There are a few erratic behaviours a car demonstrate that car owners definitely dread. However, I’d vouch that none feels quite as intimidating as a transmission-related issue. Transmissions are expected to last as long, if not longer than the engine itself. However, modern complications have made transmission reliability a top concern for car buyers.
Transmission failures can be catastrophic, leaving you stranded while racking up exorbitant repair costs. Thankfully though, transmissions rarely fail outright without any signs. Typically, they demonstrate signs of failure way before they give up entirely. By paying close attention to those symptoms, you can avert the fault from further developing.
Because there are various types of transmission, we’ll be emphasizing one of the most commonly found in most modern cars nowadays. Many of today’s cars come with an automatic transmission. In fact, some manufacturers have deviated from twin-clutch units in favour of automatic transmissions.
Inside An Automatic Transmission
It’s important to note that for an automatic transmission to function optimally, a multitude of parts has to work together in unison. First, you have the actual transmission housing itself that contains the core of a transmission unit. Inside the housing are many sets of clutch packs and various planetary geartrain.
For an automatic transmission to change gears, all the gear sets have to engage and disengage in a certain predetermined order. Just by changing the planet carrier’s drive method, 3 different gear ratios can be produced. So an automatic transmission has different combinations of planet gear sets for drivable gear ratios.
This is why an automatic transmission can remain relatively small yet be built with 10 forward gears in modern cars. To engage and disengage the various gear sets, transmission fluid pressurises and releases the clutch packs. The distribution for transmission fluid inside an automatic transmission is managed by the valve body.
The valve body is essentially a maze filled with tens upon hundreds of fluid channels. The valve body is constantly submerged and fed with transmission fluid during its operation. Inside the valve body are springs and solenoids that operate jointly to pressurise and change the path for the transmission fluid to flow through.
Of course, with solenoids, you need a computing unit to energise them. That’s the transmission control unit or electro-hydraulic controller unit. In some transmissions, this controller is incorporated directly within the valve body itself. Just by manipulating the various transmission solenoids, the automatic transmission can switch gears autonomously depending on the driving conditions and needs.
Therefore, you can imagine the shift solenoids as the heart of the transmission unit, and the valve body as the arteries. If something goes wrong with the solenoid operation, your transmission won’t be able to shift properly or shift at all.
How Do Transmission Solenoids Work
Solenoids operate by gradually building current from resisting voltage flow. The greater the current, the stronger the magnetic field. When this magnetic field is strong enough, it’ll operate a plunger which in turn changes the fluid flow path. There are also pressure regulating solenoids in a valve body that alters the fluid pressure for different conditions.
The transmission control module is sometimes referred to as an electro-hydraulic controller since the transmission solenoids are fundamentally hydraulic valves that operate electronically. The TCM has to switch the various solenoids on and off at the right time for ideal transmission shifting.
Therefore, the whole operation of an automatic transmission shifting process can actually be simplified. Your main engine computer coordinates with the ABS computer to deliver driving information to the transmission control unit. This information includes factors such as vehicle speed, throttle position, engine rpm, etc.
Then, all the pertinent details necessary to determine shift behaviour is compiled and further analysed by the transmission controller. This also includes driver demand, such as sport mode or manual shifting. The transmission controller’s job is to turn that information into solenoid operation.
It’ll switch the shift solenoids accordingly to achieve its objective. When the solenoid receives the switching signals from the transmission controller, it engages the spring-loaded plungers and alters the transmission fluid travel path. The fluid travels through the channels inside a valve body and flows toward the respective clutch packs and bands.
The pressurised clutch packs will, in turn, engage the gear sets. By designing how and when each planetary gear set engages, multiple desirable gear ratios can be derived from just a few gear sets. In transmissions with a physical transmission selector lever, the lever actually operates a mechanical valve within the valve body that also modifies the fluid flow path.
