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How To Know If a Transmission Solenoid Is Bad?

Close up bottom view of a transmission with the bottom pan off allowing one to see the solenoids and valve body in a car's transmission

Telling is a transmission solenoid is bad requires some basic knowledge of how the transmission system works on a vehicle.

Some common symptoms point to a bad solenoid, but the only way to know if a transmission solenoid is bad is through a proper diagnosis. The most common symptoms are easy to recognize.

The Top Symptoms Of a Bad Transmission Solenoid:

  • Gears getting stuck in one setting
  • Gears are skipped when the vehicle shifts gears
  • Delayed response when braking
  • Check engine light/transmission light in some models
  • The car refuses to shift gears, and RPMs continue to climb

When the transmission starts acting up, it can be slightly intimidating as the panic sets in. Mostly, the transmission is a series of gears and solenoids that help the vehicle shift so that it can travel down the road safely at a controlled speed.

Transmission solenoids are found in most modern cars with automatic transmissions. There are several ways to tell if the solenoid will need fixing soon. 

What’s a Transmission Solenoid?

Transmission solenoids are electromechanical devices that are used to control the transmission in a vehicle. They are typically located within the transmission itself and are responsible for regulating the flow of transmission fluid to different parts of the transmission.

There are several different types of transmission solenoids, each of which serves a specific purpose. For example, some solenoids are used to control the shift points in the transmission, while others are used to control the flow of fluid to the clutch or torque converter.

Transmission solenoids are an essential component of the transmission system and play a vital role in the overall performance and operation of the vehicle.

The valve body is a part of the transmission that controls the flow of transmission fluid, and the solenoids are responsible for regulating the flow of fluid to different parts of the transmission.

The transmission control module, on the other hand, is a computer that is responsible for controlling the transmission and its various components, including the solenoids.

Close up of linear shift solenoids housed inside a valve body that's been removed from a transmission and placed on a workbench

What Causes a Bad Transmission Solenoid?

The transmission solenoid fails because it gets old. The most common reason that the transmission solenoid fails is wear and tear and poor maintenance.

The transmission should be serviced regularly with the recommended fluid change intervals. Dirty fluid is a significant contributor to why transmission solenoids fail.

The Top Five Most Significant Reasons Why Solenoids Fail:

  • Damage
  • Dirt clogs
  • Metal shavings
  • Electric surge
  • Corrosion

The second most common reason is an electrical surge to the coil of the transmission solenoid. This happens for several reasons, and sometimes a vehicle owner has no control over this event.

First, all transmissions have a Solenoid that helps push the fluid around inside the transmission, which helps shift gears. Every single one is controlled by the vehicle’s Electronic Control Module, or ECM.

Maintaining the transmission is vital because tiny metal shavings flake into the transmission fluid as the transmission gears turn. Those shavings, over time, can clog the valves and other orifices on the solenoid, causing damage and clogs.

These metal shavings will also wear away at the seals of the solenoid because of the pressure they are under when engaged.

How Do I Know If I am Experiencing Shifting Delays or Irregular Shifting Patterns?

There is a shifting delay when the fluid does not eject from the valve when it needs to shift gears. Eventually, the gear will shift itself; however, it is usually noticeable before it can cause any significant damage.

The delay will start out slight at first and get worse over time. The transmission solenoid is not pushing the correct fluid out properly.

Person holding an automatic shifter inside a car to change transmission gears

What If The Transmission Will Not Down Shift?

Too much pressure is built up in the transfer case of the transmission. As a result, a solenoid is stuck open (or several), which prevents the gear from moving in the opposite direction (i.e., Downshifting). Two reasons a solenoid will get stuck open: a faulty electrical signal or debris (dirty fluid).

If the problem is the transmission fluid, it just needs to be changed. The solenoid must be changed if the issue is an electrical problem.

Transmission fluid needs to be changed at the manufacturer’s recommended intervals. Not changing the transmission fluid will eventually cause the transmission solenoids to fail.

