Your title 'Glow Plugs' is incorrect, as I understand the Caterpillar C7. It has a heater in the intake to preheat the air, not glow plugs. In cold weather, turn on ignition and a light on the dash should show the heating grid. When the light goes out, engage starter.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI