Question
i set lawn sprinkler on automatic timer the back yard comes on fine but in the front yard it comes on then shuts off

Answer
The problem is most likely in the automatic valve for the front yard.  Make sure the wiring connections are sound, that there is no corrosion in the connections.  Check the solenoid to make sure it is firmly connected to the valve.  If these don't help, you probably need to take the valve apart and check the diaphragm and clean the interior and make sure all the little holes in the valve are open.  Check out the website below for a good tutorial on valve maintenance:

http://www.irrigationtutorials.com/faq/repair-valve.htm