Help with fuel guage

HI i have a e36 bmw and recently my dash f' ed up so i had to change it. Now the fuel guage is not reading any fuel in the car. I have run a diagnostic on the dash and the fuel guage moves fine. However when i swich the car on all that happens is the guage moves a little and then stops. The reserve light is on all the time. Anybody can help???? please??
thanks