Mastering Complex Wiring Diagrams
Contents
1. WIRING DIAGRAMS Complicated wiring schematics offer a wealth of information but can be awfully difficult to decipher Dividing them into smaller more manageable bits can make your job go a heck of a lot smoother Here s how to go about it By JORGE MENCHU hen you re engaged in a tough diagnos tic battle the last thing you want to do is fight the ref erence material needed to fix the car So it s vitally important for you to be come familiar with this material what it offers and the terminology it uses One of the most important offerings of any automotive information library is the wiring diagram Many experi enced technicians use this little nugget of information as their main diagnostic guide because they know it offers so much more than simply locating grounds and pin outs Inside a wiring diagram you can often see the relation ships of circuits to one another what type of test to perform what types of signals to expect and sometimes how a particular system is designed to work To get the most out of wiring dia grams you must have a decent un derstanding of electronics and the ba sic strategies on which circuits and automotive systems are based I can not stress enough the value of under standing electricity and circuit com ponents After all this is the founda tion of all automotive control systems When working with or learning about wiring diagrams the approach you take is very impor2. vane open the fuel pump turns on Must be what this switch is for But would you expect the fuel pump to be run directly from this switch Nah that would be a lot of cur rent to handle Most likely this switch completes the circuit for a low current relay which then activates the pump So the conditions of A and B are either red and orange or green and yellow No 2 is a fixed resistor and No 3 is a potentiometer just like a TPS If you looked inside an airflow meter you could see the potentiometer and the fact that it moves with the vane The signal from this device would be a varying voltage which is one of our basic conditions No 4 turns out to be a temperature sensitive resistor simi lar to a coolant temperature sensor In this case it must be used to measure the temperature of the incoming air It also outputs a varying voltage If you understand the principles of coolant temperature sensors and TPSs you probably already have enough in formation to answer the questions of CTS Cirewit Catch a wave CTS PCM Fig 5 A coolant temperature sensor CTS circuit is actually two resistors set up in series The resistor in the computer is often called a pull up resistor what to expect at every pin of this air flow meter what type of signal you d see and how to adjust your meter Confused Let s look at these com ponents separately and then tie every thing together How many wires go to a TPS Th
3. from a Toy ota service manual Coloring it right shows that there are four basic condi tions in this system Once you have a diagram colored applying what you see is easy Using Fig 2 as an example 1 i Fig 2 The diagram on the left is from a Toyota service manual It shows all the circuits related to the vacuum switching valve VSV used to speed up the idle under certain conditions Color coding the diagram as shown at right divides it into manageable bits The colors also act as a quick reference for expected test results July 1997 MOTOR 33 Fig 3 There s enough information in this airflow meter diagram to know exactly what to expect from each of these pin outs The key is to take the right approach if you were to test between the defog ger switch and relay and read source voltage what does this mean Instead of reanalyzing the diagram we simply reference the color code to the legend Green equals ground when the circuit is complete right Since the meter reads source voltage the circuit must be open between the test point and the ground Perhaps the switch is open Close inspection of the diagram on the right in Fig 2 shows that the wire between the ECU and VSV is not col ored What color should it be Well one of our rules is that there has to be a power and ground to every load Since one side of the switch is already connected to ground the other wire must be one of t
4. he power conditions red or orange But which one Well we know the ECU controls the VSV and since the VSV is not on all the time the ECU must act as a switch Color this wire orange Referencing Fig 1 you see that the only circuit that represents the VSV is the second one power to switch load and then to ground Okay we know the ECU turns the VSV on and off but what about the wires that connect it to the other circuits in this system And what are the square boxes with the tri angles in them Since the wires to them 34 MOTOR July 1997 from the VSV are directly connected they must be the condition orange al so but what are those symbols Get out your reference manual and you ll find that they re diodes What do diodes do They re one way electrical valves that among other things offer spike protection AC rectification and logic At this point you should have more than enough information to understand the complete logic of this system If you still can t quite get what s going on what else can you do Read the description of the sys tem given in the service manual The manual tells us that the VSV is located under the intake manifold In order to control the speed at which the engine idles it increases or decreases based on signals from the ECU the taillamp and the defogger relay the amount of air al lowed to bypass the throttle valve The ECU sends signals to the VSV in ac cordance with
5. ree Fig 4 page 34 How many go to a standard CTS Two Fig 5 above Remember the rule Every circuit has to have a power and a ground Therefore the three wires of the TPS have to be power signal and ground The two wires of the coolant temperature sensor have to be signal and ground The power source of a two wire coolant sensor is in the origi nating controller What do these two sensors have in common Grounds Now let s go back to Fig 3 F has to be a ground G has to be the signal from the temperature sensor E due to the arrow is the signal for the vane s po tentiometer D is a voltage used by the computer as a reference point for the measurements of the potentiometer C is the power for the potentiometer If you wish to continue with these types of exercises the next step would be to find diagrams that include tran sistors And don t limit yourself to au tomotive schematics either The book Getting Started in Electronics in cludes many simple circuits that you can decipher and build You might al so want to expand on the color codes legend For example many digital components such as Hall sensors are very repetitive Most Hall sensors are simple switches to ground therefore it would make sense to ID them using a dashed green line An AC sensor such as a wheel speed sensor might be a dashed blue line The Bottom Line Wiring diagrams are a key piece of the diagnostic picture and I can t
