[ElGracho]

GM diesels don't use a battery isolator. They just have the batteries wired in parallel. For a diesel with the stock wiring, you have to keep your batteries exactly the same, ie when one goes bad, you replace them both. The only purpose they serve in a diesel is to give the current the starter on the diesel needs to spin the engine over fast enough to get it to run. With a battery isolator between your two batteries, the second battery can be kept in reserve, ie if you leave your lights on or play your radio too long without running your truck, the isolator won't let the second battery get used until the key is in the start position, keeping you from getting stranded. Also, if one battery is weaker than the other, it won't become a drain on the stronger one.