Reading Schematic Diagrams

Over the years many technicians have told me nobody ever taught them how to read a schematic diagram. They picked up what they know from trial and error. I can’t tell you how many times technicians have called us and asked if we had training that would teach them how to read electrical schematic diagrams. My answer is always “Yes” and “No.” Yes! We can teach you how to read a schematic diagram. No!, because reading the schematic diagram with understanding requires that you understand the parameters of voltage and electron current in a circuit, the effect resistance has at various points in the circuit and how series and parallel circuits work. Without this essential electrical knowledge, you cannot comprehend what you are reading in a schematic diagram nor understand what to look for. And we can teach you how to do that.

It seems many technicians have drawn the conclusion that if they could “read a schematic diagram” like their shop buddy Joe does, they would be able to troubleshoot and repair an electrical circuit like Joe does. They don’t realize that Joe probably understands a little about voltage and current in a circuit, the effect resistance has at various points in the circuit and how series and parallel circuits work which Joe applies to the circuit as Joe reads the schematic diagram to conclude what to do next.

It is very likely that Joe also has some understanding of using a DMM and current clamp to test and measure voltage and electron current in the circuit and from these readings determine what’s wrong in the circuit. We call this electrical troubleshooting training. This happens to be the focus of all of our training programs offered by Veejer Enterprises Inc.

How To Read a Schematic Diagram

I realize many of you reading these articles will say I already know how to read a schematic diagram but many do not. Maybe we can help all do a more professional job reading a schematic diagram. So, let’s begin at the beginning and see if we can add something more to your ability to read a schematic diagram.

Step 1:

Identify all major components in the schematic diagram. Get the big picture of what is in the circuit. This is where a composite schematic diagram can be a great asset. It clearly shows all components comprising the circuit to operate the DC Starter Motor. If using a shop manual, I would suggest you draw your own simple composite block diagram of the circuit as you plow through the pages of the shop manual. It doesn't’t have to have a lot of detail like Figure 1 but at least it will give you the big picture.

Fig. 1-1 Computer Controlled Cranking Circuit

Step 2:

Identify the main components that the circuit uses to operate or control the DC Starter Motor. In this case it is the DC Starter Motor control by the Starter Solenoid. We determine the primary device the circuit controls is the DC Starter Motor, load in the circuit.

Step 3:

Trace the wiring connected to the load terminals. There is a wire connected to the voltage side of the load and a wire connected to the ground side of the load. It doesn't’t matter if you decide to trace the voltage side first or the ground side first. The important point is that you trace wiring on both sides of the load. If you are tracing the voltage side wire continue until you reach the B+ terminal (battery positive post +BATT or generator B+ terminal). We decide to trace the voltage side of the circuit first. We begin at the + cable terminal on the Starter Motor, the voltage side of the Starter Motor. Then trace back to the positive terminal of the battery through the Starter Solenoid heavy-duty contacts.

Trace the wiring connected to the load terminals. There is a wire connected to the voltage side of the load and a wire connected to the ground side of the load. It doesn't’t matter if you decide to trace the voltage side first or the ground side first. The important point is that you trace wiring on both sides of the load. If you are tracing the voltage side wire continue until you reach the B+ terminal (battery positive post +BATT or generator B+ terminal). We decide to trace the voltage side of the circuit first. We begin at the + cable terminal on the Starter Motor, the voltage side of the Starter Motor. Then trace back to the positive terminal of the battery through the Starter Solenoid heavy-duty contacts.

Next, trace the ground side of the Starter Motor which is bolted to the engine block which is connected to the battery negative terminal (B-) by the engine ground cable. Yes, this appears simple because this is a simple electrical circuit. But you have already identified components, cables and connections that will have to be checked (tested) if there is a cranking problem. You have also learned there are no surprises in this part of the circuit which appears to be a common everyday cranking circuit.

If the circuit were very complex these two steps would provide you with insight into the circuit. Even if the circuit is more complex it is easy to begin to understand the schematic diagram following these two simple procedures of tracing B+ and B- from the load in the circuit. Now you have a picture of how the Starter Motor is wired into the circuit and have learned the DC Starter Motor is controlled by Solenoid contacts on the voltage side of the Starter Motor, not the ground side. From our previous articles on electron ground current you should find it easy to trace the electron flow from B- through the Starter Motor, through the Solenoid contacts which are connected to B+.

Step 4:

Trace the circuit for the Starter Solenoid to see how it is engaged to operate the Starter Motor. It doesn't’t matter if you trace the voltage side or the ground side first. Let’s trace the ground side wire which connects to the Starter Motor case. Of course, on some vehicles the Starter Solenoid is physically mounted on the Starter Motor which is the ground circuit for the Starter Solenoid.

(In Figure 1-1, the Starter Solenoid is mounted independent of the Starter Motor housing. This requires a ground wire which we show grounded to the Starter Motor housing. On some vehicles the mounting bolt of the Starter Solenoid to the sheet metal or case of the DC Cranking Motor serves as the Starter Solenoid ground circuit and no Starter Solenoid ground wire is needed. But it will still need to be tested with a voltage drop test as we will see later.)

The B+ supply to the Starter Solenoid is traced back to Pin 87 on the Starter Relay.

Step 5:

At this point, you encounter a previously unknown fact from tracing the schematic diagram. This vehicle has a dedicated Starter Relay. You probably would never have known this without tracing the schematic diagram from the Starter Solenoid to the Starter Relay. Pin 87 on the Starter Relay gets B+ when Pin 30 (connected to B+) connects to Pin 87 through the closed Relay contacts. The Relay contacts close (Relay “clicks”) when Starter Relay Pin 86 has B+ from the IPM Pin 22 and Starter Relay Pin 85 has B- provided by the PCM Pin 8. The topic of Relay operation by these two onboard computers will be explored in more detail in future articles.

You also discovered that there are two onboard computers that each must contribute something to operate the Starter Relay, to operate the Starter Solenoid to power up the DC Starter Motor. The IPM provides (B+) voltage to pin 86 of the Relay. The PCM provides a ground (B-) to pin 85. This is where diagnosing the circuit gets a bit more complicated. Many questions arise. What do I do if the IPM doesn't’t provide B+ to pin 86? What do I do if the PCM does not provide B- to pin 85? Before we answer these questions, we have to spend a few articles covering testing and troubleshooting the cranking circuit as it developed without the IPM and the PCM which are rather recent developments in operating the cranking circuit. They have made life a lot more difficult for us and repairs a lot more expensive for your customer.

What we have accomplished at this point is trace wires to discover all the components in the computer controlled cranking circuit and see how components are connected together. A trained electrical troubleshooting technician also begins to understand what electrical tests he may have to perform on various components while reading the schematic diagram. Understanding voltage and electron current in a circuit, how circuits work, how circuits fail, what voltage readings should appear at various points in the circuit if it is a good circuit, he can quickly eliminate different component problems with a few simple voltage or electron current tests using a current clamp.

Understanding voltage and electron current in circuits, how to test voltage and electron current in a circuit based on how the circuit is wired together and what to do next if the DMM and current clamp readings are good, too high or too low, goes along at the same time reading a schematic diagram. Technicians who can do this love schematic diagrams and do not want to get along without them. So you can see, if you want to understand how to read schematic diagrams and apply that to this computer controlled cranking circuit you have to understand electrical circuits and how to test them. You need to understand good voltage and electron current readings, what to do next if the reading is good, too high or too low. I call this electrical troubleshooting training and will be the focus of this series of articles.

Whats Next?

Go to Computer Controlled Cranking Circuits


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