342 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. Valve body Transmission pressure sensors Transmission oil temperature sensor Figure 14-3. Most transmission sensors are located inside the case, usually on the valve body. On this particular valve body, all the pressure sensors and the temperature sensor are installed in a plate. The plate is then attached to the valve body. Also note the pressure control solenoids on the outside edges of the valve body. (Sonnax) Filters Figure 14-4. The valves of a solenoid are machined to extremely small clearances and cannot tolerate the presence of even the smallest particle of dirt or metal. Most transmis- sion solenoids are equipped with small filters to reduce the possibility of contamination. These filters can plug up, especially if other transmission or transaxle parts have failed. A loss of voltage to a sensor (usually called reference voltage) will result in an inoperative sensor. Reference volt- age problems are usually caused by a defective ECM or a ground problem. Solenoid Problems Solenoid problems will vary depending on whether the solenoid is an on-off type or a pulsed type. On-off solenoids can stick open or closed. The usual result is the loss of some gears. In many cases, the transmission or transaxle will take off from a stop in a gear other than first. Other solenoid failures can cause gear skipping (shifting from first to third, for example), failure to shift into higher gears, or incorrect application of the converter lockup clutch. Occasionally, a solenoid will stick at times and work properly at other times. Typical causes of intermittent sticking include a high- resistance solenoid winding, a bad electrical connection, or buildup of sludge. Erratic shifting is a common symptom of an intermittently sticking solenoid. The transmission or transaxle may work well most of the time, with only occa- sional shift problems. In some cases, the transmission will shift improperly only when cold or only when hot. Pulsed solenoids are generally used to control pres- sures. Therefore, instead of skipping gears, a failed pulsed solenoid can cause slipping or excessively hard or soft shifts. A defective line pressure solenoid will cause problems in any or all gears. Many pulsed solenoids control pressures of a specific operation, such as converter clutch apply feel or part throttle downshift. Defects in the related solenoid will cause problems during that process only. Almost every solenoid has one or more small filters, Figure 14-4. If the filter becomes plugged, oil pressure will not be able to pass through the solenoid valve to the rest of the hydraulic system. Additionally, a torn filter can cause the solenoid to stick. ECM Problems Since it contains many complex circuits, a faulty ECM can cause a variety of problems, depending on which internal part or circuit has failed. Sometimes, the ECM will keep the transmission or transaxle from shifting. A defec- tive ECM may cause the unit to stick in one gear. Another common ECM problem is erratic shifting. Examples include downshifting at cruising speeds or occasionally failing to upshift. The operation of the ECM is often affected by heat. A cold ECM may work well when the vehicle is first driven but cause problems as it heats up. A defective ECM can hang up, or stick in one mode. Turning the ignition switch off and then back on may tem- porarily correct the problem. If the ECM controls pressures through a pulsed solenoid, failure may cause slipping, hard shifting, shudder during shifts, and other problems. In many cases, a failed ECM will also cause engine drivability problems. A defective ECM can set false trouble codes and may sometimes set codes that do not exist. A failed ECM may illuminate the MIL when there is no problem, or it may fail to illuminate the MIL when a problem is present.