A transmission shift solenoid is a type of electro-hydraulic valve that allows or restricts fluid flow inside a valve body. Depending on the design of your transmission, it can have two or more solenoids that are used to shift gears. They are operated by your transmission control unit or powertrain control module that engages or disengages a specific solenoid to shift into the required gear.
A typical shift solenoid features a spring-loaded plunger inside a magnetic coil. When your control module needs to change a shift, it sends electrical current to the right solenoid, causing its plunger to open the valve to allow the pressurized hydraulic fluid to enter the certain part of the transmission.
As a general rule, shift solenoids are integrated into a valve body. They are submerged into transmission fluid, which is why they are prone to wear and tear due to rapid temperature changes. Besides, the mechanical part of a shift solenoid can also get damaged as it is subjected to harsh operating conditions.
A faulty solenoid can cause an array of transmission related problems as well as affect operation of the whole transmission assembly. Fortunately, before this happens, you should notice one or more of the following signs of a bad solenoid.
Illuminated check engine light
Your control module monitors the condition and operation if your shift solenoids. If it detects any problem related to solenoid operation, it will turn on the check engine light.
Incorrect shifting or slippage
The shift solenoids affect the flow of hydraulic fluid in your transmission. If one of them fails, the transmission will experience a lack of pressure, which can lead to erratic shifting or transmission slippage when an increase in engine speed does not cause your vehicle to run faster.
No gear shifting happens
A faulty solenoid may restrict fluid flow into the required part of the transmission, resulting in inability to shift into the desired gear.
“Limp-home” mode is activated
If a shift solenoid fails, your control unit can activate the “limp-home” mode to prevent damage to the valve body and other transmission parts. In this case, only one or two gears will be available in order to limit your speed.
What should I do if one of my solenoids fails?
A bad shift solenoid should be replaced. However, prior to purchasing a replacement part, a thorough diagnosis needs to be done to prevent unnecessary repairs. In many cases, similar symptoms can be caused by poor wiring or a faulty sensor.
On some vehicles, solenoids are an integral part of the valve body assembly. In this case the entire assembly should be replaced. If the control module is also a built-in part of the valve assembly, its replacement will cost you a pretty penny. Besides, it needs to be reprogrammed once installed to ensure proper transmission operation.