A shift solenoid mounts to your transmission's valve body and regulates the flow of transmission fluid through the passages and circuits inside the transmission case. The plungers of the solenoid stay submerged in transmission fluid at all times, which may lead to wear and damage caused by friction or debris. Understanding the signs of impending solenoid failure and how test it may help you spot a problem before your transmission gives out completely. Here's a look at the symptoms to watch for and how to test it.
Response on the Road
The transmission's operation is dependent on the solenoid that controls gear selection and shifting. If the shift solenoid on your transmission is failing or bad, you may lose a gear in your transmission. In some cases, your transmission may be stuck in one gear or unable to shift at all.
Don't confuse slipping with solenoid failure, though. Slipping transmissions will shift, but then once the transmission is in gear, it won't have any power. If the solenoid is bad, the transmission won't actually complete the shift.
Control Module Behavior
Most cars are fitted with a transmission control module that monitors the function of the transmission through a series of sensors connect to the system. There are shift-position sensors, speed sensors and more. The wiring for these modules and the solenoids are all connected through fuses. If a fuse fails, that's going to hinder the solenoid's function. It can also cause the transmission control module to fail, which may cause your transmission to shift incorrectly or hard.
Limp-In Safety Features
The transmission control module has built-in failsafes that will detect any potential failures, including blown fuses and bad solenoids. If the problem is significant, the module will trigger limp-in mode, which is designed to preserve the transmission's core integrity while still allowing you to get the car to a mechanic.
In most systems, limp-in mode locks the transmission into second gear. You'll find that the car feels sluggish taking off from a stop and the engine rotations will run higher if you reach speeds over about 30 miles per hour. Limp-in mode is not designed for daily travel, but just to get your car home or to a mechanic. If your transmission is in limp-in mode, you need to have it looked at right away.
Engine Trouble Codes
If the control module is detecting any kind of problem in the transmission, it sends a trouble code to the engine's computer. This computer stores trouble codes for a technician to scan and read using an OBD-II scanner.
Most auto parts stores will read these codes for free if you ask, and may even be able to provide you with the manufacturer-specific code information so that you know where the problem is. In most cases, the code will even tell you which solenoid or sensor is failing. This can save you from having to test each one manually.
Manual Solenoid Testing
If you prefer, you can actually test the solenoid yourself using a volt meter. Lift the car up on jacks so that you can get beneath it safely. Then, remove the oil pan covering the transmission. To remove this pan, you'll need a ratchet set. It's secured by bolts around teh outer perimeter. The actual bolt size will vary by manufacturer, so make sure that you have several sizes of sockets. Once you remove the pan, you'll see the solenoid attached to the side of the transmission body.
Disconnect one of the two plugs just above the solenoid. Set your volt meter to 200 and make sure that it's reading ohms. Touch the black wire from the volt meter to your negative battery terminal. Then, touch the positive wire to the tab inside the plug you disconnected. There are two tabs inside the plug.
When you test the plug, the volt meter should read between 12 and 25, which is a standard reading for this type of component. If the meter reads more than this, you'll need to replace the solenoid. Repeat this test on the second plug.
If you suspect a failing solenoid, you can take your car to a mechanic to have it replaced. Most transmission repair shops can take care of it for you. It is in your best interest to replace it right away if you notice any of these signs of problems. The sooner you take care of it, the less risk you have of completely disabling your transmission.