Basically, a switch has the task of closing or breaking an electrical circuit. There are switches that break the ground line and switches that break the positive pole line.
Checking lamp switches and electric motors
Remove the corresponding switch.
Simple switches have only two terminals for connecting the wire. In this case, voltage (+) should be supplied to one terminal, and after switching on/off - to the other terminal. There are switches with several terminals. These switches, based on the circuit diagram, explain to which terminal voltage should be applied. If necessary, turn on the ignition in advance.
Check with a test light to see if voltage is supplied to the switch. If the test light comes on, turn on the switch and check if voltage is also supplied to the output terminal. If so, the switch is found to be functional.
If there is no voltage at the output terminal, the power supply wire is broken. Based on the electrical circuit diagram, check the voltage supply and, if necessary, install a new wire.
Checking switches-sensors
Switch-sensors, for example, are: a switch with a hydraulic drive, brake and coolant level sensors.
Connect a tester (test lamp/or ohmmeter) to the inlet and outlet wires of the switch by pulling the wire off the switch.
Attention! Switches that are screwed into the engine block usually do not have a wire connecting them to the housing, since the switch housing. On top of the engine block serves as a grounding pole.
When the switch is on, the probe should indicate "current flow". The best probe is an ohmmeter: when the switch is on, it shows zero Ohms, and when it is off, it shows infinity.
The quickest approximate way to check the coolant and brake fluid level warning switches is to pull the lead from the switch with the ignition on and press it to a well-grounded place, such as the engine block. If the warning light on the instrument panel comes on, the fault is in the switch. A special case is a hydraulically operated switch (brake voltage drop sensor): when the engine stops, the contact is closed (the warning light comes on), the switch opens only at a certain oil pressure.
(The text of the article was copied from the website: fordbook.ru)