I own a '97 Pontiac Sunfire GT. In the past year or more, I have had some problems with the anti-lock brakes. They have always been overly sensitive (kicking in if I just press down on them a little too hard.) However, lately whenever I hit a bump, the car really bounces. And when I press on the brakes while hitting a bump, the anti-lock brakes kick in no matter how lightly I press on them.This is EXTREMELY annoying. I don't think the shocks or struts are bad, but I am not sure how to check. Also, is there anyway to decrease the sensitivity of the brakes themselves?

Defective sensors, I would take it back and get it sorted.

The abs computer senses when wheels lock up, so when you hit a bump the car will bounce, taking weight off of the wheels which would allow them to lock up more easily. just a theory.

The sensitivity of the brakes can be adjusted, but I don't know how to do it. I would take it to a shop.

Where are these sensors? Any idea of how much it would cost?

I would say from what you are saying a damper might of given up meaning one of the wheels is pattering (bouncing up and down) meaning the ABS kicks in early. The senors themselves are either a sensor counting the ends of the wheel bolts going past or more commoningly a toothed wheel on each hub with an inductive sensor next to it.

Clean up the toothed wheels but try not to move the inductive sensor as they are distance critical.

As to tweaking the senstivity of the system that is as far as i know software controled But other things that might cause early lock up are, bad alignment, too high tyre pressure, crap tyres (nasty high milage ones).

To check the dampers it is more experience etc but bouce the car at each corner and see what happens if the car wobbles alot it might be a screwed damper. also the handling might be a bit on the [censored] side causing it to bottom out early and other nastys.

If you feel brave you could on a quite road pull the ABS fuses and try the brakes and see what happens if it locks early or pulls or some other nasty just do it at low speed and be ready for something nasty to happen. I would try that but i am used to driving vechiles that do strange things after owning some real garbage.

There will be a brake line pressure sensor somewhere in your system, probably close to (If not on) the master cylinder. The sensor will be cheap, the fitting should be 10 seconds. However, it's advisable to:

- get the brake fluid changed and the system bled
- have the brakes tested on a rig after replacement
- make sure all of your brake pads (front and rear) are OK

... which will cost according to the garage's hourly rate. The job should not be much more than about 1.5 hours altogether. Brake pads will depend on your model and manufacturer, but for my Renault I paid E55 per front brake and E 38 per rear (about E 190, or $170) for a set of pads that I fitted myself.

I think in any case it's worth paying the garage the fee to have a brake test done, which should be no more than 0.5 hours (max) and they should be able to plug into the ABS system and find out why it's apparently kicking in at relatively low braking effort/pressures. It sounds like a sensor defect to me.

There will be a brake line pressure sensor...

Wouldn't early ABSing be from a wheelspin sensor?

Depends on the system, to be honest. There's so many different systems out there trying to get around the original Bosch patent that there's no way to tell what is going to be on what car. The majority of systems look at system hydraulic pressure, and also at individual wheel speeds. Some look only at front wheel speeds. Wheel sensors are usually inductive (teeth on the hub or disk brake), hydraulic sensors are usually piezo or contact in the brake lines. There's usually a servo-actuated relief valve to drop the brake line pressure, and the CPU doing the AB looks at the line pressure to check the line pressure has been relieved, then goes to have another look at the wheel speed of the wheel it thought was in trouble. The system can usually be checked through a diagnostic at the garage. This would eliminate sensors and the CPU: it would not eliminate other failures such as glazed pads, contaminated pads, damaged (undamped) shocks, extreme wheel bearing wear, extreme alignment problems, etc. etc. etc.

Golden rule - tires or brakes, DON'T wait, get them checked!!!!!

I've just seen that Paul has said similar things in his answer, so sorry about the duplication.

Reading the post again it could be a pringled (warped) disc as that can give a similar feel to ABS kicking, another thing to look at

Wow, Pringles got verbed

Pull the main ABS fuse and be done with it. If you are a good driver you dont need ABS.

will that actually work?

will that actually work?

