The engine is like the heart of the machine in heavy machinery. It gives the power to perform the task for which heavy machinery is designed. But how to start an engine. How a small ignition key turns the engine to start.

What is the Ignition Key?

The ignition key is the switch that gives the command to start the engine or turn off the engine. This can be directed using a relay or by an electronic module. Here we’ll see the Mercedes-Benz OM904LA engine with ADM.

Electric circuit for Ignition System

Electric circuit for Ignition Key in Heavy Machinery.
Electric circuit for Ignition Key in Heavy Machinery.

The electrical circuit shown above, we have taken from Mercedes-Benz. Electric power (+24 volts) comes from the battery to the circuit breaker or fuse. Here S2 is the 3-position ignition switch. 0 is the OFF mode. 1 is the ON mode while S is the starting mode. We have already read the engine starting procedure in our previous article. Follow the engine starting procedure step by step.

When the ignition key is in the 0-position (OFF), the 24 V supply is only at pin number 30. First, the S2 switch (ignition key) has to change position-1. In this position, pin number 30 will only be connected with pin number 15 and will give the required power supply to the machine and engine before the engine starts. (The important thing to know here is that the power to the machine is supplied from the battery itself. The battery will discharge when the ignition key is in this position without starting the engine for a long time.)

After a few seconds when the engine is ready to start, the ignition key is changed to position 2. In this position, pin number 30 will be connected to both pin number 15 and 50A. Pin 50A supplies power to the ECU (engine control unit) or directly to the starter relay (depending on the engine) so that the starter motor gets energized.

Electric circuit for ECU (Engine Contro Unit)

Circuit diagram for engine starting
Circuit diagram for engine starting

As shown in the engine diagram above, K1.15 is for ADM and ECU power supplies. This power supply comes directly from pin 15 (position 1) of the ignition switch. Pin 50 in gets power when the ignition key turned to position 2 via pin 50a. When all engine interlocks and conditions are satisfied, the ECU powers the 50 out pin so that the starter relay can be activated.

Starter motor circuit diagram
Starter motor circuit diagram

As soon as the starter relay gets activate, the starter motor starts turning the engine flywheel so that cranking can begin. The power supply from the battery comes to pin 30 on the starter motor and waits for starter relay activation only. The same battery connection will charge the battery by alternator B+ connection.

Once the engine starts successfully after cranking, release the ignition key. It will automatically come into position 1 and the engine will be on. To turn the engine off, simply rotate the key in the 0-position.

This is a complete explanation, how Ignition key starts the engine in heavy machinery.

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