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Measuring parasitic current draw

Symptom: - The car will not start. - The starter motor will not turn the engine over. - Low or no battery voltage.

Measuring parasitic current draw

Fault symptom

- The car will not start.

- The starter motor will not turn the engine over.

- Low or no battery voltage.

Conditions

Low voltage can generate and store diagnostic trouble codes in the system in question.

Diagnostic help

Check the charging system. See Generator 120A, B207/Z18XE , or Generator 130A, Z19 , or Generator 155A, B284 . See Test value.

Check the battery. See Battery, 60Ah or Battery, 70Ah or Battery, 85Ah . See also Battery analyser/battery tester .

Make sure that the parasitic current draw is measured with a fully charged battery.

Connect a multimeter with a current clamp to the battery. Turn the ignition key to ON. Then turn the ignition key to OFF. Remove the key from the ignition switch and lock the car. Wait 25 minutes and then read the current consumption which should be less than 40 mA.

The diagram below shows that control module components (1) actively communicate on the bus. After about 20 minutes (2) the activity decreases and consequently also the current consumption. When 25 minutes have elapsed (3) the current should be less than 40 mA.

The control module "wakes up" (4) at regular intervals to carry out measurements and checks.

Important

Measurements and checks carried out by the control module are subtracted from the measured parasitic current draw.