It should. But I'd have to disagree about you being better off without the antilock breaks. In theory, the best drivers will be able to stop faster, but with antilock breaks you maintain control of the vehicle which is often far more important. They just have to save your ass once to really make you a beleiver.

Matthew

I would definitely agree with Acurasquirrel regarding ABS - in winter. If you have any type of sporty car, chances are it has fat tyres. In winter ABS will make you slide and slide and slide....Saw a very amusing crash once - at all of 5 mph, sliding about 15 feet into a truck. Looked like a safe distance, but the ABS just kept the brakes off.

Without ABS, you can lock which helps braking in ice and snow.

Even in the summer, heavy braking while going over ruts can trigger the ABS which can seriously stuff your stopping distance, which can be very scary if you are trying high-g manoevres, so my advice would be if you are very confident with your car's handling, and drive pretty hard, kill the ABS - the ride will be much more predictable, and safer.

Sorry can't disagree more i love my ABS i can't stop as fast as it can on decent tarmac but in snow i agree no way does it work unless you jump on the pedal then you lock all four wheels and the ABS thinks "oh the cars stopped i'm doing nothing"

Modern ABS systems the main limiting factor is how hard you can push the pedal in a new merc some of them actually sense in an emergency stop and then increase the system pressure to max.

But check out my mechanical answers as one of those might give you the answer. The brakes might be warped giving the feeling of ABS but not actual ABS action.

i recently got new tires on my vehicle and it quit doing it for awhile. However, on the road home (a 3 hour journey), it started up again. Frustrating.....>:(

any idea on which fuse it is?

Not got a clue but should say somewhere what your fuses do but have you checked the other things we listed

Have you checked the shocks yet? If you wheel is becoming airborne after a bounce (whilst you're braking), that'd definitely cause your ABS system to be reacting as it has. When you replaced your tires, did you ask the technician if there were any strange wear patterns? A constantly bouncing wheel (which happens when your shock or strut is kaput) will wear unevenly and a good tire guy (or gal) can usually tell just by looking at it.

-Zeke

I keep coming back to this post, and telling myself to keep out of it, don't start a flame war -- but I just can't do it anymore.

Without ABS, you can lock which helps braking in ice and snow.


ARE YOU OUT OF YOUR EVER LOVIN' MIND??!

Having lived in Alaska for the last 48 years ("We have four seasons just like anyplace else -- June, July, August, and Winter") I daresay I have probably had more experience driving on ice and snow than anybody else on this bbs. I am ready to wager serious money: You bring the car of your choice without ABS to my neighborhood I'll bring my ABS equipped station wagon out and we'll have a little stopping contest. Assuming comparable tires (in terms of studs or lack thereof and rubber compound) I guarantee my car will stop in less distance than yours on snow and ice.

You lock your wheels up, and you might as well have your car riding on ice skates instead of tires. Accurasquirrel noted that skillful driving can help stop on ice, and he's right: if you are skillful enough, you can pump the brakes, keep the wheels right on the verge of lockup but never allow them to actually slide, and you'll stop pretty quick. But you can't pump your brakes upwards of 20-30 times per second the way my ABS computer can.

In a true panic stop situation (I'm talking about actuall pee-your-pants panic because that fuel oil truck just pulled out in front of you and it's ICY out!) where you aren't thinking calmly and rationally but instead just standing on the brake pedal, ABS will save your ass. The car will steer, and it will slow down faster than you could do it without ABS.

There is only one situation I know of where ABS might be disadvantageous, and that is in sand or loose gravel, where locked up wheels could actually stop you quicker because the sand/gravel will then pile up in front of the tires, aiding in the deceleration.

People who disparage antilock brakes are of the same mentality as the ones who back in the early 60's were claiming seat belts were dangerous because what if your car caught on fire, they would keep you from getting out quickly.

