Lincoln/Mercury Repair: 1994 Mercury Sable speedometer/transmission problems, mercury sable, speed sensors

Question
1994 Mercury Sable 3.0 While driving down the road yesterday the speedometer quit.  After turning off the car and back on it worked again only to quit again a short while later.  This time when it quite the motor reved up.  Now the speedometer doesn't work at all.  I replaced the speed sensor(and had the old one tested and it tested good but I replaced it anyway) thinking this may be the problem and hand tested the speedo cable both are working fine.  However, when the speedometer is not working the car is shifting thru the gears very rough.  It didn't do this until the speedo quit.  Can you please help I am out of ideas and need this for school.  Thank You Jenny

Answer
Is the check engine light on? This will indicate what the problem is. There are two speed sensors on that transmission. Turbine and VSS. The VSS runs the speedometer, the turbine sensor measures engine speed. If these don't match, then the computer sees a problem and throws the car into fail safe. This is second and reverse only. It also increases transmission line pressure to max, so it will slam on engagement. It is trying to protect itself from further damage. If you continue to drive it in this mode, you will cause internal damage to the transmission and it will have to be rebuilt (I know, nasty word). I suggest you take the car at your earliest convenience to a transmission shop for diagnosing. Cottman Transmission will do this for free if you have one nearby.