please help no start

[2500HD2001]

Okay so i bought a truck 4 days ago replaced batt and alt. Ran for 3 days then tonight all i get when i turn the key is a click from the firewall it looks like a solinoid or maybe a transistor for the aux batt it is not connected. Tapped the starter still no star all the lights come on and had another vehicle running hooked up to the batt just in case my 12v wouldnt start it with another vehicle hooked up it was 14 every this has power and it wont even turn over

[00Silverado4x4]

Check all your fuses and relays? All electrical connections clean and tight?

[2500HD2001]

Yup they are all fine connections are clean and tight for the relays I switched the headlights and starter and headlights came on so relays are fine that's the only way I know how to check them I have a second batt and disconnected it just in case it was drawing every thing turns on just no turn over there is some part of the aux batt system on the firewall that does a single click when I turn to run it looks like a solinoid to me it has power on both sides so I think its good

[2500HD2001]

I'm a first year mechanic so I know where to start this needs more in depth then I know everything has power just no vroom it would be easier if it turned over I would know where to start

[2500HD2001]

so got it home had to jump the starter does any one know what the solenoid is on the fire wall right beside the aux batt

[gmtech4]

Get a test light on the starter solinoid purple wire , it should be hot when key is turned to crank ,

 

If the test light lights then the starter is done

 

 

Get a schematic and follow the wire back to the "crank " fuse ( same thing should happen there as well with a test light ). , pretty sure anyways