A visual inspection of the transmission should include a careful check of all electrical wires, connectors, and components. This inspection is especially important for electronically controlled transmissions and for transmissions that have a lockup torque converter. All faulty connectors, wires, and components should be repaired or replaced before continuing your diagnosis of the transmission.

Check all electrical connections to the transmission.

Typical arrangement of electrical components on a modern transaxle. Courtesy of American Honda Motor Co., Inc.

Faulty connectors or wires can cause harsh or delayed and missed shifts. On transaxles, the connectors can normally be inspected through the engine compartment, whereas they can only be seen from under the vehicle on longitudinally mounted transmissions. To check the connectors, release the locking tabs and disconnect them, one at a time, from the transmission. Carefully examine them for signs of corrosion, distortion, moisture, and transmission fluid. A connector or wiring harness may deteriorate if automatic transmission fluid (ATF) reaches it. Also check the connector at the transmission. Using a small mirror and flashlight may help you get a good look at the inside of the connectors. Inspect the entire transmission wiring harness for tears and other damage. Road debris can damage the wiring and connectors mounted underneath the vehicle.

Because the operation of the engine and transmission is integrated through the control computer, a faulty engine sensor or connector may affect the operation of both the engine and the transmission. The various sensors and their locations can be identified by referring to the appropriate service manual. The engine control sensors that are the most likely to cause shifting problems are the throttle position (TP) sensor, manifold absolute pressure (MAP) sensor, and vehicle speed sensor.

Throttle-position sensors are typically located on the side of the throttle plate assembly. Remove the electrical connector from the sensor and inspect both ends for signs of corrosion and damage. Poor contact can cause the transmission to miss shifts. Also inspect the wiring harness to the TP sensor for evidence of damage.

MAP sensors are usually located on top of the intake manifold or mounted to the firewall, where a vacuum hose connects it to the intake manifold. Check both ends of its three-pronged connector and wiring for corrosion and damage. Also check the condition of the vacuum hose. A vacuum leak can cause harsh shifting.

A vehicle speed sensor is often used on late-model cars to monitor speed. The function of this sensor is similar to a speedometer except that it operates electronically rather than mechanically. Proper shift points cannot occur if the signal from the speed sensor is faulty. The sensor's connections and wiring should be inspected for signs of damage and corrosion. These sensors are located near the output shaft of the transmission.