Locking Up Brakes with ABS

Quick question. Last weekend on the freeway I was under full acceleration passing a big truck, probably going 80ish+. He started coming over into my lane and I hit my brakes and dove for the left shoulder.

I locked up my brakes momentarily (you know - loud sqealing tires), longer than I thought was possible with ABS. Right after that I was off the brake altogether (and back on the gas) so I never felt the ABS come on (pulsating pedal). I have no ABS idiot light on so I assume the system is working. Has anybody else experienced this or is it impossible to lock it up with ABS?

TIA

Randy
'94 E500

[yal]

Do the rears of the E500 have ABS? If they don't then maybe that was it. Also isn't it possible to overide ABS in a panic stop (hard hard braking especially on high friction surfaces)? Anybody?

[Edited by yal on 07-27-2001 at 05:47 PM]

[David C Klasse]

No,
The ABS will come on any time a wheel begins to lose traction. There is no override other than disabling the system.
If you locked up the brakes... then it seems that your ABS wasn't operating. Could you tell if it was just the rear wheels? All wheels? Not sure? I'm pretty convinced that an 94 E500 has 4 wheel ABS. Maybe your abs sensors need to be cleaned. (If they were dirty though, I would think that would make the ABS light come on...?)

[intruder]

If you're worried about it, try a panic stop on a slick (wet or dirty or icey) surface in a safe area like a parking lot or a country road and see if you get pedal pulses to confirm that the ABS system is working.

At 80+ mph in a panic stop you may "initially" exceed the capability of the ABS system. Remember, it must sense an impending or actual lockup before it modulates brake pressure. At high speeds this may mean you get some intermittent lock-up. I used to take my corvette up to 100-110mph and STOMP on the brakes for fun; the wheels would intermittently lock-up and release until about 80mph. I can get my 1998 E320 to do what you described at 40mph on dry roads if I stomp down fast enough and hard enough. It's normal; at least my mechanic says so. He also said that below about about 12mph (20kph) that the ABS system may not work well, if at all.

What you have sounds normal. I used to fly combat military jets and their anti-skid systems were very similar, in that the "ABS" sensor at each wheel can only "read" so fast. At higher wheel speeds this means that the tire covers more ground between each "read" cycle and was more likely to lockup. Our cars work similarly.

Hope that helps.

Intruder

[David C Klasse]

Intruder,
That's a very good point. Randall, do what he suggests and stomp on the brakes in a safe place going 50 or so and see what happens.

BTW, ABS will work once passed 5 mph, and will work down to 3 mph. But you must first be a going a minimum of 5 mph for them to work at all. And then below 3 mph, they will not operate.

Thanks for the input guys. I need to do a test next time I have the car out in the rain (which may be a while here in Dallas) or find a sprinklered parking lot.

Last night I got the brakes to lock up momentarily from about 50MPH but I didn't brake long/hard enough to be able to tell if the ABS came on. It was while I was slowing down to turn onto my street. After I turned, there was a cop one house down getting out of his car and I just sheepishly drove by and turned into my driveway. What are the odds!

Randy
'94 E500

Okay, I'll ask the obvious...Does the ABS light come on with the key and then go out when you start the car?
Regards,
Randy D.

Yes. All warning lights work as designed. I'm thinking (although I have no basis for it) that in a real panic situation they momentarily lock up but then go into ABS mode. One of two things is probably happening: 1) I'm not on the brakes long enough to tell if the ABS is working, or 2) the ABS is working and I can't tell. I need to do the wet parking lot test.

The locked brakes situation is just momentary, enought to hear a short squeal (more than a chirp). But, when I was on the freeway I had no problem steering to the left shoulder as far as I could get over without leaving the pavement. I had total control. I'm sure I had my foot on the brake the whole time until the truck swerved back into his lane. Does the ABS in a '94 Mercedes cause the pedal to pulse?

Randy
'94 E500

If the ABS is working there shouldn't be any doubt at all. The brake pedal will be "hopping" under your foot. It would be a good idea to try either a gravel road or a grassy field to see what it does.
Randy D.

[bobbyv]

one way to test whether the ABS is working is the following:

* jack up the rears and place them on jackstands. Make sure they are VERY stable. Make sure the fronts are properly secured so that they do not move.

* start your car and run it through the gears until it reaches a certain speed on the speedo.

* apply the brakes. Since the fronts are not rotating, the ABS will be activated (i.e., the system will be fooled), and you will feel the pedal pulsing. I believe it will be pulsing for the fronts, because the system thinks the fronts are skidding.

Again, make sure that the car is jacked very securely before attempting this.

[Michael]

I've had occasion to use my ABS a lot (track events, but quite a few times on the road as well), and never has the ABS failed to prevent a tire, any tire, from locking for more than a brief moment

Sounds weird...try out some of the tests mentioned here, or do what I do-bring it up to speed then simply mash the brakes. You'll know if you have ABS I assure you. However, presuming your ABS might be inoperative, be prepared to slide a bit when you nail the brakes...be careful