Originally Posted by
sierrafery
You know wrong... that's a simple pressure switch normally closed to earth which opens under pressure, while it's closed it completes the warning lamp's earth path hence the lamp is on then when pressure opens it the lamp goes out... the alternator has nothing to do with this also as PhilipA said the black wire is futile cos the switch is earthed via fixing... if you have a white(with green trace) wire hanging from the alternator you have to connect it back where it belongs cos it's the alternator's protection.
EDIT: @Tombie was faster