Is my ABS working properly?

I haven't had a lot of experience driving a car with ABS in the snow until this week and I noticed one really weird thing with my car. The car in question is a 2001 Jaguar XJR with all season tires, the ones on the rear are two years newer than the ones on the front (thank you Detroit potholes). This week I was driving down a slight incline (a few degrees) on a snow/ice covered road at about 20mph. I hit the brakes and the ABS kicked in. I kept my foot hard on the brake and the car quickly slowed to a couple of MPH with the ABS pulsing, but then the pulsing stopped and the car continued sliding very slowly into the oncoming lane (the road was deserted, thank goodness). After a few seconds of very slow sliding (a person walking on the sidewalk could have kept up) I realized that I was soon going to very slowly hit the curb and that while the road was now a little steeper I wasn't speeding up. I figured that because I was going so slowly the ABS computer had simply locked up all 4 wheels and I was sliding on ice. I decided to take my foot off the brake so the front tires would starting turning again so I might be able to steer enough to avoid the curb and allow me to aim for the clear patches ahead. I didn't want the car to accelerate though, so I put the car in neutral. As soon as I put the car in neutral, it stopped. I took my foot off the brake, steered back into my lane and slowly went down the rest of the hill with no problems with the car in neutral. Being a glutton for punishment, I decided to try it again. I drove back up to the same road and slammed on the brakes and the same thing happened - the car slowed to a couple MPH then slid very slowly until I put it in neutral when it immediately stopped.

So what I figure is happening is that when I get below a few MPH the ABS computer locks up the front brakes, but it doesn't lock up the rears so the engine keep turning the rear wheels, pushing the car along despite the fronts being lock. Is this normal? The ABS works great until 2MPH and there's no idiot light telling me my brakes are screwed up, but it's a Jag, so I don't completely trust the electronics regardless of what JD Power says.

Should I have my ABS checked out? Is this normal for high powered RWD cars with ABS in icy weather? Should Jaguar's calibrators spend more time outside the UK? Should I buy a Jeep Cherokee, throw a turbo on it and have two different XJRs - one for summer and one for winter?

Any feedback is appreciated.

Bob

The only way the ABS system knows that your vehicle is moving is by reading the speed of the wheel sensors.

When these sensors show speeds that vary by more than a certain amount under braking, the ABS assumes some of them have less traction than others and starts pulsing them.

When all wheels come to a stop, the system disengages because it thinks you are at a standstill (it probably computes avg speed off the sensors and sees if it's close to 0 or something like that). If you then continue to move, the ABS system has no way to know this.

No car is high powered enough to push through the brakes at idle, so it has nothing to do with power.

Also, if your foot is on the brake pedal, your engine should not be able to power through the rear brakes until 50+% throttle (like a brakestand burnout, no different). The ABS has nothing to do with this, or.. it SHOULD not.

I dont know if your ABS system is acting up or not. The ABS system, as far as i know, should never 'defeat' your normal brake function and make your rear brakes not grab when you are at a stop. One easy way to find out would be to find a safe place and pull the ABS fuse / unplug the motor, and see if you have your rear brakes again.

Sad to say, I think that is normal in a rear drive vehicle- the idling engine will spin the rears wheels on glare ice, so the computer thinks the fronts are locking up, so it turns off the fronts. Your selection of neutral was the right choice. Eventually, you use up all your abs capacity and vacuum boost, and you can't push hard enough to stop. I generally disable the ABS on anything RWD as soon as it starts to snow. Truly terrifying- GM seems the worst of all, early Astro vans were death traps due to ABS.

But thats just my opinion, I could be wrong.

Streetwiseguy wrote: I generally disable the ABS on anything RWD as soon as it starts to snow. Truly terrifying- GM seems the worst of all, early Astro vans were death traps due to ABS. But thats just my opinion, I could be wrong.

could it be that the early astro vans had a cheap/crappy non-four wheel ABS system? I've driven alot of Ford vans from the ninties that had a two or possibly three channel ABS system and they were crash machines too. Once the fronts locked up the ABS system seemed to be convinced that the vans was stopped and the rears would do absolutly nothing, even if you were stil going 25mph down an icy hill. When you are driving a fully loaded cargo van with several thousand pounds over the rear axel its not hard to lock the fronts up, even on dry pavement.

Whoa man, on my RWD Volvo, my ABS rocks. I never liked ABS until I drove on icy junk with it. It rocks.

Once the wheels stop turning the ABS stops working. All season tires on ice and snow The theory on those rear wheel only systems was to keep the vehicle from spinning. Of course at the same time you couldn't steer since the fronts were locked up. Sheer Genius.

I have a 96 dakota that SUPPOSEDLY has rear-wheel ABS.

Of course, in 10+ years i have never verified that it works because you would have to lock up the fronts and THEN stand on the brake pedal much harder to get the rears to lock up in the first place.. and since i dont like flat-spotted tires i've never tried.

The ABS on the miata worked just fine today.

