I have a problem with my '93 Mercury Sable that doesn't seem to make any sense. The speedometer and odometer stopped working, but the cruise control still works fine.
A friend of mine who knows about cars said that if the speed sensor goes bad then the speedometer and the cruise control shouldn't function. My car has the 3.8L V6 and an automatic transmission. Any ideas would be appreciated.
The setup on your Sable is a little irregular when compared to a lot of other vehicles that were built around the 1993 model year, but the difference is fairly easy to explain.
Your car is using the good old fashioned method of driving the speedometer with the use of a manual cable routed from the transmission output shaft to the speedometer assembly. At the same time there is also an electronic VSS (Vehicle Speed Sensor). This sensor sends a vehicle speed signal to the cruise control module and the PCM (Power Control Module) which controls the engine's crucial functions.
In this situation the speedometer is irrelevant to the cruise control operation due to their separate means of obtaining the vehicle speed data. If the VSS had failed, the cruise control would have been inoperable and the Check Engine light would have come on indicating the loss of the speed signal to the PCM. The speedometer wouldn't have missed a beat.
Nowadays, the mass majority of cars are utilizing a solid state speedometer that requires the electronic VSS signal. Therefore, no speedometer, no cruise, and the Check Engine light will be illuminated on the instrument panel with the loss of the Vehicle Speed Sensor.
The possible causes of your problem are an internal failure of the speedometer/odometer assembly, a stripped plastic driven gear at the transmission, and of course the most common item, a broken speedometer cable.