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Transistorized Ignition

This component is part of Ignition System.


The 230, 250 and early 280SL ignition system is conventional with a hot wire going to the ignition coil after passing through a ballast resistor which drops the voltage down to around 8.5 volts. This resistor is in series all the time, including during start-up. With this basic system the coil current could not be increased much above 3A without seriously shortening the service life of the points, thus placing an upper limit on the attainable spark energy. Also, erosion of the points caused by arcing leads to timing drift and necessiates frequent service intervals.

It was against this background that the first transistor assisted ignition system was developed, and current through the contact points significantly reduced. Although an improvement over the basic system, despite the use of special contact breaker points they remained the most common cause of ignition faults in service on 6 and 8 cylinder engines.

A special low-resistance (0.4Ω) high-current coil with a step-up ratio of around 1:185 (rather than the more normal 1:100 for a conventional points driven ballasted coil) had to be used. The coil carried the Bosch blue color with a yellow sticker marked ‘Transistor’. This system was characterised by two ballast resistors, 0.6Ω (with metallic clamp band) in series with the coil and 0.4Ω (with a blue clamp band) in series with the ignition amplifier, the latter resistor being shorted during cranking to give increased spark energy when starting. Because the peak coil current was of the order of 7 amps and the dwell angle with points necessarily large, all the components of the ignition system generated a significant amount of waste heat, not least the amplifier unit which was housed in a cast aluminium box. On Pagodas, this box is mounted under the battery tray.

For quicker starting, a resistor bypass circuit can be easily hooked up on early cars. See Ignition Coils section for details and additional wiring diagrams.

Mercedes Benz Transistorized Ignition Wiring (Typical 113 Chassis)

The diagram shows a typical transistorized ignition system as used with the 113 Chassis, specifically the 1969 280SL. The switch gear is part of the latter W113 models' transistorized ignition system. The box contains a transistor and is encased in a resin as protection to vibrations and corrosion.

This system significantly reduces point current by using a high power transistor - located inside the module - to handle the current through the ignition coil; the points simply trigger the transistor. The diagram below shows the internal layout of the ignition module. The following text explains how the module works and how the current through the points is reduced.

In order to better understand how the system functions, one needs to understand the Mercedes Benz numbering convention used in their diagrams. The numbers used to identify the terminals on switches, modules and other components follow a specific convention to tell the reader what voltage or signal is found on the terminal. The standard convention is shown below in the table.

Mercedes Benz (Bosch) Transistorized Ignition - Internal Layout of Module

Terminal NumberDefinition
1Ignition coil negative (-) terminal
15+12 Volts switched (ON/RUN position of the ignition switch)
16+12 Volts when the engine is cranking (from starter motor)
30+12 Volts direct from the battery, not switched
50+12 Volts when the ignition switch is in the START position
R+12 Volts when the ignition switch is in the RADIO or ON/RUN position

How the Transistorized System Works

Transistorised Ignition

When the ignition switch is set to the ON/RUN position, +12 Volts is applied to terminal 15 of the ignition module through the 0.4 Ohm ballast resistor. When the ignition switch is set to the START position (engine cranking) +12 volts is applied directly to terminal 15 of the ignition module from the solenoid on the starter motor. This effectively bypasses the 0.4 ohm ballast resistor and offers higher voltage during cranking.

When the engine starts and the ignition switch is returned to the ON/RUN position, +12 Volts is again supplied to terminal 15 of the ignition module through the 0.4 Ohm ballast resistor.

A typical firing sequence is as follows.

  1. +12 Volts is applied to the ignition module through the 0.4Ω ballast resistor
  2. Assuming that the points in the distributor are open, transistor Q1 is turned off because terminal 7 of the ignition module is open (no ground from the points). No voltage is present at terminal 16 of the ignition module, thus no current flows through the 0.6 Ohm ballast resistor or the ignition coil.
  3. As the engine cranks, the hexagonal cam in the distributor begins to turn and the points close.
  4. When the points close, a ground is applied to terminal 7 of the ignition module and transistor Ql turns on. This applies +12 Volts to terminal 15 of the ignition coil through the 0.6Ω ballast resistor. The current through the points is extremely small (thousandths of an Ampere or milli-amperes) instead of 4-5 Amperes as seen in a non-transistorized system.
  5. When transistor Q1 is turned ON, capacitor C2 is discharged (shorted by Q1).
  6. When the points open, capacitor C2 begins to charge. The charge path is: +12 Volts from the ignition switch - 0.4Ω ballast resistor - C2 - 0.6Ω ballast resistor and the ignition coil to ground.
  7. As capacitor C2 begins to charge, a pulse is created in the ignition coil, which causes a high voltage to be generated and sent to the appropriate spark plug.
  8. When the points close again, transistor Q1 is turned ON and the cycle repeats.

