36 Auto Brakes Copyright by Goodheart-Willcox Co., Inc. Electrical and Electronic Testers A few decades ago the only electrical device attached to the vehicle brake system was the brake light switch. Today, how- ever, the brake technician needs a thorough knowledge of electrical and electronic theory, and a wide range of elec- trical/electronic testers. Th e following electrical testers are necessary to diagnose and service modern brake systems. Jumper Wires Jumper wires are short lengths of wire with alligator clips on each end. Th ey are used to make temporary electrical con- nections during vehicle testing procedures. Jumper wires are oft en used to bypass electrical devices, such as relays and solenoids. Bypassing an electrical device will enable you to determine whether it is defective. Jumper wires can also be used to access the computer memory on many vehicles by grounding the data link connector. Th ey are useful for mak- ing an extended electrical connection, such as connecting an engine test lead to a test device inside the passenger com- partment for a road test. Although they can be purchased, jumper wires can be made easily from a length of wire and alligator clips or other metal terminals. Use copper wire that has a large enough gage to carry several amps of current, but small enough to be easily handled. 12 or 14 gage wire is a good size for most jumper wires. Cut about 2–3′ (60.96–91.44 cm) of wire from the roll, then strip about 0.5″ (12.7 mm) of insulation from each end of the wire. Crimp the alligator clips or terminals to the wire, making a secure joint at each end. For addi- tional holding power and conductivity, the clips and wires can be lightly soldered together. Some examples are shown in Figure 3-22. Note: It is a good idea to install an inline fuse or other circuit protection device in the jumper wire. This helps to protect the circuit wiring and any electronic components should a short circuit be created accidentally. Another good source for jumper wires is the wiring harness in a wrecked vehicle. Th ese can be found easily and inexpensively in most auto salvage yards. Th e advantage to having these is they come with the original terminals, which ensures a good connection to the device to be tested. Th ey are handy for testing sensors and other parts that are dif- fi cult to access with a meter’s test leads. Remove the termi- nal from the sensor or part and then cut the terminal from the harness. Strip and crimp a terminal on the wire(s) as described earlier. Additional wire can be added if the con- nector is used in a hard-to-reach location. Th ese are handy if you only work on a few models, such as at a dealer or spe- cialty shop. Caution: When using jumper wires, make sure the current load does not exceed capacity. If the jumper wire insulation begins to discolor, blister, or smoke, remove the wire immediately using a shop towel or glove to prevent burns. Never grab a hot jumper wire with your bare hands. A severe burn could result. Never use a jumper wire to bypass a fuse or other circuit protection device. Test Lights Test lights are oft en used to check for the presence of volt- age in the brake system or to determine whether a circuit is complete. A non-powered test light, Figure 3-23, can be used to probe electrical circuits to determine whether volt- age is present. A powered test light, Figure 3-24, is used to determine whether a circuit is complete. Multimeters Multimeters are combination meters capable of measuring ohms, volts, amps, and sometimes waveforms. A typical multimeter is shown in Figure 3-25. Th e multimeter should be used when exact electrical values must be determined. Typical uses for a multimeter include checking for high resistance in an electrical connection, locating small battery drains, checking the resistance of anti-lock brake wheel sen- sors, and determining the condition of various brake system electrical switches. Ohmmeter Ohmmeters are used to measure resistance in an electrical device or circuit. Resistance is the opposition to current fl ow that exists in any electrical circuit or device. Th is resistance Goodheart-Willcox Publisher Figure 3-22. Jumper wires can be used with meters to check sensor, actuator, and circuit properties.