The transmission filter will also become dirty and clogged. Finally, as these systems slowly fail, they will reach the point of no return. The vehicle may even engage in its safe mode of operation to prevent further damage.

If the transmission fluid has not been changed and the car has high mileage, there is a chance that doing a complete flush and fill service on that specific transmission will make it worse.

My Transmission Shifts Randomly and Jumps Gears

This sure tells sign is dangerous for all drivers. The solenoid works by opening and closing a valve and moving the transmission fluid around the transfer case under pressure.

While most assume that the transmission fluid is undisturbed, the liquid plays a vital role in the transmission’s ability to shift gears properly.

When the solenoid opens and closes at random times, this creates a real danger for the driver. The vehicle could shift to a lower gear when attempting to pass a motorist on the road.

The transmission could shift into a higher gear and either accelerate or stall and jump forward, causing an accident from the unwanted acceleration. The solenoid needs to be replaced immediately, as the issues from here will only worsen. 

Does The Check Engine & A/T Lights Trigger If a Transmission Solenoid Is Faulty?

The transmission solenoid itself may not trigger the code. Still, when combined with other issues, one of the other systems may activate a code related to the transmission solenoid prompting at least a look at it.

The “Check Engine & A/T lights will often both illuminate when something is wrong with any of these engine/transmission systems.

System:Power TrainFuelEmissionsElectronics
 EngineInjectorsSensorsControl Modules
 TransmissionSensorsVacuum/pressureSending units
 Gear Box (rear)Pumps  
 SensorsFilters  

The check engine light will trigger because the sensors send information back to the computer, telling the computer that the pressure levels are incorrect in the transmission.

The sensors may even be able to narrow down the general area of the malfunction and point a mechanic in the general direction of a transmission solenoid being the most likely culprit.

Can I Drive the Car With a Bad Transmission Solenoid?

The car can be driven with a bad transmission solenoid; however, it should be noted that it needs replacing. Failure to replace a worn or damaged transmission solenoid will result in high repair bills.

In addition, ignoring the failed solenoid could result in the vehicle placing itself in fail-safe or limp mode to prevent further damage to the transmission from a faulty solenoid.

Is The Check Engine Light Flashing, and The Vehicle Driving Slow?

Some vehicles have what is called safe mode or limp mode. This severely limits all vehicles operating systems to avoid further damage to the vehicle.

For example, if the check engine light is flashing and the car is moving slowly, it is time to take it to a mechanic. The computer has limited the ability of the transmission to only allow specific movements until the issue is fixed.

Mostly limited to traveling less than thirty miles an hour, continuing the drive of the vehicle that has engaged its limp mode will cause irreparable damage to the transmission.

Close up of diagnostics lights with the AT light and check engine lights highlighted

Do I Need a Mechanic to Replace The Solenoid?

People use mechanics for the same reason they use doctors, the expertise and knowledge of the vehicle’s systems.

All the needed parts are plug n play, and some features are extremely tough to get to and require special tools and training, but the transmission solenoid alone is not that way. The job is very Do-It-Yourself friendly (on most cars).

How Much Does It Cost To Replace The Solenoid?

Replacing the transmission solenoid in a shop range between three hundred to six hundred dollars for labor plus the cost of parts.

 Do-It-YourselfShop
PartsSingle: 15-100 per solenoid Pack: 65-250+ per packSingle: Wholesale + shop markup Pack: Wholesale + markup
Labor 150+ per hour
Time-estimate2-6 hours2-4 hours

Replacing the Solenoid, the Do-it-Yourself way will always be cheaper. When factoring in the labor costs and the markup of the parts, the two things that cost the most are a lack of skill and confidence.

However, changing the transmission solenoid is an easy to medium-difficulty repair and can be done in a few hours.

How Do You Change a Transmission Solenoid?

The first step in changing the transmission solenoid is to drain the transmission fluid from the transmission. The transmission filter and the fluid need to be replaced when changing a transmission solenoid.

There is no reason to go back and do twice the work, and in some cars, it is required. The metal shavings from the fluid will only clog the new solenoid.

