Mastering Complex WIRING DIAGRAMS - …

1 When you re engagedin a tough diagnos-tic battle, the lastthing you want todo 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 itoffers and the terminology it of the most important offeringsof any automotive information libraryis the WIRING diagram. Many experi-enced technicians use this little nuggetof information as their main diagnosticguide, because they know it offers somuch more than simply locatinggrounds and pin-outs. Inside a wiringdiagram you can often see the relation-ships of circuits to one another, whattype of test to perform, what types ofsignals to expect and sometimes how aparticular system is designed 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 andautomotive systems are based.

2 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 working with or learning aboutwiring DIAGRAMS , the approach you takeis very important. As with anythingcomplex, DIAGRAMS are easier to dealwith in manageable bits. Since there aremany aspects to WIRING DIAGRAMS , thereare many approaches. Regardless ofwhat approach you take, keep the fol-lowing key concepts in mind: Complex WIRING DIAGRAMS are madeup of many individual circuits. Some aredirectly interrelated and others are not. There will always be a power sideand a ground side for every load. In every circuit there is a load. There are five basic voltage of these concepts are fairly30 July 1997 Mastering ComplexWIRING DIAGRAMSC omplicated WIRING schematics offer a wealthof information but can be awfully difficult todecipher.

3 Dividing them into smaller, moremanageable bits can make your job go a heckof a lot smoother. Here s how to go about The word load refersto the device that performs the workin a circuit. In Fig. 1 (shown on page33), the load is represented by a resis-tor symbol. Contrary to what somemay think, the switch is not the load!The legend in Fig. 1 shows the fivebasic conditions that represent voltagelevels found in a circuit as read by ahigh-impedance voltmeter. These con-ditions are the result of the arrange-ment and state of the circuit. It s im-portant to understand the differencesamong them, since the principles theyillustrate are key for proper (red) refers to that part ofthe circuit that is hot all the (yellow) refers to that part ofthe circuit that is ground all the are the easy ones!

4 Now look at the second circuit inSchematics: Jorge Menchu31 July 1997 Fig. 1. Notice the unique condition be-tween the switch and the load. The or-ange color identifies it as power whenthe circuit is complete. This meansthat when the switch is closed (com-pleting the circuit), it serves as a directpath to the power source. Your meterwill display source voltage. When theswitch is open, it will read zero is that ground? No,because of the resis-tance of the load, whichis between the test pointand ground!The next circuitshows a condition that isground when the circuitis complete (green). Inthis instance, the switchserves as a direct path toground. Testing be-tween the load and theswitch when the circuitis complete will displayzero volts on your me-ter.

5 Open the switch andit will read source volt-age. Again, because ofthe load between ourtest point and the powersupply, it does not fit in-to the power final condition isa varying voltage (blue).This is a signal thatchanges voltage levels ina linear fashion. A throt-tle position sensor is a classic exampleof a device that does just Diagnostic Starting PointComprehending these five basic con-ditions is certainly a good startingpoint for diagnosis, but there aremany variations. Getting further intothe dynamics of circuits is a matter ofunderstanding the behavior of elec-tricity and how components affect might sound complicated, butit s really as we have manuals for particu-lar vehicles, we need manuals for elec-tricity and basic circuit it comes to books and referencematerial, my motto is, you don t have toknow everything, just where to find itwhen you need it and how to under-stand it enough to apply it.

6 One of myfavorite manuals is Getting Started inElectronics by Forrest Mimms (avail-able at most Radio Shack stores). I alsohave an extensive book collection, andcertainly use the library when WIRING DIAGRAMS goes far be-yond the examples in Fig. 1. I colorthem on a regular basis, especially whenI m in a situation that involves dealingwith Complex circuits. I simply copy thediagram, make a color legend and colorit using highlight markers. This savestime because it forces me to analyze thediagram, the colors give me a quick ref-erence for testing and it divides themost Complex DIAGRAMS into manage-able building diagram on the left in Fig. 2below, a vacuum switching valve (VSV)system for idle control, is from a Toy-ota service manual.

7 Coloring it (right)shows that there are four basic condi-tions in this system. Once you have adiagram colored, applying what yousee is easy. Using Fig. 2 as an example,33 July 1997 Mastering Complex WIRING DiagramsFig. 1. These colors represent the basic voltage con-ditions that exist in most circuits. Using markers tocolor-code different circuit conditions, as shownhere, makes dealing with Complex DIAGRAMS 2. The diagram on the left is from a Toyota service manual. It shows all the circuits related to the vacuum switchingvalve (VSV) used to speed up the idle under certain conditions. Color-coding the diagram (as shown at right) divides itinto manageable bits. The colors also act as a quick reference for expected test you were to test between the defog-ger switch and relay and read sourcevoltage, what does this mean?

8 Insteadof reanalyzing the diagram, we simplyreference the color code to the equals ground when the circuitis complete, right? Since the meterreads source voltage, the circuit mustbe open between the test point and theground. Perhaps the switch is inspection of the diagram onthe right in Fig. 2 shows that the wirebetween the ECU and VSV is not col-ored. What color should it be? Well,one of our rules is that there has tobe a power and ground to every one side of the switch is alreadyconnected to ground, the other wiremust be one of the power conditions,red or orange. But which one? Well,we know the ECU controls the VSV,and since the VSV is not on all thetime, the ECU must act as a this wire Fig.

9 1, you see that theonly circuit that represents the VSV isthe second one power to switch, loadand then to ground! Okay, we know theECU turns the VSV on and off, butwhat about the wires that connect it tothe other circuits in this system? Andwhat are the square boxes with the tri-angles in them? Since the wires to themfrom the VSV aredirectly connected,they must be thecondition orange al-so, but what arethose symbols? Getout your referencemanual and you llfind that they rediodes. What dodiodes do? They reone-way electricalvalves that, amongother things, offerspike protection,AC rectificationand logic. At thispoint, you shouldhave more thanenough informationto understand thecomplete logic ofthis you still can tquite get what s going on, what else canyou do?

10 Read the description of the sys-tem given in the service manual. Themanual tells us that the VSV is locatedunder the intake manifold. In order tocontrol the speed at which the engineidles, it increases or decreases (based onsignals from the ECU, the taillamp andthe defogger relay) the amount of air al-lowed to bypass thethrottle valve. TheECU sends signalsto the VSV, in ac-cordance with sig-nals from varioussensors, to causethe engine to idleat the a picture isdeveloping. Thesystem descriptionbasically tells usthat the VSV can beturned on by any ofthree circuits. Andwe know from thewiring diagram thatthe three circuitsmentioned shouldnot turn each otheron. For example,the taillight circuitcan turn on the VSV, but not the defog-ger.