I have the wierdest problem with my lumina. I have a check engine light and when it's on.. the engine has a high idle and the speedometer doesn't work.. then.. sometimes.. when I drive it the check engine light goes off.. the idle goes back to normal and the speedometer works!
The input speed sensor which is probably screwed in the transmission. Check the connector. Sounds like a bad electrical connection. If there is no speed sensor screwed into the transmission, then it's the speed sensor associated with the vehicle timing, probably near the flywheel or distributor. The check engine light and the high rpm says the computer cannot determine that the engine is running and at what rpm.
90 Lumina check engine light
problem with my lumina. I have a check engine light and when it's on.. the engine has a high idle and the speedometer doesn't work.. then.. sometimes.. when I drive it the check engine light goes off.. the idle goes back to normal and the speedometer works! 
