Remove the distributor cap and disconnect the spark plug tips.
Caution: When doing this, grasp the tip, not the wire.
Measure the resistance between the individual contacts in the distributor cap and the contacts of the corresponding spark plug tips. Specified value: 6±1.2 kOhm.
If there is a deviation from the specified value, unscrew the spark plug tip from the ignition wire. The resistance of the spark plug tip should be 1 kOhm. If this is not the case, replace the spark plug tip; otherwise, replace the spark plug wire or distributor cap.
If the resistance is increased, clean the wire connection contacts and repeat the test. If necessary, replace the wire.
Check for cracking by bending the spark plug wires at a large angle near the spark plug tips. If necessary, replace all spark plug wires at once.
Check the distributor rotor.
Connect the ground wire to the battery.
Checking the distributor rotor
During normal spark formation, the resistance is (measured in ohms) should not be elevated.
Shine the distributor cap.
Connect the ohmmeter to the distributor rotor. The specified resistance value is about 1 kOhm +300/-100 Ohm.
Information taken from this resource: BMWman