Chevrolet Repair: 01cheverolet hd 3500, starter solenoid, rock quarry

Question
Why will the truck crank if i bump the key over,however,it wont if i hold the switch over? this truck has spent its whole life in a rock quarry.this truck has alot of dust and dirt in it.just thought i would throw that out there for some insight of our conditions.

Answer
HI POWDER,ONE OF TWO THINGS GOING ON HERE. FAULTY IGNTION SWITCH OR AND FAULTY STARTER SOLENOID OR STARTER ITSELF...WHEN YOU ARE HOLDING THE KEY OVER TO CRANK SEE IF THERE IS POWER ON THE SOLENOID WIRE,,FULL 12 VOLTS FROM THE BATTERY [ PURPLE WIRE[SMALL WIRE ON SOLENOID]]IF THERE IS POWER WHILE HOLDING THE KEY OVER TO CRANK THE STARTER THE STARTER OR SOLENOID IS BAD..IF NO POWER TO THE STARTER, POSS IGN SWITCH OR WIRING PROBLEM ???