You have a abs fault in the abs system(probably a wheel speed sensor). the system will have to be scanned for codes to find out exactly what the fault is,a scanner with data strem capabilites is required to do this,if you have a auto zone parts store near you they will scan the computor for codes although i'm not sure if they can scan abs systems,they can scan other codes,give it a try.if you dont have one near by then a good mechanic can this for you.
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