What's the solenoid on an automobile?

Whats wrong with an 88 ford aerostar when you replace the starter battery alternator and starter solenoid and it still wont start just clicks?

• Answer:

  Whenever a starter motor in any vehicle clicks it is telling you that it is not getting enough current.At this point I would probably replace the cables and make sure that the ground cable is attached to clean metal and grounded to the engine and a secondary cable is grounded from engine to the frame. Over time these connections corrode ,sometimes from the inside out. A starter motor generates around 20hp so they require a lot of current and will refuse to start when cables are not up to the job.