Help!- 1994 mercury sable

i have a 1994 mercury sable and i need help with things to do with the speedometer and over heating issues my first problem is when i get above 35 mph my needle starts bouncing and i know it about 10 mph off when i went to go and fix this problem no one could tell me if i have a speedometer cable or if it's a sensor when i bought the car the manual wasn't with it please help my second problem it the over heating it dores not like to sit in one spot at all not even for a red light it starts to over heat a friend said well change the thermometer and i did but it still does it i have everything else updated for the most part i don't know what else to do please help me

 

If it don't overheat on the highway, but does it basically when at red lights or in slow traffic, it's a good chance that your electric fan is not coming on.

If it were the thermostat, it would also overheat on the highway and would even get hotter under load.

If the radiator was plugged - to test - if you left out the thermsotat, it would still overheat, even at highway speed, when you don't even need your fan to cool the engine, as the air coming through the radiator, at 55-60, would be cooling it.

 

Also an outside chance you have a head gasket problem. If you have the 3.8L engine, they were quite prone to blown head gaskets; the 3.0L was better.

Don't quote me, but I do believe that is an electronic speedo and not cable driven; should be a vehicle speed sensor mounted in the tranny. Any decent tranny shop can answer that question for sure.

 

thanks for the help. but the fan does kick on and it still over heats and now it's eating through anti freeze but ill have the things that you guys mentioned checked out.