Your Hummer is built to withstand a lot of tough road conditions, and its electrical system will also endure extreme weather ranges - but eventually your Hummer alternator will begin to fail. When one of components of your Hummer alternator begins to fail, it is recommended that you replace the Hummer alternator as a unit, instead of trying to diagnose and replace each part as they fail. Diagnosing a failing Hummer alternator can be a tricky business, since often the car can run without any problems until the alternator dies completely - and then the car does not run at all. When the alternator begins to fail, the car might run off the battery's stored energy, until the battery dies. A Hummer alternator can fail slowly, so at first you might not be able to run some of the energy-sucking accessories that depend on an alternator. You might not notice any energy issues at first because the battery has reserve power stored in it, and can run for a while on those stored reserves. Often when an alternator is failing, simply turning off an accessory like the stereo or air-conditioning will preserve enough energy to keep the car running for a while. One sign of failure: the charge indicator light (or 'idiot light') comes on, although that may also represent a faulty fuse in the system, and not a bad alternator.