Dear Dr. Bill

A solenoid and a relay perform the same function. They switch electrical contacts as a result of some external signal. I seem to have it stuck in my tiny little brain that the difference is that in a relay the signal is electrical and in a solenoid it is mechanical. Thus, the pressure switch that turns on the submersible pump in my water well is a solenoid, since the signal that controls it is water pressure on a diaphragm in the switch and the power to the pump is switched. Your description of the solenoid in a car suggests that it is controlled by an electrical signal through the keyed ignition switch. Perhaps it is merely that the term solenoid is just an older term that has become fossilized in automotive usage, much as the capacitor somewhere in there is known by the older term condensor. Any ideas?