Does anyone know what this is? One of the wires crosses the engine bay by the firewall and is unattached by the relay on passenger side firewall. Wondering what the wire is suppose to be attached to?

 

The device the wires are attached to is just a basic solenoid (think starter solenoid, just not attached to a starter). Which wire runs to the passenger side - the heavier cable or one of the smaller wires?

If your truck is a diesel it could be the glow plug solenoid.

 

Interesting...... the one wire in not attached to anything and hanging in engine bay. I haven't had any trouble starting or running truck. I was worried it fell off somewhere. Maybe its ok?
i have the 5.7l v8
the smaller wire runs to passenger side red/black

 

Is the unhooked wire near the battery? These types of solenoids are used for conducting heavy current. It was most likely for an aftermarket accessory that was installed/removed by a previous owner. The only things I can think of are a snow plow, bed mounted fuel tank/pump, winch, or a lift gate.

I’m not sure which wire is red/black (can’t make out the colors), but the wires that hook up to the top & bottom posts would have been the ones that operated the accessory. The 2 smaller wires on the sides would’ve been the control to engage the solenoid. I can see they are taped together but one should run to ground and the other to a switch that may be in the cabin.

If it’s not hooked up and you aren’t finding any accessories that the cable hooks up to, you’d be fine to remove the solenoid and wiring unless you can think of something you’d want to use it for.

 

The wire is too short to reach the battery. I think your right and your explanation makes sense. Thanks a lot for the help. I won't worry about it anymore.

 

You were correct. It is a selenoid for the second battery. Probably had some accessory attached at one time