Starter Solenoid AND Mag Switch?

I'm redoing the engine wiring harness on my '89 379 with a cummins 444xt

The starter is a Denso, I can read that much of the tag. Too much stuff in the way to read a part number. The sticker is on the side next to the frame rail. It looks like this:
Welcome to Sears.com

Do I need a separate solenoid/mag switch with that starter? I'm looking over wiring diagrams and can't figure out why there is an external solenoid. It doesn't look like it's switching a higher current circuit. All the wires going to the solenoid are the same size.

 

I get why it might be needed. I just can’t tell if it really needs to be in there. The wiring has been hacked up and 12ga wires with butt connectors have been starting the truck fine for the five years I’ve owned it. The solenoid is mounted on the firewall above the passthough connectors. I’m thinking that an extra foot or two of wire wouldn’t make that much of a difference.

I have the wiring diagram from super miller. It shows the mag switch as being an option but none of the schematics I have explain which satarters need it.

I also have the build sheet for my truck and it lists the mag switch being installed when the truck was built.

The starter looks like it’s been replaced though and I can’t find a spec sheet or installation instructors for the starter that’s in there.

 

235083

When the dealer brings it up there is some confusion about which truck it is. I guess There’s another truck out there with the same last six. Mine is the ‘89.

 

You would think it would be as easy as picking between two trucks, but you also have your wits together. Whenever I go to the dealer to track down parts he shows me schematics of the other truck. I have to tell him "No, it's the other truck" more than once in the same conversation.

I think it's the other way around with the mag switch. The build sheet shows one being installed. At some point the starter got swapped out and they left it in. The starter that's in there now isn't a gear reduction starter.

 

I checked the resistance on the coil of the mag switch and on the starter solenoid.

The mag switch/solenoid is 17.3 and the starter solenoid is 7.7

So the mag switch is drawing about .8 amps and the starter solenoid draws 1.8 amps at 14 volts.

A 1.8 amp draw doesn't sound like a whole lot to me, unless I'm not understanding something.