A starter solenoid like the kind you see mounted on the fender well (photo 1), or starter motor housing of some vehicles is used to bring current to a starter motor when the key is in the start position. This type is designed to pass as much as 400 amps.
The length of time this high current passes through a solenoid like this is limited. The limited time is the key to their usefulness. These solenoids use a coil with only 3 to 4 ohms of resistance. If this type solenoid was turned on and left on continuously, it would destroy itself from heat due to the amount of current passing through the coil winding. These solenoid windings pass close to 6 amps when operating.
Continuous duty solenoid (photo 2) windings will draw less than 1 amp when operating. These solenoids will have approximately 15‐30 ohms of resistance across the coil winding. This allows the coil to run much cooler allowing it to pass well over 100amps continuously without failure.
Never replace a continuous duty solenoid with a regular starter solenoid. The starter solenoid will burn out in a very short period of time if used for “continuous duty.”
Note: Any solenoid is nothing more than an “overgrown” relay. They work exactly like a relay. They have a coil winding that creates magnetism when energized, and this magnetism causes a “contact disc” (photo 3 cut-a-way) to move from its off position to its on position. In photo 3 you will see a spring in the center of the contact disc. This spring is not pushing on the disc. It is pulling the disc up away from the copper contacts attached to the ends of the studs sticking out of the solenoid. These studs are used to hold the heavy cables that carry the current from the battery positive (+) terminal to the load which could be a starter motor or whatever else engineering decides to turn on.