A Mercedes-Benz does not use an external ignition control module. It has a distributorless ignition and uses one coil and two spark plugs per cylinder. While the C230 and the E320 use the same system, there are some minor differences. For example, the C230 has one coil located on the top of one of the spark plugs and has a companion wire to the second spark plug. The E320 series and up use a single coil with separate plug wires to the individual plugs.
The Mercedes-Benz uses individual drivers and transistors within the computer that are dedicated to the individual cylinders. These individual drivers and transistors control the firing of the coils. Mercedes is unique in that its computer can either fire both spark plugs simultaneously or fire them separately in a timed sequence. The firing choice is dependent on load and atmospheric pressure. This makes the engine more effective at reducing emissions in all altitudes and temperatures. It also enhances power and fuel economy through all driving conditions.
The computer uses the crankshaft sensor for rpm determination and for position of the crankshaft relative to top dead center on the number 1 cylinder. It uses the mass air flow sensor and intake air temperature sensors for calculation of altitude above sea level in terms of air density. One oxygen sensor is in front of the catalytic converter, and the other is behind the catalytic converter. These sensors measure the amount of free oxygen in the exhaust system. All of these signals are used by the computer to determine the fuel-air ratio and the ignition timing necessary to burn as much of the fuel as possible. The computer uses these signals to tell the ignition module which coil to fire and when to fire.
The signals from the oxygen sensors are called counts. The computer compares the counts on the front oxygen sensor with those on the oxygen sensor behind the converter. The front sensor should ideally show a much more rapid change in counts, and its counts should be a lot higher than the rear sensor's. This indicates how much less unburned fuel is coming out of the converter than went into the converter. It helps control emissions, the ignition (including the ignition control module) and fuel injection. A knock sensor sends a signal to the computer if it senses any damaging pre-ignition, and the computer reacts by retarding the spark.