Shift Solenoids Symptoms or Failure
Obviously, the transmission solenoids play a huge part in a transmission’s operation. It’s not acceptable for even one of the solenoids to fail, as it will cause improper shifting and rough operation of the transmission.
However, in most cases, solenoids rarely do fail outright. Over time, shift solenoids start to wear out and might not manage optimum capabilities. When this happens, the plunger’s switching time might be delayed or just undesirable. Therefore, shifting action might not feel as smooth and prompt compared to a new transmission.
Poor Shifting Quality
When a transmission solenoid fails, you’ll most definitely notice the impact on shift quality. Each solenoid is responsible for different tasks, and the transmission only shifts neatly if everything happens together. This means there isn’t a solenoid for every gear, so you just lose that gear in particular.
What actually happens when a solenoid isn’t functioning correctly is a single clutch pack might not be engaged or disengaged on time. This clutch pack might be responsible for multiple gears in an automatic transmission.
When a clutch pack isn’t being pressurised to specifications, it translates directly to the shift operation. What you’ll perceive are delayed shifting, jerky shifting or even gears slipping out. In some cases, the car might not shift into certain gears at all.
If your transmission is slipping in every single gear, this can mean that there’s trouble with the pressure regulating solenoid. In severe cases, a failure with this solenoid can cause your transmission to give up entirely.
Typically, the effects are felt in both ways. This means that it can also affect the downshifting of the transmission. The transmission might refuse to downshift altogether, leaving you stuck in neutral. With that said, a solenoid that’s only starting to go bad might not produce any symptom at all.
Check Engine Light (and Automatic Transmission Warning Light)
If there’s something that’s obviously wrong with your car, then it should trigger a check engine light. In some cars though, there will even be an automatic transmission warning light. By this stage though, you should definitely visit a local workshop to get it sorted.
With that said, if you own an earlier car, especially one built before the advent and popularity of clutch-operated transmissions, there might not be any engine code defined for transmission faults. This is because earlier transmissions are generally mechanical, with very few electrical components inside. This means that transmission abnormalities are not monitored as thoroughly.
This is in contrast to modern automatic transmissions. There are more electrical parts within with an external transmission control unit monitoring the transmission performance. Factors such as clutch filling times and pressure are a major element for ideal transmission shifts.
These are monitored so the transmission controller can adapt to clutch pack wear and tear. If the filling times are out of specification, it’ll flag a trouble code. On newer automatic transmissions, the transmission controller is typically incorporated into the valve body or within the ECU.
The transmission error code will also help your technician to determine the problem quickly and effectively. Therefore, if your choppy shifts are accompanied by a check engine light, then a workshop visit is way overdue.
It’s also worth noting that when there’s an automatic transmission related issue, there’s a good possibility that your car will enter ‘limp-home mode’. When this happens, the automatic transmission will default to a single drive gear, normally gear 2 or 3, so you can drive it as long as possible to the workshop.
This is mainly to prevent further damage to the transmission. It’s easy for a slipping transmission to start overheating due to the excess heat produced by friction from the clutch packs. This ingenuity of a default drive gear is made possible thanks to the properties of solenoids. A solenoid can be normally open and closed.
If those solenoids are not energised, the transmission fluid flow then defaults to a safe state. The fluid pressurised the necessary clutch packs to shift to the default gear.
Low Power, Odd Car Behaviour
A poorly performing transmission would also behave strangely in general. This can heavily impact the drivability of your car. In some cases, the transmission might excessively rev out a single gear in particular before shifting to the next.
It’s also possible that your car will shift into the gear fine, but it’ll constantly slip out of gear whenever you apply power. This is a sign that the transmission fluid pressure is insufficient to keep a clutch pack engaged under load.
When the transmission does shift into gear, it might be very rough and sudden. It’ll feel like a manual transmission car when you release the clutch too quickly. Sometimes this might be mistaken as an engine that’s hesitating or struggling, when in fact it’s a transmission fault.
