The transmission is one of the most critical components of your vehicle. It’s also one of the most expensive to replace. Ensuring it’s well maintained is essential to keep it in good working condition as long as possible.
With time, some of the internal parts will start to wear, and repairs will be needed. To help you with that, we created a list of the most common transmission problems and what you need to do to fix them.
Table of Contents
- 1 How does an automotive transmission work?
- 2 Common warning signs that your transmission is about to fail
- 3 10 most common transmission problems and how to troubleshoot them
- 4 How does a transmission code reader help to diagnose transmission problems?
- 5 How to prevent future transmission problems
- 6 Last words
How does an automotive transmission work?
To understand transmission problems, you first need to learn the basics of how it works.
The transmission’s main job is to reduce and transfer the power produced by the engine to the wheels. On FWD vehicles, the differential is inside the transmission unit.
The flywheel connects the crankshaft to the transmission, and both turn at the same speed. The torque converter will then transfer the power to the input shaft, which will, in turn, engage all sorts of hydraulic components.
In comparison to the automatic transmission, the manual transmission is much simpler. It uses nothing but gears and a selector to perform the same task.
Common warning signs that your transmission is about to fail
A warning light is often the first sign that a problem is lurking. If the light is yellow, you should drive your car to the nearest auto repair shop when possible. A red light will let you know that you need to stop the vehicle as soon as you can. Keeping on driving may harm you or damage expensive transmission elements.
Lack of responsiveness
You should have your transmission checked whenever the gear changes start to feel sluggish. Various failures can explain such a condition, but in all cases, ignoring the problem will only make it worse.
Gear shift delays
The same advice applies here. If your vehicle shifts later than it should, think about seeing your local mechanic. This condition usually happens in first and second gear or when accelerating on the highway.
An unusually low fluid level on the dipstick often means that it may be leaking somewhere. Re-adjust the level and inspect it regularly. If there’s a leak, the level will go down again.
Newer cars often lack the refill cap feature, and adjusting the fluid level may require special equipment. When in doubt, have your car checked by a knowledgeable person.
Read more: What Transmission Do I Have?
10 most common transmission problems and how to troubleshoot them
Not all car problems are equal. Some are easy to fix, while others are way more complicated. And expensive.
Finding the source of transmission problems could save you a lot of money in the end. Even if you can’t always fix the problem yourself, giving clear indications to your mechanic can only speed up the troubleshooting process. Keep in mind that the faster they find the problem, the cheaper it is for you.
The term “slipping” refers to when the transmission seems to stay between gears when shifting. Slips often occur when the transmission fluid level is lower than recommended. The same problem can happen because of incorrect oil pressure from the result of using old, burnt, or contaminated fluid.
An incorrectly calibrated transmission module can also cause a transmission to slip from time to time.
Driving with a slipping transmission will cause permanent damage to the internal components. If the fluid level is right and the transmission software is up-to-date, the transmission may need to be replaced.
Signs to look out for
Slipping starts small and gets worse with time. Take warning of slight whistling sound when shifting. A strong burnt smell is also a clear sign that your car’s transmission may be slipping.
The first thing to inspect when troubleshooting a slipping transmission is the fluid level. If the level is low, adjust it following the manufacturer’s recommendation. A quick test drive will let you know if you fixed the problem.
If the transmission is still slipping, the next step would be to check if there’s an update available for the TCM. Make sure the transmission calibration is correct, reset the mapping, and test the car once again.
At that point, if the problem is still present, you will need the help of a transmission specialist. After a thorough inspection, further disassembly may be necessary to find the problem and replace the faulty component.
Leaks are pretty common on cars, especially old ones. Gaskets are made of silicone and rubber, which tend to dry out with time. After a while, seals will start to crack, and oil will start leaking.
It’s the same with all rubber hoses and threaded plugs. Metal and braided lines could also rust and crack.
Signs to look out for
Any sign of a greasy film around gaskets and seals will warn you that something is leaking or about to. Depending on the leak, you may also notice spots under your vehicle after leaving in the morning.
In worst-case scenarios, transmission hoses could even break completely, and a severe leak may occur. Thick white smoke will appear if transmission fluid drips on the exhaust system.
Whenever you suspect a transmission leak, try leaving a big piece of cardboard under your car overnight. Inspect it for oil deposits in the morning. If you find dark spots on the cardboard, look right above to locate the leaking part.
Is the cardboard still clean? It doesn’t necessarily mean you’re out of the woods yet.
