In my 2003 2900 both switches have to be on 1 to start. I think the batteries and switches are dealer installed based on the battery option purchased by the original owner. So they may vary from the manual.
Here’s my best guest at their rationale...
The leads to the motor/load is on the common, so power flows “in” to the switch from either 1 or 2. “1” comes from the battery and “2” is a jumper to the other switch (also “2”) so you can pull power from the other side.
In this way, during an emergency, you start by routing power from the good switch to the jumper by setting the good power source switch to “both”. Then on the bad side you select “2”, routing power from the jumper thru the common to the engine/load, which keeps the bad battery isolated from the good supply. By selecting “both” on both, you are connecting both batteries together.
Think of 1 as the primary power source and 2 as the backup jumper power source for each engine.
Note, I’ve heard running with 2 alternators ties together is a bad idea. It’s best to start the engine and then switch from “2” past “both” back to “1”. If you don’t cross the “off” then the power will not be interrupted with engine running. Then the alternator will charge the bad battery.
this setup also allows for a house battery and house loads to be placed on the common rail jumper using only 2 selector switches. In this scenario, you have the option of running each engine off the primary or house, and also be able to combine 2 or 3 together to supply either engine or house load.
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