352 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. damaged wiring, obvious component defects, or problems in related areas. For instance, if your preliminary investi- gation indicates a problem with the MAP sensor, do not assume the sensor itself is bad. First, check the vacuum hoses to the sensor and all nearby vacuum connections for damage (splits, cracks), kinking, and clogging. Make sure that hoses are not disconnected or connected to the wrong parts. Next, look at the wiring connector and check the sensor for physical damage. Making Electrical Tests Caution: Some procedures call for energizing a solenoid or other device with battery power. Always consult the proper factory service manual to be sure that the device is designed to operate on full battery voltage. Some electrical devices will be destroyed if battery voltage is applied to them. When using jumper wires to operate a solenoid or other device, be sure that the wiring to the ECM is disconnected. Full battery voltage to some ECM connectors can destroy the ECM. Note: Before making ohmmeter checks, be sure that the device or wiring is disconnected from all sources of electrical power. Ohmmeter readings taken on an energized device or wire are useless. Electrical power can also damage the ohmmeter. Solenoids can be tested by making a continuity check, Figure 14-18. Additionally, jumper wires can be used to apply power to the solenoid while observing its operation. Most solenoids do not have a visible plunger, and the techni- cian must listen carefully for a click to determine whether the solenoid is operating. Some solenoids and other compo- nents can be checked through the wiring harness without removing the part from the transmission. Sometimes, the part can be checked though the transmission or transaxle wiring connector, and the oil pan does not need to be removed. However, you must refer to the service manual to determine which connector pin energizes which compo- nent, Figure 14-19. Although most solid-state components require the use of specialized test equipment, basic checks can be made to some solid-state components. Common checks include using an ohmmeter to check a terminal for proper ground- ing. This check must be made very carefully to avoid dam- age to the unit. A few solid-state components can be checked for proper operation. When checking any electrical component for proper voltage or resistance, consult the manufacturer’s service literature to determine what the readings should be. On some vehicles, the solenoid amperage draw can be read and compared to the manufacturer’s specifica- tions. Some pressure control solenoids can be checked by comparing amperage draw to the corresponding line pressure. A typical amperage-pressure chart is shown in Figure 14-20. Testing by Substitution In many cases, the only way to determine whether a component is defective is to replace it with a unit that is known to be in good working condition. When testing by substitution, however, the technician must keep the follow- ing things in mind: • Obtaining a known good part may be expensive and time-consuming. In many cases, especially when sub- stituting an electrical or electronic part, the part will not be returnable. • If the actual problem is a short or high amperage draw somewhere else in the related circuit, the heavy current flow may ruin the replacement part. • If the part requires a great deal of labor (time) to replace, this time will be wasted if the suspect part is not the problem. Be sure to eliminate all other sources of a problem before substituting parts. Always consult the manufacturer’s manual to determine whether a part can be tested before replacing it. Technicians can partially offset the problem of obtaining substitute parts by acquiring a stock of commonly needed replacement parts. Figure 14-18. Many solenoids can be checked with an ohm- meter. To accurately check a solenoid, you must know what the correct resistance reading should be. If the solenoid resistance does not match the specifications, the solenoid should be replaced. (Nissan) Ground Line pressure solenoid Red White Blue Black Shift solenoid B Shift solenoid A Overrun clutch solenoid DISCONNECT 1 2 3 4