These 6.2L GM diesels use 12v starting systems. The starting wiring system is the same as a gas powered chevy except they have two engine batteries, one under the hood driver's side and another under the vehicle below the driver's seat.Problem: Won't Start
If the starter and batteries tested ok, then it's battery cable connection problems or the starting circuit that comes from the ignition switch through the neutral safety switch down to the starter. First try cleaning your battery connections. Especially at the battery under the hood were the cables from both batteries connect. Check voltage at the purple wire on the starter while trying to start the engine. Then check the voltage on the battery cable at the starter while cranking. They both should read close to battery voltage. The blue finned box under the hood is a battery isolator that isolates the house batteries from the engine batteries from the charging system and isn't part of the starting system.
Contributors: Billy J. Brower