662 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. and testing new electronic systems. Th ey can sometimes be helpful in the shop to do pinpoint testing when the source of an electrical problem cannot be found. A breakout box is connected in parallel with the circuit wiring harness. An inline connector is provided for plugging the test box into the wiring harness, usually at the ECU connector. Th en, a multimeter is used to touch specifi c terminals on the breakout box. Th ese measured circuit values are com- pared to manufacturer’s specifi cations. If the measured values are not correct, you have found the defective component or wiring problem. For example, if a reference voltage out of the tested circuit is low, say only three volts (should be fi ve volts), the ECM is bad, the wiring between the ECM and the break- out box is faulty, or a sensor has partially failed. Cranking Balance Test A cranking balance test is done to check the engine’s mechanical condition. It can be used to isolate a cylinder with low compression due to a burned valve, worn piston rings, or other problems. Th e scan tool will show how much current is drawn by the starter motor as each cylinder goes through its compression stroke. High current draw (high display line) means high compression stroke pressure. Low current draw (low display line) means that cylinder has low compression. Look at Figure 34-10. Checking ECU Terminal Values Electronic control unit (ECU) terminal values are tested at the metal pins of the unit. A digital VOM and special test connector can be used to read terminal voltage and resistance values to evaluate the internal circuitry of the ECU. Th ese read- ings can then be compared to known good values to isolate the problem with the internal circuitry of the device. Caution: Never connect a low-impedance (resistance) analog meter or test light to an electronic circuit unless instructed to do so by the service manual. A low-impedance meter or tester could draw enough current to damage delicate electronic devices. To be safe, only use high-impedance instruments and test lights during diagnosis. Using a Breakout Box A breakout box allows you to check electrical values at specifi c points in an electronic circuit. It is typically used as a last resort to fi nding electrical/electronic faults that can- not be found by other means. It is one of the last tools used in diagnostics, because it is time-consuming, Figure 34-9. Breakout boxes are oft en used by engineers when designing Figure 34-9. A breakout box is usually the last tool used to find performance problems. It is connected to the wiring harness in the computer system. Then, a multimeter can be used to check terminals on the breakout box for actual operating voltages, resis- tance, and current values. They can be compared to known good values or to datastream values to find the cause of the problem. Figure 34-10. Cranking balance tests are used to check general engine compression. If any cylinder does not load the starter motor as much as the others, it has low compression pressure and a possible leak. A—All bar graphs are at the same height, so all cylinders have the same compression. B—The number 5 cylinder has a low bar graph indicating less compression pressure. (Snap-on Tool Corp.) B A