OK, let the flames begin....

tanstaafl.

I agree 100% with you. However I will grant that when ABS fires prematurely (think hitting a big pothole when braking hard on dry pavement to make that ramp you're almost missing at 80 mph) the brake effectiveness goes way, way down. This is really scary & unnerving. This has happened to me about twice in 15 years. I'd put the score at about 4598 ass savings to 2 frighenings.

-Zeke

I like the ABS on my Golf but I drove someones saturn once a while ago and I slid all the way across a street when trying to stop at a stop sign with the ABS pulsing all the way across. I don't think all ABS systems are created equally. I think sometimes it can engage to easily

I'd put the score at about 4598 ass savings to 2 frighenings.

You've had to have your ass saved by ABS brakes 4598 times?

Note to self: Never ride with Zeke...

Mine engaged the other day in my WRX when i barely touched the brakes while going down VERY steep hill in Sausalito at about 10mph. Scared the shite out of me since the braking seemed to barely work all of a sudden, and i was going down an extremely narrow steep road towards a turn. Never had that happen before.

when ABS fires prematurely (think hitting a big pothole when braking hard on dry pavement to make that ramp you're almost missing at 80 mph)

I'd argue that rather than your ABS firing prematurely that perhaps your brain cells have been firing late. If you're not with it enough to avoid a pothole then perhaps you shouldn't be driving that fast. If you're not with it enough to notice your exit early enough to make it safely then perhaps you shouldn't be driving at all.
Of course, I'd never do anything like that. But my point is that you should never expect your cars safety systems to dig you out of shit that you willingly and knowlingly put yourself into. So I make that 4598 for and 0 against.

Like other folks said, though, sometimes ABS seems to kick in for no reason at all. I've had this happen to me a number of times backing down one of my friend's driveways, which is fairly steep (30% grade or so), and not being able to stop is scary, even when it's a quiet neighborhood. This is on totally dry concrete.

The problem is not so much that the technique is invalid, but the implementations can occasionally leave something to be desired.

"If you're not with it enough to notice your exit early enough to make it safely then perhaps you shouldn't be driving at all. "

Perhaps you shouldn't wear a blue wig. Whatever! In a strange city running very tight for a meeting and realizing, 'oh crap, there's the exit' there's not a lot of time for deliberation. The pothole in question was also 4 feet wide, a bit tough to miss on a 1 lane exit ramp. I'm _sure_ nobody _here_ drives too fast somtimes. All I'm saying is that premature ABS firing is spooky as hell. [/ranting]

BTW: I drive 120 miles per day to/from work in central NH over a highway with a 1200' elevation gain/loss frequently over the rain/ice/snow line. It's a little slippery in winter time. I regularly rely on my safety systems (steering wheel, brake pedal and accelerator) to help me keep control of the car.

-Zeke

"Note to self: Never ride with Zeke... "


-Z

I have a feeling our opposed viewpoints here may be all down to the difference between weather in Scotland and weather in Alaska - (ie our snow and ice probably don't compare)

Certainly in typical winter conditions here I can stop way faster with ABS off when on ice and snow. On ice with ABS on I get zero traction, whereas with ABS off I can control direction and braking fairly well. On snow the difference is not quite so marked, as the ABS works a bit, but you are still looking at a couple of seconds difference stopping from 30mph.

It is distinct enough, and consistent enough that I always pull my ABS fuse in winter now. It's just too scary/dangerous otherwise. And believe me, I have tested this extensively, being a devout member of the sideways-driving-is-good religion!

Edit:

It could be down to a similar issue you mentioned with gravel " where locked up wheels could actually stop you quicker because the sand/gravel will then pile up in front of the tires, aiding in the deceleration." as the snow certainly piles up in front of wheels.

That wouldn't explain the improved deceleration on ice - which I'm tempted to believe is down to locking up melting the ice and droppping the tyre through to the surface of the road. Again, this maybe doesn't work on ice more than a few mm thick.