What Should You Do If You Think You Have A Bad Shift Solenoid
It can be daunting when you’re faced with a transmission-related issue. Especially when you’re unfamiliar with the mechanics of an automatic transmission. This can be problematic especially if your car remains seemingly drivable. It tricks you into a sense of security.
When you’re dealing with a transmission related issue, it’s best that you consult a workshop as soon as possible. This is made easier if your car can still be driven. Don’t attempt to use your car as it is for your day-to-day commute.
It’s advised so because a transmission issue can quickly snowball into catastrophic transmission failure. By that time, a simple solenoid replacement or even transmission fluid change may not remedy your issue anymore. Due to the nature of an automatic transmission, it’s quite delicate.
This stands true if you have an older car without transmission-related error codes that can appear. You should recognise the symptoms of a faulty transmission and consult a local workshop as soon as possible. That said, older band-type transmissions with mechanical valves can last a long time even when neglected.
Once you experience the aforementioned symptoms, drive gingerly to the workshop. Do not put too much load on the transmission. If needed, you can always manually select a gear that’s not slipping and stick with that gear. It’s important that you avoid overheating your transmission when you have low fluid pressure.
A quick check you can perform is to inspect your transmission fluid level. Make sure you still have adequate transmission fluid and it’s not overfilled. Either can cause serious shifting issues. That said, on some newer cars, manufacturers have eschewed the transmission dipstick, some even removed the tube altogether. You need a diagnostics scanner or a special tool to check the transmission fluid level in this case.
If you have low fluid levels, check whether if you’ve developed a transmission fluid leak. If it’s alarmingly low then it’s possible that you might’ve hit your transmission fluid pan and caused it to crack. You should stop driving and ask for a tow service when this happens.
What Can Cause A Bad Transmission Solenoid Or Shift Issues
It needs little reiterating, but failure comes with age. The transmission solenoids reside within an intricately self-contained system that can be tough on any electrical system. Over time, the solenoids, being continuously bathed in transmission fluid, can clog up with foreign particles. This causes performance issues with the solenoids, changing how the fluid flows.
The issue with an automatic transmission is that the valve body itself consists of tiny oil channels. The tolerances for dirt or excessive metal shavings within the oil channels are minimal. Shift solenoids themselves incorporate a small filter web to reduce chances of failure. Regardless, over time the buildup of minuscule particles on the solenoids can cause shift issues.
Although rare, it’s also entirely possible that the solenoids have just stopped working. Coil windings can degrade over time, reducing the magnetic strength. It can even be an internal open circuit or short within the solenoid causing a failure.
It’s worth noting that your transmission issues may begin to surface the moment you change your automatic transmission fluid. This can happen from reasons to improper fluid level, incompatible fluid, or just dirty transmission fluid being introduced. Be sure to check the fluid level yourself once you get it changed.
Newer transmissions might require a bleeding process once you’ve performed a repair on your transmission, or just replaced the fluids. This is typically accomplished via a diagnosis system. This is required because air within a hydraulic valve system can actually airlock the system. Automatic transmissions do have a vent that facilitates self-bleeding processes though.
Also, this might not be entirely obvious, but fluid contamination can also be a huge problem. Most people would think that this wouldn’t be a problem, as long as you keep tab on your transmission fluids. However, some transmissions implement a transmission fluid cooler into the coolant radiator.
Therefore, when this transmission fluid cooler starts to leak, coolant can quickly enter and circulate within the transmission. As you can imagine, transmission fluid stops being effective once it’s contaminated with coolant. When this happens the transmission fluid turns into a milky color.
Determining Whether You Have Shift Solenoids Symptoms
The issue with transmission related issues is the fact that the system works as a whole, and once there’s a problem with it, it’s difficult to pinpoint the issue. The process can be simplified if it’s a modern automatic transmission with error codes that can help determine the fault.
