mmarshall

I've seen a number of Internet complaints (though not many here on CAR CHAT) about a noise that occurs at low speeds after start-up. Typically, It will occur after the car has been parked awhile or sitting overnight. Upon start-up, shifting into gear, and taking off, you'll hear either a dull muffled-clunk or a grating sound like you've just run over an aluminum soft-drink can. (I myself have owned a few cars that did this). The sound usually just occurs once during each driving cycle, lasts only a split-second, then is gone.

This, apparantly, is normal, and does not indicate any problem (though I wondered about it myself a few times). I did some research, and, from what I could find, that is partly how the ABS computer works. On an initial start-up after the car has being shut down awhile, the ABS computer does a self-test at low speed, just for a split-second........usually around 10-12 MPH, though the speed may vary by manufacturer. So, if you hear a light crunching noise at that speed every morning on first pull-away, just consider that as a routine part of a very important safety system in today's cars. You don't necessarily have to run to the service-shop to have it checked, especially if the yellow ABS warning-light doesn't stay on indicating a computer-malfunction. The ABS light and other warning lights, of course, (usually displayed inside the speedometer or tach), will normally come on for a few seconds when you first turn on the igniton, but then go off after the engine starts, indicating that those systems are OK.

Of course, the more silent the car otherwise runs, the more likely you will be to hear it. You will probably be more likely to hear it, of course, in an otherwise tomb-quiet car like a Lexus LS or Mercedes S-Class than, say, driving off with the top down in a buzzy Mazda Miata or Honda S2000. But, of course, on the other hand, the sound insulation in the LS or S-Class may be so effective that it muffles out even the ABS-computer-test, too.

BertL

I'm pretty particular on the details and have not heard such a start up clunking noise in my existing or previous Lexus, BMW or Acura's -- including my SC and 335i starting up with the top down in the garage or outside. There are of course the odd sounds after turning off my hybrids from time-to-time that are to be expected, but that isn't your point. I honestly can't remember back to the Toyota and Fords I owned in the 80's and before in what some may consider the dark ages.

bitkahuna

iTrader: (20)

yup, abs pump priming can do that.

lucasb7

It is pretty noticeable in the RX. It's a sort of high pitch sounds exactly like what it is, a very quick actuation of something. Not so coincidentally, you can hear the same noise when ABS & VSC activate (which is especially easy to hear when driving slowly on snow).

I can't hear the noise on the GS or MDX, though. The GS, however makes some strange hybrid noises. For example, you can faintly hear the brake booster priming when you open the door. And after shutdown you'll hear the evap. pumps running various functions for several hours.

mmarshall

Toyota and Fords of that vintage (80s) didn't have factory-designed/installed ABS to start with, so they wouldn't be an issue. Modern electronic ABS, as I remember, was first used in luxury/premium-grade vehicles (specifically Mercedes, Volvo, BMW, Audi and Cadillac) starting in the late 1980s, with traction/stability systems, which use ABS components, coming a few years later. These manufacturers shared the technology needed to first develop modern ABS.

BTW, I usually drive carefully enough, especially on slick roads, that I don't need the ABS for real. But, a couple of times, on rare occasions, it has helped prevent an accident when someone has pulled out in front of me, on a wet surface, at the last second.

swajames

What you're describing sounds like light oxidation on the brake rotors (if the wheels were wet or there was overnighr moisture the rotors can lightly oxidize). The clunk and grating may well be the brakes and rotors breaking the light layer of rust and it all wears off the minute you use the brakes. Can happen on pretty much any car with iron rotors which has been parked a while or where the wheels/rotors were exposed to moisture. If you wash the car the rotors lightly oxidize in minutes, it really doesn't take much.

mmarshall

The difference will be that, with the one-time split-second grinding on the test-cycle, you probably won't see a light (a light generally doesn't cover that function). But, when the ABS is being activated for real (such as on the snow or ice you mention) the vibrations/gringing noise will last (and a dash-light will stay on) as long as it takes for the tires to regain normal braking-traction. This ABS-use-light, of course, is not the ABS-malfunction-light, which is on a different circuit, and comes on at the start-up ignition-check or to indicate a malfunction.

Even with an ABS-malfunction-light on, of course, you won't necessarily have lost your regular service-brakes, which are on a dual safety system with cross-hydraulic brake-lines and a split-master-cylinder. You just won't have anti-lock protection, that's all. If the brake fluid leaks out of one side of the master-cylinder, you will still have a functioning brake on one front wheel and on the opposite rear one. That's to minmize the chances of the car being unstable or veering to one side.

mmarshall

Apples and oranges. There's a difference. What you say is true, but it has nothing to do with ABS, or what I'm actually describing here. As you note, you do get some scraping/grinding sounds with the first couple of brake-applications after you have, say, washed the car, or had it sit in in damp weather. But that only happens in damp conditions, after the rotors get a light layer of rust, and it takes the pads a couple of applications to scrub off the rust and get the brakes back to normal. And, with wet/rusted rotors, the sound is continuous as long as the brake-pedal is depresssed (until the rust wears off)....whereas, with the condition I'm describing in this thread, it comes independently, dry rotors or wet, only lasts once for a split-second, and happens whether your foot is on the brake or not.

rld14

Oh sure, my Ford and Lincoln make all sorts of clunking noises

GiantsFan

iTrader: (9)

Hmm I've never noticed this on my IS250...