FIRST QUESTION ??? DOES THE ABS LIGHT GO OUT AFTER STARTING?? IF THE LIGHT DOES GO OUT & COME BACK ON W/DRIVING THEN AT LEAST ONE OF THE WHEEL SPEED SENSORS AREN'T BEING "READ" BY THE COMPUTER. CHECK RESISTANCE W/OHM METER AT EACH CONNECTOR.
IF THE LIGHT DOESN'T GO OUT AFTER STARTING YOU PROBALLY HAVE A VOLTAGE SUPPLY PROBLEM.