However, it always helps to start from the easiest issue first. The transmission fluid is behind the main operation of any automatic transmission. Therefore, always check your transmission fluid first. That said, it might not be possible to check the fluids yourself. As mentioned prior, you might need a special dipstick that can be bought online or a scan tool.
If possible, drain your own transmission fluid so you can get an idea of your transmission’s condition. You can also use a paper towel to sample your fluid condition to compare it against other examples online. You shouldn’t find too much metal shavings on it, which can indicate internal transmission damage.
If your transmission fluid seems fine, then a diagnostics scanner is needed in most cases to perform in-depth diagnosis. In earlier automatic transmissions though you can find wiring information online and attempt to diagnose the actuation of the solenoid by directly applying voltage to it.
Otherwise, diagnostic scanners can be used to remotely activate the solenoids. A solenoid that’s operating normally should produce an audible clicking sound. This can rule out the fact that you might have a malfunctioning solenoid, although in newer cars that should trigger a fault code.
If the solenoids operate fine, then a valve body removal is necessary for further diagnosis. Removing the valve body can reveal a lot about your transmission. Some might prefer to leave this job to the workshop though. It’s a bit more involved than replacing the transmission fluid.
In general, automatic transmission issues are relatively scarce. If you start experiencing shift issues early into your car ownership, then you should suspect that it’s a recurring concern with the automatic transmission itself. You can find this information online, just search for similar complaints from other owners.
It might be a design flaw that the manufacturer itself has issued a service recall for. In some cases though, the problem might just be widespread enough that you can find it online. A case like this can be observed with Volkswagen and their DSG 7-speed transmission. The dry clutch within failed so frequently that Volkswagen had to acknowledge there is an error with the transmission design.
Is Replacing A Bad Transmission Solenoid Diy-friendly
Thankfully, if you’ve narrowed the fault down to the transmission solenoid, the job to replace it is not all too difficult. You have to ensure that you have the appropriate spare part and tools before committing to the job though. A diagnostic scanner is typically not mandatory when replacing a shift solenoid. You’ll need a floor jack and jack stands for this.
A workshop manual will be a big help here. It’ll provide information on the tightening torque, specific steps required, filling capacity, etc. With that said, even without documentation, it shouldn’t be too tricky to do on your own.
Before you undertake the job, make sure you have a drain pan at hand. You should also prepare towels as it can get quite messy. You’ll want to replace the transmission fluid and filter while you’re inside the valve body too. Spare an afternoon to carry out the job as well. If you’re ready, jack up your car and prepare the drain pan.
The first step is to drain off your transmission fluid. You should be able to source a guide on this for your particular car model online. Normally, all you have to do is to remove the drain nut from the transmission fluid pan. Some cars (notably Mercedes-Benz models) might have an overflow pipe that you need to poke out to fully drain the fluid though.
After you’ve drained the fluid clean, you’ll have to remove the transmission fluid pan. Depending on the car model, this can be either simple to relatively complicated. Some cars have very limited access to some screws affixing the pan to the transmission housing.
You’ll reveal the transmission fluid filter. Remove the filter to gain access to the valve body. Before you eagerly remove the valve body, disconnect the main transmission electrical connector on the outside. Ensure you remove the selector lever as well while taking note of the position.
There will be several bolts holding down the valve body, typically at the same length and possibly colour-marked. Remove these screws and carefully drop the valve body. It’ll be quite heavy.
Ensure that work performed on the valve body is done somewhere clean and sterile. Wear a pair of clean lint-free gloves while removing the transmission solenoids. While you’re replacing the faulty solenoid, it makes sense to remove the other solenoids for a clean. Just rinse them in clean transmission fluid.
All in all, it’s not an overly complex job. Make sure you have the correct solenoid, normally these are also colour-marked. Be mindful and avoid dirtying the valve body, it’s an intricate part. If you’re mechanically competent, this job is entirely achievable in your own shed with only basic hand tools.
These tools have been tried and tested by our team, they are ideal for fixing your car at home.