meilers

I've got an 02 WRX and it has the infamous ABS problem where the anti-lock is triggered by light braking on an uneven/bumpy surface. However, I've noticed something disturbing the last time this happened: it took me over twice the distance to stop with the ABS going off (shudder, crunch) vs. stopping without the ABS. I do the exact same commute every day, and have done so for 3 years; I must have pulled up to that light over 1,000 times so far, so I know when to brake for it. Normally I can stop without an issue, but a few days ago the ABS went off and suddenly I found myself with the entire nose of the car hanging out into the intersection.

I've changed the wheels (17x7), tires (Goodyear Eagles), pads (Hawk HPS) and rotors (DBA 5000 2-piece), so that's got to be pretty damn confusing to the wheel-speed sensors that trigger the ABS. But why would it take the car so much longer to stop? It makes me want to pull the fuse and disable the ABS; yeah, I might lock up the wheels and slide, but at least I'll be able to control when that happens vs. the ABS just taking away all brake feel and modulation...

Wingless Wonder

iTrader: (7)

Like most early GD owners, I'm also familiar with the hop-bump-skip actuation of the ABS under certain conditions.

Was your braking distance ever any better after the tire/wheel and brake parts upgrades? Has there been any gradual falloff of stopping ability or has it been consistent?

After upgrading to better tires, pads, fluid, and rotors, my overall braking is better but I still get the usual hair-trigger ABS actuation when braking over bumpy surfaces or manhole covers or the tiniest bit of gravel, although it doesn't seem any worse than before the changes.

meilers

After reading my post I realize I wasn't doing a very good job describing what was going on. OK: I brake to a stop at the stoplight WITHOUT the ABS going off, I stop in a normal, reasonable distance. I brake to a stop at the same light (with the same speed coming up to the intersection) and ABS goes off, and suddenly it takes me twice the distance to stop. In either case I never came close to my friction limit (no screeching tires). So, all things being equal, when the ABS goes off my stopping distance increases, rather than decreases.

I'm just wondering if that is to be expected, or whether I need to be worried about problems with the ABS system (for example a bad wheel-speed sensor) itself.

Wingless Wonder

iTrader: (7)

I'm not sure if Subaru considers this to be normal behavior for ABS, but this has been my experience with my WRX. Unlike other cars I read about in test reviews, my car's stopping distance greatly increases once the ABS kicks in. All it takes is to brake moderately hard over just a momentary bump in the road surface. The ABS stays activated for several moments. I think that most people are able to modulate their braking in a situation like that better than the factory ABS, at least in a straight line on dry pavement.

Did you ever take up SOA's offer of a replacement ABS unit for our model year, supposedly with revised calibration to address this very issue? I had already modified my brakes and did not approach the dealer for a new ABS unit since the bulletin specified the free replacement was intended for cars with unmodified brake systems.

meilers

Ah, well at least I'm not imagining things!

I don't know about the recall part... I took my car to the dealership for oil changes (only $35 for Castrol Syntec, and they rotate tires and check fluids too) for the first 40k and they've done recall work on my car without even asking me first (seat rail bolts, etc.) so it is possible they pulled the ABS unit. We went through two wheel speed sensors in the Legacy GT in two years; that's not really relevant (it was a '98) but it shows that ABS bugs aren't unique to the Impreza.

I'm wondering what causes the ABS to go off prematurely. I keep my front tires at around 36psi, and I have stiffer springs as well, so I have changed almost every factor having to do with front traction except the front endlinks (Whiteline 20mm front swaybar as well). The wheel speed sensor is looking for a significant difference in the rotation speed of the wheel as compared to the forward speed of the car (a much slower speed, as in the wheel isn't turning when it should be) before it goes off. Despite all the changes my wheel diameter is exactly the same as the factory wheels, so the rotation speed of the wheel is still factory stock... I'm just thinking aloud here, but I wonder if there is something I can do to minimize the problem? Ending up 1/3 of the way into a busy intersection was NOT my idea of fun.

In response to your original question, my rotor/pad/wheel/tire upgrades GREATLY improved my stopping distance and pedal feel, although slotted rotors still feel odd when braking compared to smooth-surface rotors. My pads are in very good shape (as checked last oil change) so it has to be the ABS that is the factor here.

Have any of the 08 WRX reviews mentioned the stopping distance (60-0 test)? It would be interesting to go find an empty parking lot and see how I do in comparison.

Wingless Wonder

iTrader: (7)

The dealer may or may not have replaced your ABS unit. It wasn't a recall, just some sort of service notice to dealerships (and possibly Impreza owners) that offered dealer replacement of the ABS unit for a unit with different calibration:

https://www.i-club.com/forums/suby-shopping-maintenance-warranty-92/02-03-abs-issues-warranty-fix-110318/

I did not get one of these mailed notices because the dealership had incorrect mailing address on file and did not do the correction as I requested.

Other than running the stickiest tire for given conditions, it seems like the slightest bump in the road under moderate to hard braking may trigger the ABS into much longer stops. I think that what is happening is that the ABS unit is overly sensitive to minute changes in speed between the different wheels on the car and takes over modulation prematurely (in my opinion). I haven't yet installed my aftermarket springs so I don't know what effect lowering and a stiffer spring rate will have on ABS response.

There may have been some complete road test reviews with braking distances for the 2008 WRX but I have not seen them.