Transmission solenoids are typically located within the transmission itself, either on the valve body or on the transmission control module.

Depending on the specific vehicle, the transmission solenoids may be located in different places within the transmission, but they are generally found either on the valve body or on the transmission control module.

With the fluid drained, remove the transmission oil pan and gasket. The next step is after locating the valve body, which controls the flow of the hydraulic fluid in the transmission. The Solenoids are color-coded and should be easy to disconnect and remove from the valve body.

Car mechanic removing the bottom pan of the transmission to change the automatic transmission fluid exposing the solenoids and valve body

Check The Manufacturer’s Requirements Before Changing The Solenoid(s).

Some transmission solenoids must be replaced in groups. In addition, some manufacturers require that all the solenoids be replaced simultaneously based on the engineering design of the transmission.

Not following these recommendations from the manufacturer will lead to more severe issues. However, most of the time, a single transmission solenoid can be replaced after draining the fluid.

Re-assemble The Transmission After Replacing The Solenoids

Apply a little bit of transmission fluid to the solenoid before inserting the new solenoid. This way, the solenoid is not firing dry the first time it is used.

After reconnecting the wiring harness to the transmission solenoid and re-installing any brackets removed, it is time to put the transmission oil pan back onto the transmission.

Since the transmission is already open, a new filter and fluid are highly recommended. Put in the new transmission filter and use the gasket that came with the filter.

Using a regular gasket and not a silicone-based liquid gasket is highly recommended, as those gaskets tend to be hazard-prone. However, if using a liquid gasket, do not use too much because it will cause transmission issues later down the line.

After installing the filters and aligning the gasket onto the pan, it is time to mount the transmission oil pan back in place.

Tighten the mounting bolts and rub a small amount of transmission fluid on the outer rim to check for any air bubbles or gaps in the gasket. Then re-fill the transmission with fresh fluid and allow the gasket material to cure.

Some vehicles will require resetting the Transmission control module, and each manufacturer will have instructions for accomplishing these tasks. The steps to follow may even require manually placing the car into safe mode before resetting the transmission control module.

Bottom view of the automatic transmission with the pan removed and the solenoids highlighted

The Final Step Is To Drive The Car

The solenoid needs to be used a few times to get the solenoid in sync with the other parts of the vehicle. For example, while driving, the operator wants to go at least ten miles and cycle through each gear (including the “Reverse” gear) at least twice.

This method should introduce the new solenoid into the adaptive memory of the transmission control module, placing the new solenoid in sync with the rest of the solenoids.

Will Replacing The Transmission Solenoids Help My Car Be Less Sluggish?

A car can run sluggish for various reasons, and a transmission solenoid could be a contributing factor.

As the hydraulic fluid travels through the valve body and into the appropriate solenoid to create the pressure needed inside the transmission having a faulty transmission solenoid can add to the problem.

The other consideration with the transmission solenoid is the sensors sending and receiving the signals from the transmission. Again, depending on the vehicle’s design, these sensors report to the transmission or engine control module.

These modules and sensors tell the computer when and where to send the fluid needed to help the transmission solenoids that need the fluid at specific times.

Reasonably, the transmission control module will malfunction from time to time and need to be replaced over the transmission solenoids. Therefore, the transmission control module must be replaced instead of the transmission solenoids.

The only way to determine if this is the case is to run a proper diagnostic using the correct scanning tools.

Close up of a valve body that goes inside the automatic transmission with the solenoids installed

Conclusion

The transmission solenoid is a normal part of maintenance and should not be ignored. The solenoid itself is embedded into the transmission’s valve body, however, not properly maintaining the transmission is what causes most transmission solenoids to fail.

Therefore, after replacing the solenoid, it is crucial to follow the manufacturer’s instructions for resetting the transmission control module and cycling through the gears to complete the repair.

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References:

Mister Transmission

Rx Mechanic

Axle Wise

How Do You Reprogram A Transmission Control Module: Location & Procedure

Motor Hills

Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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