Thread: ABS Variables?
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Old 04-16-2002, 08:40 PM   #18
romanom
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Join Date: Mar 2002
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2002 WR WRX wagon

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Quote:
Originally posted by Zak
The WRX system is not as "dumb" as stated here. IMO that might be exactly why we experience the problem. The WRX incorporates a G sensor that monitors vehicle deceleration. I don't know if anyone has found out how the ABS computer actually uses this information. One post mentioned that the ABS computer calculates what the vehicle speed should be based off the G sensor and then compares it to the wheel speed sensors. This would cover the situation of all wheels skidding in snow/ice/etc. Most G sensors I have seen are very touchy when subjected to bumps and tend to oscillate a bit. If the ABS computer relies on this calculated speed for anything, the computer is going to freak out when the G sensor gets any sharp bumps unless there is very good filtering of the signal. IMO, this may be why the "no brakes" situation only happens under very specific conditions. If the ABS computer software did not take into account one specific set of driving conditions, the computer may not know what to do or at the least may do the wrong thing. This seems to fit the experience that lifting and reapplying the brakes usually solves the problem.

Zak
Bumps would not effect a long-A sensor as it doesn't care what is happening in the vertical. But long-A sensors are usually only used with AWD AND ESP systems.

The long-a maybe used by the AWD system, but it wouldn't be worth the extra (high) cost to have one just for ABS. ABS doesn't really need it.

ABS only works with your foot on the brake, so all it needs is you braking and wheel locked up. Know anything else would be useless to an ABS only ECU. It's not as if ABS can self activate like traction control or ESP.

Long-A are used to distribute power on AWD drive cars. So the Subaru probably has one, but it's used to figure out where to send engine power.

Now if the Impreza had a full ESP system, one that acts on torque delivery and can activate the brakes without driver input then a long-A (and a bunch of other sensors) would be of use. And long-A sensor is almost always used on AWD cars with ESP as it is possible for an AWD to be sliding while all 4 wheels are acting normally in relation to each other because all 4 are being powered. On two-wheel drive this is not possible because as soon as the non-drive wheels lose traction they stop spinning.

The other sensors in an ESP are also useless to an ABS only ECU, lateral-g, yaw rate, steering wheel angle, throttle body position, brake pedal position, and on some a ride height sensor.
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Last edited by romanom; 04-16-2002 at 09:29 PM.
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