Automatic Transmission Shift Solenoid Fault Codes
Automatic Transmission fault indicator lights may be stand alone, or may be shared with the engine light. Some vehicles have an OD (overdrive) light that may blink. When the light is on, it means the computer has detected a problem and has set a diagnostic trouble code, which can be read using a diagnostic scanning tool. Diagnostic trouble codes can be set in 1 of 3 ways. The first is a persistent code that alerts the operator with the indicator light and may also set the transmission into fail safe, if it is a transmission code. This code remains set, even if the vehicle is shut off and restarted. The second is similar to the first, but is reset if the vehicle is restarted. A history of the code is kept in the ECM on some vehicles. We use the generic term TCM, but some use ECM, Computer, Controller, etc. Lastly, some codes do not illuminate the indicator light, and is often reset when restarting the car. When bringing a car or truck to a shop because of a persistent or intermittent automatic transmission service fault code light, you should consider calling ahead and then keeping the vehicle running at the shop until it is checked by the mechanic.
There are different categories of automatic transmission fault codes. Some are concise and can generate an immediate, accurate diagnosis; others can be vague or completely misleading. Any fault code can be primarily classified as either a circuit fault or not one. Those that are not can be further classified as gear ratio errors or shift / performance error codes.
Electrical Circuit Fault Codes
Electrical circuit fault codes are set when a transmission control processor within the TCM or ECM identifies a voltage drop across a circuit which is determined to be out of acceptable range. This usually indicates an open or a short in either a specific electrical component or in the associated wiring completing the circuit. Circuit codes usually identify the affected component by name, making a diagnosis easier. Automatic transmission shift solenoids, speed sensors, and shifter lever position sensors can all be affected. Poor wiring harness connections and improper body grounding are occasionally the culprit, and in some rare cases, the electrical problem may lie within the automatic transmission control module itself. If an electrical fault code is set soon after major mechanical or bodywork, it is less likely to be an internal automatic transmission electronic component, so other areas should be checked first.
Vehicle and Transmission Input and Output Speed Sensors
Engine, transmission, and drive axle RPMs are constantly monitored by the control modules using speed sensors. Speed sensors are generally composed of a coil of fine wire, with hundreds of windings wrapped around a metallic or magnetic core, secured in a plastic housing. The coil reacts to changes in inductive reactance as the metal teeth located on the spinning engine and transmission parts move past it. While speed sensors have no moving parts, they commonly fail due to inferior manufacturing methods. A defective speed sensor usually produces a circuit error, but can also create an automatic transmission gear ratio error, especially if it is contaminated with metal particles that stick to the metallic or magnetic core of the sensor.
Gear Ratio Error Fault Codes
An automatic transmission gear ratio error fault code can also be caused by slipping or broken components inside the transmission. These codes are generated when the ratio between the input and output speeds of the transmission do not match the currently commanded gear. Gear ratio error codes usually identify the specific gear that is malfunctioning and may be accompanied by automatic transmission shift solenoid performance error codes. Performance codes are set when the computer sends out a command, such as a 2-3 shift, but the transmission does not respond properly. Any solenoid code that is not a circuit error is likely going to be a performance or malfunctioning error.
Electronic Transmission Solenoids
A transmission solenoid is an electro-mechanical device that controls the flow of transmission fluid through the hydraulic circuits within the transmission. With this knowledge, most would assume that a performance error means that there is a problem with the mechanical operation of the solenoid, but very often this is not the case. Problems with any transmission component that adversely affects the actuation of a clutch pack or band can return a solenoid performance code, including the valve body, sealing rings, pump, and rubber seals. For this reason, solenoid performance codes should not be taken at face value. The best tool for diagnosing solenoid codes is knowledge and experience. An experienced transmission technician knows which solenoids often fail and which ones rarely do. If the solenoid has a high failure rate, than the money that would be spent on further diagnostic time should instead be applied towards the cost of replacing the solenoid. If the solenoid has a low failure rate, then more time should probably be spent looking for other causes of the problem.