1993 Crown Vic ( Check Engine Light on)

It is likely that yur vehicle speed sensor has failed preventing the ECM from knowing how fast your are traveling. This not only screws the speedometer but, the transmission, the engine emission controls, the antilock brakes and everything else that needs a speed reference. Have the fault codes read from the ECM and go from there. Once this is properly repaired, you will notice a major improvement in other vehicle performance areas. Until this is done, the light will remain on and your vehicle is limping along using more fuel, emitting more polution and a general PITA to drive.