1991 Mercury Sable Transmission Problems

This is most likely a speed sensor problem. 1991 was the first year for the AXOD-E (electronically controlled AXOD). Trouble codes for the transmission do NOT light up the "check engine" light. You need to have someone check the trouble code(s). Since this is an OBD1 car, you may have to look around for the tools or people to read the codes, but it is a better way to diagnose than by changing parts.