Technician Handbook 652 Body Electrical Diagnosis Technical Training 159 Technician Handbook 652 Body Electrical Diagnosis Introduction In step #3 of the six-step troubleshooting plan, you analyzed all the symptoms that were confirmed through your preliminary checks Based upon these symptoms, you could make a conclusion as to the type of electrical problem that the circuit has: •  An open circuit •  An unwanted parasitic load or short-to-ground •  A high resistance problem •  A feedback from another circuit In this section, we will concentrate on diagnostic strategies and techniques that should be used to isolate each of these problems You’ll find that using the “right” tool for each type of problem will save you a lot of time when working to pinpoint location of the circuit problem Diagnosing Open Circuit Problems Of all the types of electrical problems, open circuit problems are the most common Open circuits are typically caused by: 1.  Disconnected connectors 2.  Bad switches 3.  Poor terminal contacts 4.  Cut wires 5.  Blown or defective fuses You can assume that you have an open circuit problem whenever there is no visible sign of operation You can use a number of tools to find the location of an open circuit Each of the tools has its advantages and disadvantages, so it’s probably best to use a combination of the three, depending on the situation 160 Technical Training Technician Handbook 652 Body Electrical Diagnosis Using a Voltmeter on Open Circuit Problems The voltmeter’s advantage is that it is easy to use, and cannot cause circuit or fuse damage An available voltage test (positive probe at terminal, negative probe connected to a known good ground) will verify continuity in the circuit to the +B source 1.  Use the EWD to determine where to make the checks and if any switches/relays need to be closed 2.  Remember that the EWD will not tell you how much voltage you should have at every pin in the circuit You need to apply your knowledge about circuits to determine what the correct voltage should be 3.  Connect the negative probe of the voltmeter to a known good ground, and use the positive probe to check the various pin voltages with the circuit ON NOTE Technical Training Inspect the connectors/locations that are the easiest to access, then check the harder ones, if necessary 161 Technician Handbook 652 Body Electrical Diagnosis On the circuit diagram above, write what voltage you expect at each of the test points? NOTES: 162 Technical Training Technician Handbook 652 Body Electrical Diagnosis What does this test result tell you? NOTES: Technical Training 163 Technician Handbook 652 Body Electrical Diagnosis What does this test result tell you about the problem with this circuit? What result would you expect if the switch were turned OFF? NOTES: 164 Technical Training Technician Handbook 652 Body Electrical Diagnosis What does this test result tell you? NOTES: Technical Training 165 Technician Handbook 652 Body Electrical Diagnosis What result did you expect here? What does this test result tell you? NOTES: 166 Technical Training Technician Handbook 652 Body Electrical Diagnosis What result would you expect with the switch ON? With the switch OFF? NOTES: Technical Training 167 Technician Handbook 652 Body Electrical Diagnosis High Resistance Problems High resistance circuit problems are very similar to open circuit problems But instead of an infinite amount of resistance stopping current flow entirely, a high resistance problem adds series resistance into the circuit to restrict current flow This restriction can cause the load in the circuit to: •  Operate erratically •  Operate partially (such as a dim bulb) •  Not work at all (insufficient current flow or voltage) Causes of a High Resistance Problem The best connections and conductors always have a certain amount of resistance As you learned earlier, there are five factors affecting the resistance of any conductor The condition of the conductor is at the heart of all high resistance problems •  Corrosion at connections The effects of weather, road salt, and moisture can take its toll on a terminal and harness Although weather sealing on most terminals has improved greatly, terminal corrosion can still be a problem •  Cut or chafed wiring Any reduction in the diameter of a wire also adds resistance When any of the strands in a wire are cut, series resistance is added Also, a hole in the wire’s insulation allows moisture to corrode the wire, adding resistance to the circuit Because of the wicking action of the wire, this corrosion will eventually affect a large area of the wire, not just the area where the insulation is damaged 168 Technical Training Technician Handbook 652 Body Electrical Diagnosis Intermittent or Continuous? Determine if the short to ground is intermittent or continuous •  If you don’t know whether the fuse is blowing intermittently or on a continuous basis, (and if a supply of replacement fuses is available), replace the blown fuse with a new one, and retest the circuit •  If the fuse is blowing intermittently, find out the exact conditions which cause the fuse to blow This may point you directly to the problem circuit Instead of replacing and blowing fuses, connect an appropriate load in the place of the blown fuse With the short to ground condition present, the load should be ON HINT If the problem is intermittent, consult the power source fuse charts to operate circuits that are switchable for the fuse that blows If you have to turn something on to blow the fuse, the problem is after the switch Remember, that a short-to-ground that blows a fuse is always before the load Disconnecting Component Connectors Your next step in this process of elimination is to disconnect individual connectors Using the Power Source (Current Flow Chart) Section of the EWD, determine which components are connected to the blown fuse If the components are accessible, and there aren’t too many, disconnect them one by one until the headlamp goes dim This can be a quick means of finding the problem or at least eliminating some of the possible causes But if the problem is in the harness, you will have to use the “mapping current flow through the junction blocks” technique 186 Technical Training Technician Handbook 652 Body Electrical Diagnosis When performing this type of diagnosis, refer to the Troubleshooting section of the EWD for hints on disconnecting connectors NOTES: Technical Training 187 Technician Handbook 652 Body Electrical Diagnosis If the load turns off, the short to