Measurement technique (BMW 7 Series E32)

Voltage measurement

The presence of voltage in a circuit can be checked with a simple test light or probe, but to determine the magnitude of the voltage, a voltmeter is needed.

First of all, you need to determine the measurement range. Usually, the voltage in the car's electrical system does not exceed 14 V, with the exception of the ignition system, where the voltage reaches 30,000 V. This high voltage is measured by special devices. In normal measurements, the upper range of the measured voltage should not exceed 15-20 V. When measuring lower voltages, for reliability, you need to switch the voltmeter to lower measurement limits in turn to obtain the required accuracy of the results.

The voltmeter wires must be connected in parallel to the consumer as shown in the figure, observing the polarity of the connection. The black (minus) wire of the device is connected to the ground of the car on the cylinder block or on the body, and the red (positive) wire - to the wire coming from the positive contact of the battery.



Example: if the starter turns the engine too slowly when starting, then you need to check the battery voltage. To do this, connect the voltmeter wires to the battery. Then the assistant should turn on the starter. If the battery voltage is less than 10 V (temperature is about 20°C), then the battery needs to be checked and recharged.

Measuring current

Such measurements on a car are carried out quite rarely, for example, when searching for the cause of self-discharge of the battery. For this, an ammeter is needed, which is included in the multimeter (tester).

Before taking measurements, you need to set the range of currents to be measured. If the expected current value is unknown, it is better to switch the measurement limits from higher to lower.

To measure the current strength, you need to break the current circuit and connect the ammeter to the circuit break. To do this, you need to, for example, disconnect the plug and connect the red (positive) ammeter wire to current-carrying wire. Black (negative) the wire must be applied to the contact from which the wire is disconnected. The ground connection between the consumer and the plug must be made using an additional wire.

Note: A regular ammeter cannot measure high currents, such as those in the starter circuit (up to 150 A) or in the glow plug circuit of a diesel engine. High currents can damage a regular ammeter. A special inductive ammeter is needed for such measurements.

Measuring resistance

Before measuring resistance with an ohmmeter, you must make sure that there is no voltage in the circuit and disconnect the plug. Turn off the ignition, disconnect the wire or part, or disconnect the wires from the battery. If you do not do this, you can disable the ohmmeter.



The ohmmeter is connected to two contacts of the consumer or to two ends of the electric wire. In this case, the polarity of the ohmmeter connection does not matter.

Resistance measurements on a car can be divided into 2 types:

1. Checking a resistor or component in a circuit.

2. Checking the integrity of the electric wire, the presence of a switch contact or heating coil. In this case, it is checked whether there is a break in the wire, due to which the corresponding device does not work. To check, connect the ohmmeter to both ends of the wire being checked. If the resistance is close to 0 Ohm, then there is contact and the wire is OK. If the wire is broken, then the resistance will be infinite.