I've heard that turning off the ignition switch while a diesel engine is still running can damage alternator diodes. Is this true? I do know that turning off the battery switch can damage diodes but is that also true for the ignition switch? We have a Ballmar alternator and voltage regulator, circa 1998. A few days ago the alternator stopped charging the batteries. At first I thought the regulator was at fault and replaced it but with no success. The alternator/regulator/cable are now at the alternator shop to be diagnosed and repaired - no word from them yet. We have, a couple of times accidentally, turned off the engine using the ignition switch key. I'm just wondering if that could be the culprit. Anyone know?