1995 Mercury Grand Marquis mileage: 124,000. The ABS system activates below 45mph with no warning lights and no error codes. The dealer could find no problem with sensors or sensor rings. Ford TSB says to check for poor or corroded connections which dealer says can take several hours of trial and error to track down. I know that one or both of the front rotors is warped, could this cause one of the sensors to give a false output at lower speeds? I am not convinced about the poor connection theory as that usually leads to erratic or sporadic problems and this problem seems to be speed related.
You may have a faulty ABS computer. We would obtain a known good computer and substitute it for the one in the car first to eliminate the computer factor. If the computer proves not to be the problem, the connectors would need to be checked for corrosion or high resistance. Warped routers would not cause the problem you described.