The answer is true! You are supposed to shut the ignition switch off first, then the battery switch off.
See attached picture of the inside of the door on our M816 wrecker. You can see that behind the window crank it says "CAUTION POSITION BATTERY SWITCH TO OFF ONLY AFTER ENGINE HAS STOPPED". FWIW, this is a truck with original factory paint inside the cab. It has not been repainted.
The reason for the specific shutdown sequence is to protect the relay in the protective control box, and to protect the regulator in the alternator. There is a relay in the protective control box that breaks the circuit from the alternator to the batteries. When the battery switch is off, the relay is open. When the battery switch is turned on, the relay closes, and once the engine starts and the alternator is charging, it can reach the batteries.
When the battery switch is turned off before the ignition switch, or even very quickly after the ignition switch is shut off but the engine is still turning, the alternator is still charging. If there is current flowing to the batteries when the relay in the protective control box opens, there is more arcing, more carbon buildup, and the relay will fail earlier. Also, if the alternator is still charging when the circuit opens and the alternator is disconnected from the batteries, the regulator will go to full charge on an open circuit and will eventually kill the alternator. This is much the same reason why it is not a good idea to jump start and run a vehicle equipped with an alternator without any batteries, or to disconnect the battery on a vehicle equipped with an alternator while it is running.
The M939 series five tons have a protective control box as well, and the reasoning for the shutdown sequence is the same on both vehicles.
If you forget and shut off the battery switch first a few times it won't kill it. Personally, I always shut off the ignition switch first, let the engine stop turning, then turn off the battery switch.
Joe
