1967ihtruck

We have a 1086 with the digital tach. Just put a used sensor in and it is still showing erratic reading. For example when it is idling it shows 9xxx. What causes this?

Will the ac valve that is on top of the compressor off of a 70s ford truck work on a 1086. I need a new one and was wondering if one off of a truck would work

My dad has an original style tach on his 986 and it does not work at all and I was wondering what it could be. We also have a 1086 with a tach with all the fancy buttons ad it gives a false reading all the time too it lights up but it just gives a false reading