Antilock brakes engage while stopped?

[Earendil]

My buddies IX has an issue where the antilock brakes engage when he's at a standstill (pulsing/gritty pedal), but operating fine while moving. Any ideas would would be causing this? I'm aware a bad sensor can make the car think a wheel is stopped when it's not, but can a bad sensor make the car think it's moving when it's not?

I have zero experience with the ABS system, as mine has never worked.

[jlevie]

If the car was other than an IX I'd say that the ABS module is probably bad. But I seem to recall that an IX has a somewhat different ABS system, so it could be something else.

[nando]

accelermometer sensor broken/shorted? only thing I can think of. but it should throw a fault.

[Earendil]

For whatever reason I wasn't alerted to these replies. Sorry about the late reponse!

The system throws no faults, and continues to work just fine at speed. I didn't realize there was an accelerometer involved in the calculation, I thought it was just speed sensors on each wheel? Is the accelerometer located in antilock break computer, do you know?

[nando]

it's just behind the left front strut tower, near the fuse box. it's black with 2 shear bolts and a 2 pin connector.

[Earendil]

it's just behind the left front strut tower, near the fuse box. it's black with 2 shear bolts and a 2 pin connector.

Thanks! I'll look into how that works. Maybe it's shorting out or something. Or hanging by the wires and swinging back and forth inches off the ground ;-)

[ahab]

Don't want to hijack but since this thread is specific to the iX ABS system I'll risk asking a few questions instead of making another one. Plus the answers may help the OP with his problem. I'm troubleshooting an ABS light fault (goes out at start and lights after rolling) and have read a lot of posts here about the issue, so I'm suspecting the sensors. I also get a stiff pedal with pulsing and poor braking on or about the first application of the brakes, sometimes, which I've seen discussed too. Is the sensor test that jlevie has outlined many times applicable to the iX too or does the accelerometer invalidate that method? I've disconnected 3 sensors and the light still comes on after moving, does that still mean that sensor is good per the test? Also, assuming the test method works for this car I can't find where to disconnect the passenger front sensor so I can move on to individually testing the others. Finally, the sensors show as iX specific, is that a physical restriction (due to length or something) or is there a functional reason (resistance, performance, etc)? Same for the ABS computer, I suspect it HAS to be from an iX and not a regular E30?

Thanks in advance

[nando]

if working normally, the accelerometer is in a "neutral" state - IE, it doesn't affect anything. the standard sensor test should work, the differences in the ix sensors are simply the length, and the ix has 4 (plus the accelerometer) instead of 3. I don't think the accelerometer typically goes bad, but I could see the plug corroding or something.

It sounds like a bad sensor to me, in any case.

also - the accelerometer is really just a mercury switch (or works the same way). it doesn't give variable accel/deccel load like a modern accelerometer. But it still functions very well. I watched ~2005 Subaru slide down our hill with the brakes totally locked up. How the hell can a car that's supposed to be the best AWD/snow car on the market not have compensation for the ABS locking up on ice/snow? It doesn't make me feel very confident in our Subaru.. glad i have an ix too. :|