Computer Controlled Cranking Circuits: Introduction & Getting Started

With the gradual introduction of electronic control the cranking circuit has grown very complex and has undergone a major technological evolution.

In the early days of the development of automotive electrical circuits, cranking circuits were simple and very effective at cranking the engine. Then automotive technology grew more complex eventually resulting in computers controlling most vehicle functions including cranking the engine.

As a result, our job has become more difficult and the cost to the customer for repairs has gone sky high.

Figure 1-1 below is a composite and generic schematic diagram of a modern cranking circuit based on computer control technology.

Fig. 1-1 Computer Controlled Cranking Circuit
Learn how to read schematic diagrams

We call this a composite schematic diagram because all components responsible to operate the Starter Motor are shown. By having all components on the same schematic diagram we can provide some clarity on what components are included, how they are interconnected and what components may affect the operation of the circuit.

Besides the Starter Motor and Starter Solenoid we discover a mechanical Starter Relay. The Relay is controlled by two independent on-board computers which will be discussed later on in this series of articles.

Take some time reviewing this generic schematic diagram in Figure 1-1. We have purposely left out the color code of wires, wire diameter markers and numbered connectors completing a wire between two points on the schematic.

This was done to keep the diagram from being too cluttered so we can focus on the components in the circuit and discuss how each contributes to the operation of the DC Starter Motor.

These schematic items are present in shop manual schematic diagrams and of course are essential when tracing a circuit on an actual vehicle but not necessary to explain how this circuit works and principles of circuit troubleshooting.

In a shop manual this circuit may require several pages to depict the circuit shown in Figure 1-1. For example, the battery and DC Starter Motor may appear on one or two different pages in the shop manual. This makes understanding the connections between these two important components a little ambiguous rather than show the clarity that exists between them.

The PCM will likely appear on another page in the shop manual and may or may not show the fuel pump Relay on the same page. The IPM (Integrated Power Module as we call it) will be on another page of the shop manual and so on. To follow the flow of circuit operation using the schematic diagram may require flipping through several pages in the shop manual. These pages are often separated by pages highlighting other circuits on the vehicle.

In our experience we have found the shop manual method of laying out vehicle circuits through several pages interrupts our thinking and hinders our attempt to understand and analyze the circuit. Components spread out over multiple pages makes it difficult to see the circuit in its entirety.

A composite schematic diagram helps to develop a coherent troubleshooting procedure. When a circuit on a vehicle has a problem, and you are not familiar with this circuit on this vehicle, a composite schematic diagram helps the circuit stands out from all the other diagrams. A composite schematic diagram like Figure 1 also makes it easier to explain the circuit as it evolved into the complex electronic circuit it is today.

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