Yesterday, I learned that the Fit has an alarm that goes off if you do a handbrake turn. Seems like if you are moving at anything over a very slow crawl with the e-brake on it alarms. Startled me as I was playing in the parking lot snow.

I was playing around with the ABS on the mrs 325XiT and it's damn near flawless. Smooth, controlled stops - I even tried to mess with it by keeping two wheels on clear pave and two on packed snow in a turn. No drama at all. Uncanny. No matter how rough I was on the pedal it would not get out of shape.

It takes all the fun out of a snowstorm.

Lucky for me I have a 7000lb, 650 ft/lb of torque duramax 2500HD with an empty bed and LRR highway tires to make my hair go white. It certainly does hone one's think-ahead organ.

Speaking of ABS, yesterday in the snow, about halfway to work, my G20s ABS light came on, letting me know that using the brakes in the snow would mean instant sideways. Thing is, its never done that before, and the ABS actually kicked in twice that morning before it took a dump. I pulled over, shut the car down, did the ignition on/off without starting the car x3 to reset the warning lights. Sure enough, upon restart, the ABS light went out, and ABS works again. What can cause it to do that? Vacuum leak? 12 year old pump dying on me? Im not familiar enough with ABS, because, as with others, this is my first DD with ABS, so Im a little ill equipped to diagnose this singular episode.

4cylndrfury wrote: Speaking of ABS, yesterday in the snow, about halfway to work, my G20s ABS light came on,

Start simple. Wheel sensor? Its fine until you start moving - then it gets confused by faulty/no signal?

Check them and the encoder wheel in the hub for corrosion.

Check resistance of the sensor with a digital ohm meter. I don't know the spec on yours but they should all be close to the same. While the ohm meter is hooked up move the front wheels lock to lock and see if you can get an open reading. Next pull and tug on the wire and see if it goes open. I usually give the wire a good tug figuring if I can bugger it up it was weak and as a safety item I want to find any weakness.

I had almost the same thing happen in my Subaru last year. It was 100% caused by my tires in my opinion. Worn out all seasons on solid ice on a slope = slide.

i won't post my resume here, but i will say that i've been a brake systems engineer in Detroit since 1993, primarily on ABS and Traction Control systems.

I did not work on the Jaguar system, but all systems work on the same basic principles, within the same laws of physics.

to the OP's situation, ie downhill (albeit not a steep grade), with 2 years less tread depth on the front tires compared to the rear, I will speculate that the shallower tread depth on the front contributed to the fronts engaging ABS at a pressure lower than they would have if all four tires were the same. In other words, the fronts had done all the braking they could do without ABS intervention, but the rears had not. When you felt the ABS pulsing, you assumed it was active on all four corners when it was really on the fronts only, so you didn't push the pedal any harder.

In this situation, it is possible that the combination of gravity (pulling the car downhill) plus drive torque (yes, even at idle), was enough to keep the car moving downhill.

to address the individual replies:

Vigo, your analogy of a brakestand burnout is pretty close, but must be adjusted to consider how little brake pedal force is required to activate ABS on a slippery downhill with less-than-great front tires.

Streetwiseguy, (1) he wasn't on glare ice, (2) he still had his foot on the brake, and (3) he was on a downhill grade. As far as the Astro = Deathtrap comment, perhaps you could cite some reputable numbers linking Astro ABS performance to crashes? And a question for you: How do you "use up all your ABS capacity"?

To HappyAndy and Vigo, rear-wheel-only ABS (aka RWAL or RABS) does not monitor, control, or depend upon the front brakes to properly control the rear brakes. I would suggest to Vigo that you've probably had your RWAL system activate a lot more frequently than you think, but because that system does not have a pump you did not receive the traditional ABS feedback through your brake pedal foot or through your ears. These systems do not wait until the front brakes have locked up before they activate. And I would suggest to HappyAndy that in his example of a very tail-heavy vehicle, once the fronts locked up the driver could have still pushed the pedal harder to get more rear brake output. Up to the point that the pedal bottoms on the floor, the master bottoms out, or the rear brakes just aren't big enough to force the RWAL / RABS to cycle with that much normal force on them.

To everyone hating on rear-wheel ABS, I will say that a properly functioning RWAL / RABS system from model year 1998 and newer never limits pressure to the rear wheels unless and until and only during the time that it has seen (or still sees) wheel slip on the rear wheels greater than what the surface estimation algorithm says the surface can support. I can't go deeper than that in public.

Similiar issue in my 740T years ago. All wheels were locked up, so ABS quit working. Bounced off a few snow banks and a parked car. Nuetral, Drive, gas, brake...nothing would stop my gravitational pull down the hill. But I'm from SoCal, so my snow/ice skills have a lot to work on.

I think AngryCorvair probably summed up your situation.

If there is no friction at the tires, you'll keep going no matter what. 've seen a car in gear with the emergency brakes on slide down a hill. Likewise, on wet, smooth ice, I've hit the brakes on an ABS equipped car and not slowed down at all, since the brake force was enough to lock up the wheels pretty much in an instant.

