Tachometer creep on '96 Q45

[soloQ45]

The tachometer on my '96 Q45 is behaving strangely.
When I turn the engine on, the tach comes up to the nominal idle speed (~1.5Krpm). As I start to drive, the tach starts to increase....then continues increasing until it finally pegs the bottom of the instrument.
Turn the engine off, if comes back to zero.
I suspect the speed sensor, or the ECM...The shop manual refers to two ECM's (one is the ECCS relay; the other is the ECCS control module)...which would would need to be replaced?
Where is the speed sensor? How would you test it...or the ECM?
How come there's nothing wrong with the speedometer?
Any suggestions?

[Skibane]

I suspect the speed sensor, or the ECM

The speed sensor measures vehicle speed, not engine speed. If it was defective, your speedometer (among other things) would be acting up - not your tachometer. Similarly, if the ECM was broken, you'd probably be seeing other problems besides the tachometer.

The fact that everything except the tachometer is operating normally is a clue that the problem probably lies in the tachometer itself (or in the connections to it).

[life602]

My 02 q45 is doing the same thing it also idles at 1000 rmp when I let go of the gas and slowly goes down to like 700 rpm then when I step on the brake it goes to loke 100 rmp and stays there ..does urs do the same?ohh and there is no check engine light as well..any clues will help cause its also killing me in the mpg I'm getting 18 mpgs and when I got it I was gettin 20 to 22 mpgs