speedometer and odometer, HELLLPPPP!!!

I have a 90 nissan maxima that I just purchased from a cousin for a 100 bucks. Nothing is wrong with it except the speedometer and odometer dont work. Any Ideas? I am just trying to figure out what it is. I work at checker auto and parts dont make a difference. Just need to know what to replace. Any help is appreciated. It is a automatic transmission.

...Just need to know what to replace. ...

- ppl say sensor tends to pop out; nothin to replace, push down.
- test sensor output: http://www.cardomain.com/ride/748507/9
- wiring to gauge, gauge connection in the assy, gauge itself