Point wear and stress is significantly reduced since the transistor is handling the current through the coil (4-5 Amperes). The points were designed to switch high currents repeatedly for tens-of-thousands of miles, therefore, when switching a very small current, they should last for a much longer time. Tune-up intervals (due to point failure) become longer and vehicle maintenance becomes cheaper.

But (yes, there's always a "but"), we still have the points. Points are mechanical, subject to bounce and vibration and must be actuated - mechanically - by the camshaft in the distributor. Whereas the wear on the point contacts has been reduced, the fiber pawl, which rides between the points and the distributor camshaft, can still wear out or break.

Electronic Ignitions provide solutions for this.


The switch gear component is installed under the battery box. Remove the battery and the battery frame plate. The switch gear box can then be seen.

If the switch gear fails, the entire unit needs to be replaced... MB Part No. 000 545 18 32

Switchgear box

Switchgear circuit diagram

Verifying if your switching unit is bad

This can be done using this procedure. It takes a bit of guidance to find the right wires and bypass the switching unit if it is not functioning correctly. The car will run with the unit bypassed, if the points are good and there are no other obvious ignition problems.

Symptoms for a bad switching unit:

The car will either start right up and run cleanly, especially when cold. Suddenly it will stumble and cut-out after only seconds (or after 10 minutes of driving). Once the problem occurs it seems to get progressively harder to keep running for more than a few seconds until it finally just cranks and seems to show either no spark or very weak spark. Troubleshooting it today from cold engine condition, it started right up smoothly, then died after 20 seconds. On all subsequent starting attempts (basically still with a cold engine) it starts up and then stumbles very erratically in idle until it dies after several seconds, sometimes it races up to fast idle for a second and then dies. Fuel is plentiful and pressurized. It seems like the ignition is arbitrarily cutting out. I know this could sound like the ballast resistors which I have both checked and replaced, as well as the coil, high-tension ignition lead and more. I also replaced the transistorized ignitions "black box" with another one. Do all of these symptoms together sound familiar to anyone? Otherwise I'm not quite sure where to go next except for checking wiring shorts or breaks.

Transistorised Ignition System testing

Switchgear testing diagram

Testing of Transistorized Coil Ignition in Vehicle with the Engine Stopped

  • Measure voltage on primary winding of ignition coil with a voltmeter.
  • Switch-off ignition, check whether breaker contact is closed. If not, keep rotating engine until contact is closed.
  • Connect black cable of voltmeter to ground, red cable to terminal 15 of ignition coil (see diagram above, Fig. 15-20/3).
  • Switch on ignition and read voltage on voltmeter. Rated value: 2.6-3.5 Volt
  • Open make-and-break contact. The voltage should return to zero.

Testing of Ignition Coil

  • Disconnect all connections on ignition coil. The primary resistance between terminal 1 and 15 is 0.38-0.43Ω at 20° C.
  • Connections 1 and 15 should not be connected to ground.
  • Measure with a conventional resistance measuring bridge. The ohm ranges in a standard multiple tester are generally too inaccurate for such tests.
  • At an ignition coil temperature of approx. 80° C the resistance measured is approx. 25% higher.

Testing of series resistances

  • Disconnect connecting lines.
  • Test terminals for body contact. Measure resistance with a measuring bridge.
  1. Rated value 0.4Ω ± 0.05Ω at 20° C. Resistance (2) between ignition switch and switch-gear.
  2. Rated value 0.6Ω ± 0.05Ω at 20° C. Resistance (4) between switchgear and ignition

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