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Shift solenoid malfunctions can be the result of a variety of both mechanical and electrical faults. These solenoids are typically used to regulate a valve body, or to control clutch packs in a more direct manner. One cause of shift solenoid malfunction is if the solenoid itself becomes stuck, though a plugged valve can also result in various issues. Electrical problems can also cause a shift solenoid malfunction, since these components require battery voltage to operate. Any wiring or component between the solenoid and the transmission control unit (TCU) can theoretically cause a shift solenoid malfunction.
In modern electronically controlled transmissions, the purpose of a shift solenoid is to facilitate the changing of gears. This can be accomplished in a number of ways, which are largely automated by a transmission control unit or other similar computer module. The shift solenoids are responsible for automatically switching gears as the driving conditions change, but they can also be activated when the driver selects overdrive, low gear or another option. Since both mechanical and electrical components can be involved in the operation of shift solenoids, there are many different factors that can lead to malfunctions.
Many common shift solenoid malfunctions are mechanical in nature, such as a stuck plunger. This type of shift solenoid malfunction is typically caused by fluid contamination that results in the plunger not moving when it receives a signal. The vehicle will usually fail to shift properly when this occurs, and a trouble code might be set in the computer. Stuck plungers typically cannot be repaired, so the whole unit usually has to be replaced. This typically requires removing the transmission pan, and the valve body sometimes must be lowered as well.
Various electrical problems can also be the cause of a shift solenoid malfunction. A shorted out coil winding is one common issue that typically indicates that the solenoid requires replacement. Bad electrical connections at a shift solenoid or anywhere else in a wiring harness can also cause a failure. These issues are usually rectified by tracking down the bad connection or frayed wire and repairing it.
It is also possible for a transmission control unit to be the cause of a shift solenoid malfunction if it fails to send the correct signals at the proper time. These control units are responsible for the operation of all the electronic components in a transmission. Troubleshooting procedures are typically similar to other computer diagnostics, and can include the use of scan tools or a voltmeter to check output voltages to the solenoid.