Probably the cheap black plastic odometer gears(2) if you have set in center. If you have odometer on the left and trip odometer on the right then it is two sets...four gears. Usually when one breaks the other is not far behind in the two set type. There are several online suppliers who can provide parts for the repair.
There is no fuse for the odometer. If it is mechanical and not working it most likely had a stripped or broken gear.
2004 mercury grand marquis fan stop working
troubleshoot head lamps on 1988 grand marquis? Not working...fuses OK
A grand marquis wiring diagram can be found in a Chiltons or Haynes auto repair manual.
tripometer doesn't work either.
i have no heat in my 1994 mercury grand marquis. what could cause the heat to stop working
Try Autozone.com - they very likely have an on-line illustrated repair guide for the '93 Grand Marquis.
According to my Chilton's Auto Repair manual : A 1988 Mercury Grand Marquis has a ( 20 U.S. gallon ) gas tank
According to my Chilton's Auto Repair manual : A ( 1987 ) Mercury Grand Marquis has a ( 20 U.S. gallon gas tank )
Then take it to a professional for repair of the SRS. This is not a repair job for a novice. Serious injury can occur when working with the SRS.
If both odometer and speedometer are not working I would blame the vehicle speed sensor. If just the odometer isn't working the gauge cluster assembly needs to be replaced with a reconditioned one.
Replace the flasher relay if none of them are working.