Basically, the task of a switch is to close or open an electrical circuit. There are switches that switch the ground wire, and switches that interrupt the supply current (pluses).
Checking lamp switches and electric motors
Remove the corresponding switch.
Simple switches have only 2 terminals. In this case, voltage is applied to one of the terminals (+), and on the second terminal it appears after the switch is turned on. There are also switches with multiple terminals. With regard to these switches, find out from the wiring diagram which terminal should be energized, if necessary, before switching on the ignition.
Check with a probe whether voltage is applied to the switch. If the lamp lights up, turn on the switch and check for voltage at the output terminal. If so, the switch is normal.
If there is no voltage at the input terminal, then there is a break in the current lead. According to the wiring diagram, check the voltage supply and, if necessary, lay a new wire.
Checking the sensors-switches (signaling devices)
Switch sensors are, for example, oil pressure sensors and coolant or brake fluid level sensors.
Connect a probe or ohmmeter to the sensor terminals by removing the wires from them.
Attention: As a rule, sensors attached to the engine block do not have a ground wire, since the sensor housing connected to the block itself serves as a ground pole.
When the switch is closed, the instrument should indicate a short circuit. In this sense, an ohmmeter is best suited. When the switch is closed, the ohmmeter should show 0 ohm, with the switch open, ∞.
The functioning of the sensors-switches of the level of coolant and brake fluid is most quickly checked by removing the wire from the switch with the ignition on and connecting it to ground, for example, on the engine block. If the control lamp on the dashboard lights up at the same time, the switch is faulty.
A special case is the oil pressure switch: When the engine is not running, the contact is closed (control lamp on), the switch opens only at a certain pressure.