what is this part for what does it do?
 

Starter would not shut off in the run position of the brand new starter and ignition switch and tumbler?
Had an electrician guy come and look at the car.He said had to be a relay.Couldn't find anything really in the manuals.He listened to each one (5) on the firewall.Took each one out and each time same problem.Then he disconnected the black thing on the firewall labelled pressure sensor (numbers on it are F-201 small #s 8601 to the right and slightly lower,then E1T11574? Problem went away,starter now shuts off after starting.Anyone have any idea what this switch does? Can't find anything in the manuals about it?