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Ignition system

The design of the ignition system of the Mercedes Benz 230, 250 and 280 SL is discussed here. The ignition system is a major component of the Electrical Systems.

Definition

The ignition system provides the high voltage required by the spark plugs to ignite the compressed air fuel mixture at the right time to produce power.

It consists of the Battery, Ignition Switch, ballast resistor, Ignition Coil, Distributor, Spark plugs, and low and high tension leads. Later Pagoda models (late 280 SLs) also have a transistorised module to reduce load on the contact Points and increase their service life.

Function

In order to ignite fuel in the cylinder, a spark is needed. The spark is delivered by the Spark plugs, by passing a very high voltage (tens of thousands of volts) across the terminal of the plugs. The ignition system creates the high voltage in the Ignition Coil, and passes it to the correct spark plug via the Distributor.

For a great description of how this all works together follow the Engine Starting Aid Tour.

Ignition System Basics

This content is courteously provided by John Hassell, Texas, 15 April 2002


Figure 1-1: Standard "Kettering" Ignition System for a 1969 280SL


A man named Kettering developed the fundamental ignition system more than 70 years ago. Even today, the simple system consisting of Battery, Ignition Coil, Points and Condensor is referred to as the Kettering system. Figure 1-1 depicts the standard system as installed in a Mercedes Benz, 1969 280SL (113 chassis). The terminal numbers are those shown on Mercedes Benz wiring diagrams for the 113 Chassis.

How it Works

When the ignition switch is turned to the ON/RUN position, terminals 30 and 15 are connected. This connects 12 volts from the battery to terminal 15 (input side) of the ballast resistor. The other side of the ballast resistor is connected to the positive (+) terminal of the ignition coil.

The negative (-) terminal of the ignition coil is connected to the points inside the distributor. If the points happen to be open, no current flows through the coil and no spark occurs. If the points are closed, current flows from the battery, through the ballast resistor, through the primary winding of the coil, through the points to ground, thus completing the ignition circuit.

When the ignition switch is turned to the START position, several things happen. The starter motor is engaged via 12 volts from the battery applied to the starter solenoid/switch from terminal 50 of the ignition switch. The solenoid pulls in or closes and current is supplied to the starter motor, which begins cranking the engine.

Simultaneously, 12 volts is applied directly to the positive (+) terminal of the ignition coil via terminal 50 of the ignition switch. This effectively shorts or bypasses the ballast resistor. Thus, in the START position, the engine is cranking and full battery voltage is applied to the ignition coil.

As the engine begins to turn, so does the center shaft in the distributor. This causes the points to begin to open and close. When the points are closed, a high current flows through the primary winding of the ignition coil to ground. The condenser is also shorted or bypassed when the points are closed.

When the points open, the magnetic field of the coil primary collapses and a high voltage is induced in the coil's secondary winding, which results in a high voltage (15,000 - 25,000 volts) to be routed to a spark plug via the distributor rotor. The spark fires the fuel/air mixture in the cylinder and the engine starts to run.

The Condensor serves two purposes. First, it reduces arcing between the points and second, it slightly increases the energy in the spark. It's primary function, however, is to increase point life by reducing arcing. When the ignition switch is released to the ON/RUN position, the bypass across the ballast resistor is removed and power to the positive terminal of the ignition coil is supplied through the ballast resistor. At this point, the voltage applied to the positive terminal of the ignition coil is less than full battery voltage. This reduces the current through the points and extends their operational life, however, it also reduces the amount of high voltage available for spark.

As can be discerned from the above discussion, the points are the weak link in the system for two reasons, namely:

  1. Mechanical wear caused by the open/close action created by the distributor camshaft. In actuality, the fiber pawl on the points wears out. Usually, however, the points will require replacement before this happens.
  2. Burning and pitting caused by switching high coil current. This is the main reason for point failure.

To examine why this happens, let's review a bit of simple Ohm's law. The current flowing through any circuit is a function of voltage and resistance. The basic relationship is:

 I = E/R

Where: I = Current in Amperes, E = Voltage in Volts, R = Resistance in Ohms

In a typical Kettering ignition system, the resistance of the primary winding of the ignition coil is about 0.4 ohms. This is also the primary resistance of the standard "blue" Bosch ignition coil used in most 113 chassis cars.

Normally, when an engine is running, the battery voltage will be about 13.2 volts (due to the charging action of the alternator). So, if the battery exhibits 13.2 volts and the coil has a resistance of 0.4 ohms, the current through the points is...

 I = 13.2/0.4 = 33 Amperes.

That's a lot of current. In reality, we must also add the resistance of the wires to/from the battery and ignition switch to the calculation. We can assume that the total resistance of the wires and switch will be about 0.5 ohms. Therefore, the total resistance of the coil and the wiring will be about 0.9 Ohms. This gives us...

 I = 13.2/0.9 = 14.6 Amperes or less than half what we calculated above.

But, when the ignition switch is released to the ON/RUN position, the ballast resistor is now placed in the circuit. The ballast resistor usually has a resistance of about 0.6 Ohms. This makes the total resistance of the circuit - coil primary + wiring + ballast resistor (0.4 + 0.5 + 0.6) equal to 1.5 Ohms. If we put this into the calculation we have...

 I = 1.32/1.5 = 8.8 Amperes

Now, we must also include the fact that the points are not always closed, they open and close rapidly as the engine runs. Thus, the amount of average current flowing the 6 coil and points determines wear on the points. In a normal Kettering system using a ballast resistor, the average current through the coil is about 4-5 Amperes. This is a function of point "dwell", or, the amount of time the Points remain open. Remember, the ignition system "fires" or generates a spark when the points open. If the points open too soon or too late, timing is adversely affected.

Other Cause for Points Failure

When the ignition switch turned to the ON/RUN position (before the car started or to listen to the radio) it is possible that the points in the distributor are closed. Thus, we would have about 8 Amperes of current being drained from the battery and the points would begin to heat from the current. In order to avoid this, a "radio" or "accessory" position was added to the ignition switch of most vehicles. In the radio position, no power is applied to the ignition system, battery drain is significantly lowered and the points do not wear due to heat.

Automobile manufacturers began to employ transistorized ignition systems in the late 1960s and early 1970s to eliminate or reduce point failure.

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