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I have a Chevrolet S-10, according to the the Ignition System Circuit Diagram the ICM gets an Ignition Control Signal from the PCM and sends a Switching Signal to the ignition coil.

As the Ignition Control Signal from the PCM is just and on/off signal and given that the ICM does not get any other signals I'm wondering what the ICM does? What is needed for? Why can't the coil get the signal directly from the PCM?

What would happen if I connect the PCM signal cable directly to the coils switching signal terminal?

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While the Ignition Control Module (ICM) does pass along the signal for the coil(s) from the Powertrain Control Module (PCM or ECM or management system), it also controls the voltage input to the coil(s). This is a higher current then you want flowing through your PCM. If you tried to hook it directly from your PCM to your coils, you'd most likely fry the PCM.

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