360 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. Most modern ECMs have a provision for changing the information in the programmable read only memory (PROM). Often, the original ECM programming was not perfectly calibrated to the vehicle. Sometimes actual oper- ating conditions cause minor drivability problems with the original PROM programming. When this occurs, the manufacturer issues updated PROM information. The only way to determine whether updated information will cure a transmission or transaxle problem is to check the manu- facturer’s service bulletins or other update information. Deciding on Work Needed After locating a defective part, never assume that it is the root cause of the problem. Always try to determine what caused a part to fail. For example, if the ECM is defective, test all sensors and output solenoids for excessive current draw that could have caused the module to fail. Careless or incomplete diagnosis can allow further damage to the elec- tronic components. Sometimes, a missed electronic defect can cause the transmission or transaxle to slip, destroying the holding members. Summary Many electronically controlled transmission or trans- axle problems are the same as those encountered in hydrau- lically controlled transmissions. Other problems are unique to the electronic control system. Many computer control Figure 14-32. One method of checking voltage output from the ECM. Be extremely careful when making checks like this, as the current flow through the meter could damage the ECM. (Nissan) system problems are referred to as pattern failures, since they occur regularly on a particular system. In many cases, the transmission or transaxle is react- ing to problems in the engine. Sensors are the most com- mon cause of electronic transmission/transaxle problems. Many sensor defects first show up as engine performance problems. If a solenoid controls an entire system, failure may cause a transmission defect in all gears. If a defective sole- noid controls only one gear, the problem will show up in that gear only. A failed ECM can cause slipping, hard shifting, improper shift speeds, shudder during shifts, and other problems. A failed ECM may also cause engine drivability problems. Wiring and connector problems are common sources of computer system problems. The voltages used to operate the sensors are usually much lower than battery voltage, and a slight increase in resistance can cause problems. Defects in the ground circuit are commonly overlooked. The seven-step diagnosis process is very helpful in determining the exact cause of a problem. It is sometimes difficult whether the problem is in the electronic control sys- tem or in the basic hydraulic and mechanical components. Always make the same preliminary checks that you would with hydraulically controlled transmission and transaxles when troubleshooting an electronically controlled trans- mission or transaxle. Before performing complex electronic tests, make some basic electrical checks. Check fuses, wiring, and ground wires. If these checks do not reveal a problem, use a scan tool to check the system. The scan tool allows the technician to access trouble codes and obtain information on shift speeds, converter clutch application speed, and transmission/transaxle temperatures and pressures. The scan tool can also provide information on engine RPM and temperature readings. Multimeters are used to read electrical values such as voltage, resistance, and amperage. Many multimeters can read the waveforms produced by the system components as they operate. Scopes can also produce these readings. The easiest way to diagnose the operation of an elec- tronic control system is by using the proper scan tool. If pos- sible, drive the vehicle and observe the scan tool readings. During the test drive, note the transmission or transaxle shift pattern, as well as any slippage or noises. To test individual electronic transmission and transaxle components, you should make visual checks, perform elec- trical tests, and test by substitution. Begin by looking for dis- connected or damaged wiring, obvious component defects, or problems in related areas. Then make electrical tests to sensors and output devices as needed. As a last resort, test by substituting a known good part. Most throttle position sensors can be tested with an ohmmeter. Some throttle position sensors are transducers and, therefore, must be checked with a voltmeter. To check a speed sensor, use a multimeter to check for the presence of an ac waveform. Speed sensors can also be Tester probe Thin wire