(ABS RANT) The ABS in my Forester XT simply stinks if I am driving on a bumpy surface (like in a rallycross or off road). I don't know why, but if there are bumps during braking, the ABS pulses rapidly and the car doesn't slow down. When I know I'm going to be driving on terrain like that I just pull the fuse. Locking up the wheels and sliding a bit and having to use my driving skills beats hitting a tree. (ABS RANT OFF)

Probably because when going over the bumps and braking, the wheel briefly stops rotating, which is the same as locking up, so the ABS kicks in and prevents the other wheels from locking up. On a slippery road, it could take a Long time to stop.

Perhaps the ABS hates rough surfaces because the wheels come off the ground momentarily and the ABS sensors think that tire is locking up?

EDIT: kb wins.

Angry,

Thanks for providing some expert feedback. I've got a 98 Frontier, and I'll have to say that the ABS performance is pretty excellent for an old truck with rear drums. I do notice that when ABS engages, there is a big power draw...the headlamps and interior lights dim briefly. This always makes me wonder: how would having a weak battery/alternator affect ABS performance?

Sorry for the threadjack.

Thanks AngryCorvair (I was pretty sure you were a brake engineer but wasn't positive) and everyone else for the answers. I'm used to a FWD car with snow tires on fairly flat ground, so this RWD on all season tires in the mountains of North Carolina is a little new to me. I was actually just watching the news and they showed a Jeep doing the exact same thing that I experienced. It was inching along on an icy road and the fronts would lock up while the rears kept turning, then the driver took his foot off the brakes and the fronts started turning again, then the brake lights came on again, the fronts locked and the rears kept turning.

I wish there was some way to datalog what my ABS is doing, just because I think it would be cool to see how quickly it reacts on slippery roads. Does anyone know if any tools are available that can do that (besides OEM development or high-end service tools)?

Thanks again, Bob

Schmidlap wrote: I wish there was some way to datalog what my ABS is doing, just because I think it would be cool to see how quickly it reacts on slippery roads. Does anyone know if any tools are available that can do that (besides OEM development or high-end service tools)?

you might be able to get an OBD2 scan tool that would tell you the individual wheel speeds are at each corner, but I'm almost 100% confident that you will not be able to see actual ABS control signals, as (in my experience) they're not broadcast on the vehicle network.

kb58 wrote: Probably because when going over the bumps and braking, the wheel briefly stops rotating, which is the same as locking up, so the ABS kicks in and prevents the other wheels from locking up. On a slippery road, it could take a Long time to stop.

You are right about the momentary lockups due to the wheel losing contact with the bumpy surface. But ABS controls each wheel independently, so it will only reduce fluid pressure to the wheel which has momentarily locked up. On a bumpy surface, that will inevitably be more than one, which is why the deceleration feels weak.

An algorithm could be implemented that would look at the rapid and random changes in wheel speed that characterize rough road braking, and could then alter some of the control strategy to allow better braking on rough surfaces (ie perhaps allow longer delay before dumping pressure). Like any performance situation, there are tradeoffs which must be made, and the combination of attributes that makes a Sedan DeVille driver happy will probably bum out an STi driver, at least occasionally.

In reply to AngryCorvair: That is the best description I've seen on ABS in laymans terms that I've seen, Thanks, learned from that and describes what I've seen. I've never locked up the rears on my 95 F-150 w/rear only ABS however it isn't difficult to lock the fronts on dry pavement just by pumping the brakes a couple times. Love the ABS on my RX8 and my wifes 2010 Sonata (even though you probably didn't have anything to do with them ) and the ESC/DSC. Came in very handy with the near record snowfall we've had here. Took a while to get used to the ESC & ABS on the RX8 though, it will apply brakes to the slipping wheel. Discerning till I figured out what was happening. Have to turn ESC off to get any noticeable wheel spin to play in the snow.

AngryCorvair wrote: You are right about the momentary lockups due to the wheel losing contact with the bumpy surface. But ABS controls each wheel independently, so it will only reduce fluid pressure to the wheel which has momentarily locked up. On a bumpy surface, that will inevitably be more than one, which is why the deceleration feels weak. An algorithm could be implemented that would look at the rapid and random changes in wheel speed that characterize rough road braking, and could then alter some of the control strategy to allow better braking on rough surfaces (ie perhaps allow longer delay before dumping pressure). Like any performance situation, there are tradeoffs which must be made, and the combination of attributes that makes a Sedan DeVille driver happy will probably bum out an STi driver, at least occasionally.

Good points! As I recall, some "real" cars (ws it Porsche?) have a disable button for the ABS, with the understanding that under certain conditions (snow and mud) it's actually better to have the wheels lock, so that said snow or mud piles up in front of the tires and helps it slow faster. Otherwise, on a slick surface, the car could go literally 100s of feet, dutifully keeping the tires from fully locking.

Subaru definitely has a weird setup that turns a little slip at the very end of moderate braking into a 6 ft jump into the scenery.