Checking switches (BMW 5 Series E39)

Most electrical consumers are switched on and off manually via a switch. In addition, there are switches that operate automatically. These switches include hydraulic switches for the lubrication system and a brake fluid level sensor switch.

The main function of a switch is to close and open an electrical circuit. There are switches that open the ground wire and switches that open the positive voltage wires.

Checking lamp switches and electric motors

Remove the corresponding switch.

Simple switches have only 2 wire connectors. In this case, positive voltage (+) is always supplied to one connector, and after the switch is turned on, positive voltage supplies the other connector. There are also switches with an increased number of connectors. According to the electrical diagram, you should find out to which connectors in such switches voltage should be supplied, if necessary, first turn on the ignition.

Use a test lamp to check whether voltage is supplied to the switch power connector. If the test lamp lights up, turn on the switch and check whether voltage is supplied to its output connector. If voltage is present, the switch can be considered to be in good working order.

If there is no voltage at the switch power connector, then there is a break in the current-supply circuit. In accordance with the electrical diagram, check the current-supply circuit and, if necessary, lay a new wire.

Checking the sensor switches

These switches include hydraulic switches for the lubrication system and sensors-switches for the level of coolant and brake fluid.



Connect the device to test the switch (a test lamp or ohmmeter) to the supply and return wires of the switch, for which one wire must be removed from the switch.

Note: Switches that are screwed into the engine block usually do not have a ground wire, since this function is performed by the switch housing.

When the switch is on, the test device should show the conductivity of the switch. This is best determined with an ohmmeter: when the switch is on, it should show 0 Ω, and when it is off, it should show ∞ Ω.

The fastest way to check the functionality of the coolant or brake fluid level indicator switch is to remove the power cable from the sensor with the ignition on and connect it to a reliable ground, such as the engine cylinder block. If the indicator lights up, the switch is faulty.

A distinctive feature of the hydraulic switch: when the engine is not running, the switch contact is closed (the indicator is on). Only at a certain pressure the contact opens and the alarm goes out.