees that this is possible.
I've noticed this many times. I believe a minimum speed threshold has to be crossed before ABS is active.

ABS in reverse? Can't verify in an MB, but I recently slid backwards down a very steep icy hill in an ABS-equipped Ford Expedition and can verify it's activation in reverse. I can also verify that once 5,500 lbs. of truck decides to slide backwards, a pair of Depends comes in handy.
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  #6  
Old 01-01-2003, 12:50 PM
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So the procedure , when parked on sheet ice... is to accelerate in reverse so as to get up to min activation then slam on the brakes so as to have a controlled slide and turn (backwards) into the street....WOW.... a heart starting way to start a commute....
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  #7  
Old 01-01-2003, 03:55 PM
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here's what I think

First of all, there is a threshold. I know, in my mothers Bonneville its 5 MPH. I've always heard ABS cheks all 4 wheels to see if any of them are traveling at drasticly different rotation speeds. If one is rotating extremely slower than the others, the system "thinks" the wheel is locked up, and pulses the brakes. So, think....If all four wheels lock up at the same time.... (as on ice) the computer thinks nothing is wrong. So it doesnt pulsalte the brakes, they just lock up! This is just my theory, im not sure, just makes since to me.
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Old 01-02-2003, 12:32 AM
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I also believe that their is a threshold. For my Chevy Astro it is around 15 MPH, but it may be only activated in forward. Once the threshold is meet then the ABS will work in reverse.

When backing out of a garage, the minimum threshold is not meet since the car is not traveling fast enough. I do not know the minimum threshold for Mercedes or if the threshold can be activated in reverse.
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  #9  
Old 01-02-2003, 03:52 PM
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My ABS definitely engaged while reversing down my icy driveway last week. I don't know about any speed threshold, but I'm sure I was doing well below 5mph as I knew it was going to be slick. It worked too - it stopped me clean so I could then adjust my course around the ice patch (which led down into a rock wall!)
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