6. s a classic example of a device that does just that A Diagnostic Starting Point Comprehending these five basic con ditions is certainly a good starting point for diagnosis but there are many variations Getting further into the dynamics of circuits is a matter of understanding the behavior of elec tricity and how components affect it This might sound complicated but it s really not Just as we have manuals for particu lar vehicles we need manuals for elec tricity and basic circuit componentry When it comes to books and reference material my motto is you don t have to know everything just where to find it when you need it and how to under stand it enough to apply it One of my favorite manuals is Getting Started in Electronics by Forrest Mimms avail able at most Radio Shack stores I also have an extensive book collection and certainly use the library when necessary Coloring wiring diagrams goes far be yond the examples in Fig 1 I color them on a regular basis especially when I m in a situation that involves dealing with complex circuits I simply copy the diagram make a color legend and color it using highlight markers This saves time because it forces me to analyze the diagram the colors give me a quick ref erence for testing and it divides the most complex diagrams into manage able building blocks The diagram on the left in Fig 2 below a vacuum switching valve VSV system for idle control is
7. sig nals from various sensors to cause the engine to idle at the appropriate speed Now a picture is developing The system description basically tells us that the VSV can be turned on by any of three circuits And we know from the wiring diagram that the three circuits mentioned should TPS Circuit Latch a wave can turn on the VSV but not the defog ger The defogger should also be able to turn on the VSV but not the taillights The diodes must be used to control this logic Their one way valve action allows for all three circuits to be tied to the VSV without affecting one another Some wiring diagrams offer more de tail than others Those that do not offer much especially the ones with inte grated circuits and controllers can be very tough to deal with even when you have a system description For example there are times when you need to know when and under what conditions an ac tion in a circuit is supposed to take place Every now and then the answer can be determined by deciphering the diagnostics in the manual Do Some Investigating Sometimes the key to understanding involves a little detective work like I did with my first bad airflow meter Once I determined that it was bad I pried the cap off to expose the internal circuits By reading the component description in the manual and examin ing the wiring diagram I was able to make the connection among the physi cal part the descrip
8. stress enough the importance of mastering them Understanding diagrams means understanding electricity which leads to understanding the information your test equipment offers This is paramount for our success today and in the future Remember being an automotive technician means being an investiga tor Use those investigative skills to build a solid foundation a foundation based on understanding knowledge and insight Go for it For a free copy of this article write to Fulfillment Dept MOTOR Magazine 5600 Crooks Rd Troy MI 48098 Additional copies are 2 each Send check or money order 36 MOTOR July 1997
9. tant As with anything complex diagrams are easier to deal with in manageable bits Since there are many aspects to wiring diagrams there are many approaches Regardless of what approach you take keep the fol lowing key concepts in mind Complex wiring diagrams are made up of many individual circuits Some are Mastering Complex directly interrelated and others are not There will always be a power side and a ground side for every load eIn every circuit there is a load eThere are five basic voltage con ditions Most of these concepts are fairly 30 MOTOR July 1997 straightforward The word load refers to the device that performs the work in a circuit In Fig 1 shown on page 33 the load is represented by a resis tor symbol Contrary to what some may think the switch is not the load The legend in Fig 1 shows the five basic conditions that represent voltage levels found in a circuit as read by a high impedance voltmeter These con ditions are the result of the arrange ment and state of the circuit It s im portant to understand the differences among them since the principles they illustrate are key for proper diagnosis Power red refers to that part of the circuit that is hot all the time Ground yellow refers to that part of the circuit that is ground all the time These are the easy ones Now look at the second circuit in July 1997 MOTOR 31 Schematics Jorge Menchu Caich a wa
10. tion in the manual and the diagram I could even see the problem area which was related to Pale Ea TPS not turn each other on For example the taillight circuit Fig 4 A throttle position sensor TPS is nothing more than a simple potentiometer It usually has three wires power signal and ground which supplies a varying voltage the skewed output signal that con demned the meter Fig 3 on page 34 is the internal di agram for an airflow meter This one is rather simple but at the same time has much to offer There s enough in formation in this diagram for you to know what signals to expect from every pin A G As you examine this diagram keep in mind that the same rules mentioned in the beginning of the article apply and so do the voltage conditions listed in Fig 1 The first step is to identify the components in the diagram If you look these up in an electronics refer ence manual you d find that there are three types of resistors and one switch Don t confuse the switch and resistor symbols for a relay A relay contains a coil whose symbol is a se ries of loops as opposed to the saw tooth appearance of the resistor No 1 in the diagram represents a simple switch We stated earlier that every circuit has a load Therefore this switch switches the unseen load either to ground or power Anyone who has played with a hooked up airflow meter with key on engine off knows that as soon as you move the
11. wel Cacut comditions Color codes He Het af the time CO Ground ail the time D Power when the circuit is complete EE Ground when the circuit ie complete MM Varying voltage Fig 1 These colors represent the basic voltage con ditions that exist in most circuits Using markers to color code different circuit conditions as shown here makes dealing with complex diagrams easier But is that ground No because of the resis tance of the load which is between the test point and ground The next circuit shows a condition that is ground when the circuit is complete green In this instance the switch serves as a direct path to ground Testing be tween the load and the switch when the circuit is complete will display zero volts on your me ter Open the switch and it will read source volt age Again because of the load between our test point and the power supply it does not fit in to the power category The final condition is a varying voltage blue This is a signal that changes voltage levels in Fig 1 Notice the unique condition be tween the switch and the load The or ange color identifies it as power when the circuit is complete This means that when the switch is closed com pleting the circuit it serves as a direct path to the power source Your meter will display source voltage When the switch is open it will read zero volts a linear fashion A throt tle position sensor i
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