1995 gmc yukon i have change the fuel assmbliy

You can manually actuate the relay if you want. Take the cover off, and push the contact over with a screwdriver and see if you can hear the pump. Have you checked to see if there's power to the relay, both the control side and the side providing power to the pump? There's two different circuits involved here, so you have to make sure they're both ok. If you have power before the relay, and power coming out of the relay headed to to pump, then your problem falls between the relay and the pump. Other things to consider, are the fuel pressure sensor, and oil pressure sensor. Some vehicles will not turn the pump on if they sense no oil pressure after a couple of seconds. Secondly, if the fuel pressure sensor is telling the pump that there's plenty of pressure, (even if there's not) it'll shut the pump down. Hope this helps you.