Starter won't engage heat gauge goes across to hot when trying to start truck. What causes this?

Voltage drop? I'm guessing you don't have a voltmeter, so turn on the headlights and observe them as someone tries to start the truck. If the lights go out, bad connection at battery, starter, or ground. If the lights get dim, probably the starter-it's drawing power, just not working. If the lights don't change, check for power going to the starter solenoid on the small wire on the starter; voltmeter or test light, check if the small wire to the solenoid has power when key is held in start position-in crank position. There should be battery voltage there when the key is turned to crank. If so, the starter or solenoid is bad. If no power there to solenoid, power is lost somewhere between ignition switch and the starter wire.