ground is after the connector Reconnect it and proceed to the next connector NOTES: 188 Technical Training Technician Handbook 652 Body Electrical Diagnosis When the load turns OFF, you have disconnected it from the unwanted ground point This means the short to ground is farther along in the circuit Continue disconnecting connectors until you find one that does not turn the load OFF NOTES: Technical Training 189 Technician Handbook 652 Body Electrical Diagnosis If the load stays ON when you open the circuit, the ground must be before that connector Look for the short between this connector and the one before that turned the load OFF NOTES: 190 Technical Training Technician Handbook 652 Body Electrical Diagnosis Here the tail fuse powers multiple circuits through the junction block When diagnosing this type of circuit, the first thing to is identify the branch that is shorted to ground Remember, when you disconnect a connector, you’re disconnecting many wires at once Therefore, the reason the problem goes way may be different from what you think NOTES: Technical Training 191 Technician Handbook 652 Body Electrical Diagnosis It is important to disconnect individual circuits one at a time In this case, it means you have to remove wire 2G terminal 15 from the connector to determine if the short is in that branch Disconnecting the entire 2G connector does not tell you anything about the short to ground in the taillight circuit If the 2G connector is disconnected from the junction block, the light goes out (it does not dim) because the 2G connector also contains the wire that powers the taillight circuit NOTES: 192 Technical Training Technician Handbook 652 Body Electrical Diagnosis When the 2K terminal 19 wire is disconnected, the light dims because the short to ground is eliminated from the circuit That means the short to ground has to be in the green wires or the lights/light sockets on the 2K terminal 19 circuit The problem could not be in the white-black wires because they are ground circuits and cannot be shorted to ground NOTES: Technical Training 193 Technician Handbook 652 Body Electrical Diagnosis Feedback Problems Feedback problems are probably the strangest electrical problems you can encounter on a vehicle At one time or another, you have probably seen an electrical problem that just did not make sense, with seemingly unrelated circuits affecting each other like “magic”: •  Operating the right side turn signal causes the side markers to flash •  When the rear defogger is turned ON, the radio turns OFF •  When the horn is operated, the high beam indicator turns ON As you know, there is no “magic” in electricity These circuits have to be related through parallel connections on either the +B or the ground circuit For example, an open in the circuit to a ground point (in a circuit that has no redundant or alternate ground path) will force current flow to find another path to ground This other path to ground can be through any load or resistance that has a parallel connection to the problem circuit Diagnosing a Feedback Problem 194 With a feedback problem, tracing the path of current flow is extremely difficult You have to think backwards as you try to guess where current flow is going Since feedback problems don’t happen very often, figuring out how everything is happening could be a very time consuming process Technical Training Technician Handbook 652 Body Electrical Diagnosis Quick Checks for Lighting Circuits Fortunately, there are some quick checks that you can make to catch just about any feedback problems you will run into One of the most common areas of feedback problems is in the car’s exterior lighting circuits When working on a lighting circuit feedback problem, check the following: •  A shorted light bulb (by a blown filament) •  The customer installed light bulb of the incorrect type or wattage •  There is an open in one of the light bulb grounds Quick Checks for General Feedback Problems When diagnosing feedback in any body electrical circuit: 1.  Check the fuses Look for a blown fuse on any circuit The lack of a voltage in a circuit can sometimes allow voltage from one circuit to feedback into another 2.  Check for open ground wires A simple disconnected ground wire can cause the worst of feedback problems 3.  Check for an open circuit isolation diode In many circuits, a diode is used in the circuit to prevent feedback problems from happening If you are operating a circuit that has an isolation diode, and the feedback condition occurs, it’s a good place to check Technical Training 195 Technician Handbook 652 Body Electrical Diagnosis If the Quick Checks Do Not Find the Cause If these quick checks not find the cause of the problem: 1.  Thoroughly check each circuit individually Make your inspections to one circuit at a time, and make sure current flow is going exactly where it’s supposed to Check each connection and section of wiring with an ohmmeter, making sure that there is continuity to the correct locations as specified in the EWD Apply this check to both the +B and ground sides of the circuit 2.  Technical Assistance With a database of solved problems from throughout the country, Toyota Technical Assistance can be of great help for this type of problem When you reach the “wall” in diagnosing the problem, call your Region/PD’s assigned TAS 196 Technical Training Technician Handbook 652 Body Electrical Diagnosis You are going to be asked to use the 6-step diagnostic process to isolate the causes of several electrical concerns Review the 6-step process before you begin NOTES: Technical Training 197 Technician Handbook 652 Body Electrical Diagnosis Use this space to write down any questions you may have for your instructor NOTES: 198 Technical Training Technician Handbook 652 Body Electrical Diagnosis Technical Training 199 Technician Handbook 652 Body Electrical Diagnosis This page intentionally left blank 200 Technical Training ... 162 Technical Training Technician Handbook 652 Body Electrical Diagnosis What does this test result tell you? NOTES: Technical Training 163 Technician Handbook 652 Body Electrical Diagnosis What... 164 Technical Training Technician Handbook 652 Body Electrical Diagnosis What does this test result tell you? NOTES: Technical Training 165 Technician Handbook 652 Body Electrical Diagnosis What... Technical Training Technician Handbook 652 Body Electrical Diagnosis What reading did you expect at this test point? NOTES: Technical Training 171 Technician Handbook 652 Body Electrical Diagnosis