Need help - my tach goes up and down at highway speeds - why?

[DavidG]

I have a '92 Mercury Cougar 5.0 auto and thought maybe somebody on here could help me out.

At highway speeds the tach will move up and down (it's a digital factory tach). Usually at 70mph it'll run 2000rpm but it moves up to as high as 2800rpm at times and then it will go up and down off and on. I don't think the actually engine rpm is changing (it's hard to tell at highway speeds though) it's just the tach that is moving. Anyone know what might be going on? Oh, the car is stock and runs fine aside from the moving tach.


Thanks

-David

 

[proxses]

Do you know if it has a lockup torque converter? That could explain what is happening.
Does the car feel any different when this happens?
How long has this been going on?

 

[DavidG]

I doubt it has a TC. I don't feel anything at all when it happens. It's been happening for about 2-3 months. And just now it's starting to go up and not come back down as far (stays up around 2300rpms when it should be at 2000rpms).

 

[proxses]

Then the gauge must be going bad. If the engine is not changing, and the torque converter does not lock, then the only thing left is the coil or the gauge. And if the coil was bad, the engine would not run.
So, bad factory gauge, no surprise.

 

[DavidG]

the '92 cougar has one of those full digital displays - mph, rpms, gas, temp, everything is on the same display. Could a single gauge go bad or would the whole set of guages go bad?

I friend of mine who works as a auto tech told me it was something to do with a sensor from the throttle body that was probably sticking or something. Is that possible?

 

[proxses]

There are no sensors that are connected to the tach on the throttle body. There is a signal wire that runs from the coil. That, at least, is how mine is hooked up.
So I doubt it has anything to do with the throttle body.