Chapter 25 Computer System Service 425 Copyright Goodheart-Willcox Co., Inc. source, only after all other potential sources of trouble have been eliminated. It is sometimes possible for an integrated circuit, transistor, or other electronic part in the computer to fail and upset system operation. Measuring Computer Output If the computer is not tested, a defective computer is often identified through the process of eliminating the sensors, actuators, and related wiring as the cause of the problem. In the process of sensor and actuator testing, it was necessary for you to measure the computer’s output. A computer output can be a reference voltage to a sensor or a supply voltage to an actuator. Use a voltmeter to make sure the correct refer- ence voltage is being sent to a sensor. Most comput- ers produce a reference voltage of about 5 volts. If the reference voltage is not correct, check the wiring before condemning the computer. You can also mea- sure voltage to make sure the correct voltage output is being fed to the actuators. Each computer input and output passes through an individual metal terminal, or pin, which plugs into the vehicle wiring harness. Pin numbers identify the location and purpose (electrical value and internal connection) of each terminal in a computer wiring harness connector. You may have to probe computer terminal pins to find the source of complex problems. This will be discussed in a later chapter. Again, always refer to the service manual for exact procedures when testing a computer system. One wrong electrical connection can destroy the computer. Caution Never use an ohmmeter to check a computer, as it will damage the computer’s internal circuitry. If it is necessary to use an ohmmeter to check the continuity of a wire or circuit in the computer harness, disconnect all wiring harnesses from the computer before testing. Saving Memory Saving memory can be done by connecting a memory saver to the accessory port (lighter port) or the data link connector before disconnecting the vehicle’s battery. The memory saver supplies enough power to keep the clock, stereo, and computer from losing the information stored in their memories. Some memory savers use a 9-volt battery and plug into the vehicle’s accessory port. Other memory savers contain a rechargeable 12-volt battery and plug into the vehicle’s accessory port or the data link connec- tor. See Figure 25-11. In some cases, a battery jump start box can be used as a memory saver. A specially designed cable connects the jump start box to the vehicle’s accessory port or data link connector. Before using a memory saver, turn off all accesso- ries (radio, blower, etc.). The current drain from these devices, combined with even the smallest voltage drop, could cause electronic devices (computer, clock, radio, etc.) to lose their preprogrammed data. You are still disconnecting the vehicle battery for safety. The memory saver cannot supply enough current to cause an electrical fire or operate the starting system. Computer Replacement Before disconnecting the battery and removing the computer from the vehicle, you should scan the computer and obtain the PROM identification number or the EEPROM calibration number. This information Memory saver Memory saver-to-data link connector cable Memory saver-to-accessory port cable OTC/SPX Corp. Figure 25-11. This memory saver has cables for your vehicle’s accessory port and for the data link connector.