Why the “Mag Switch”? OR as it should be called… “Starter Control Relay”
Many new starter units are coming supplied with solenoid control relays. These are the small solenoids mounted closely to the starter solenoid. The 39MT series Delco come with the control relay mounted to the starter housing, but in the case of Cummins Marine engine they are usually mounted below the starter on a bracket on the engine.
The control relay is powered from the main starter solenoid battery post. When the control relay gets a start signal from the ignition switch, power is transferred through to the main starter solenoid switch post. Both the control relay and main starter solenoid are grounded back to the starter ground post in the back cap.
The main reason for using a starter control relay is that the signal needed for the main starter solenoid switch post is much higher than can be properly delivered from a key switch or thru off engine wiring harness safely & reliably. The relay should me mounted close to the battery source (main solenoid positive battery post) as the switch is feeding (main solenoid switch post) ensuring there is as little voltage / current drop in power from wiring as possible.
The standard switch circuit may become quite long, routing from the battery through a fuse panel and the key switch and eventually ends up many feet long. This distance as well as connections in between may lead to a weak signal when it finally gets down to the starter switch post. The main starter solenoid not only transfers power through to the starter, but also physically engages the starter drive requiring a strong, solid signal. Use of a control relay ensures that the main starter solenoid is getting the strong signal it needs, and since the control relay does not physically move anything, it can function on a weaker signal (typical draw is 1 AMP, vs. 30-75 AMPS for the starter solenoid) .
The starter control relay does not have to be mounted to the starter, but the closer it can get the better.