Put back the cardboard under the car and start the engine. Press and hold the brake pedal and shift from P to D1. Bring back the shifter to P, wait 20 seconds and repeat the same thing. Keep on doing this for 5 min then turn the engine off.
Take another look at the cardboard. Transmission hoses and coolers often only leak when fluid is circulating. Smaller cracks can even drip only when the engine is hot or cold.
Every car and every situation is different.
Unusual noises are the hardest transmission problems to diagnose. The main reason is that almost any part can start producing a strange sound when damaged. The fact that you can’t open the transmission and inspect it doesn’t help at all.
Transmission noises can come from slipping belts, damaged gears, and loose components. If something is broken, it will make a sound at some point.
Types of transmission noises
With a little practice, it’s easy to categorize car noises. Grinding metal noises are usually related to bearings and pulleys. Squeaking, on the other hand, is often a result of a slipping rubber or plastic part.
The moment at which the noise is happening is also essential. For example, if the noise is only heard when the transmission is in P, the problem can come from the input shaft. If the condition only occurs when the lever is in D, the faulty component may be related to the output side.
When trying to identify a noise coming from your vehicle, always start with a test drive. Pay attention to details and take notes of as much information as you can.
The first thing to look out for when your transmission starts to be noisy is, once again, the fluid level. Never forget that oil pressure is what makes everything move inside an automatic transmission.
Once you’ve made sure the unusual sound is not fluid-related, you’ll need to stop for a minute and think. There’s no definitive procedure for this one.
Analyze the data you have, the sound’s characteristics, and write down some of the suspected components. Your vehicle’s repair manual should be your go-to tool here. Exploded view diagrams will help you learn the location of each component.
A stethoscope may also come in handy to confirm your diagnosis. Raise your car on a hoist, put it in D, and let it run while you sneak under it. Move the stethoscope around the transmission and place it over key locations. Inspect all bearings and main components.
In most cases, replacing the problematic element will require the disassembly of the transmission.
Lack of power
Experiencing a lack of response when accelerating can come from incorrect TCM programming. Faulty transmission sensors can also bring false readings producing the same result.
Start by scanning the vehicle for DTC codes. Most programming and software communication problems will record a trouble code. The code will give you a better idea of which sensor or which module may be faulty.
Codes related to “data out of threshold” will let you know that a sensor needs replacement. Contaminated transmission fluid can sometimes cause pressure-related codes too.
Get a copy of your vehicle’s repair manual and find the correct troubleshooting chart. Every situation will be different based on what sensor or module is the problem. Make sure to follow all the steps and perform every test as demonstrated in the chart.
Troubleshooting charts are usually your best bet to find answers to specific problems. If you plan on diagnosing complex transmission issues, get your car’s repair manual first. They are easy to find online, quite inexpensive, and they will save you a lot of time. Trust me.
Transmission fluid is recognizable by its distinct odor. Its smell can enter through the vehicle’s air vents and fill the cabin when burning. A burnt sugary smell usually appears whenever transmission fluid is dripping on the exhaust system. A slipping gear belt or overheating can also create the same unpleasant odor.
The easiest way to find the source of an unusual smell is by using your nose. Open the hood or jack the car and try to locate where the odor is coming from. Look out for smoke clouds, fumes, and black dried-out oil spots on hot components.
Once you have located the source, verify the surroundings to see where the fluid is dripping from and fix the leak.
If you can’t find anything leaking, the smell may come from inside of the transmission. When it’s overheating, fumes may come out of the differential vent. Greasy deposits around the vent will let you know that something is not working correctly.
Be aware that an overfilled transmission can leak through the vent and create the illusion of an overheating condition. If your transmission fluid level is too high, remove the excess fluid using a syringe. Clean the exhaust system and tranny housing with a good degreaser, rinse and try again.
Slow gear changes
Slower than usual gear changes are often caused by an incorrect transmission fluid grade or contaminants. Anything related to the thickness of the fluid could make the pressure valves go nuts.
First, make sure that this problem is not related to any sensor issue by scanning the TCM for codes. Perform the usual fluid level check and make sure the tranny is not leaking.
Take the car out for a drive and look at the transmission live data using an OBD2 scan tool. Anything out of the ordinary will let you know that either the oil pressure is not quite right or the valve body is malfunctioning.
Try replacing your transmission fluid completely and see if the readings are back to normal. If not, you’ll need to have your transmission checked by a specialist. Valve body repairs are not for amateurs. If you don’t know what you are doing, you better leave it to a professional, or you could end up doing more harm than good.
Yellow lights lighting up your dashboard are always stressful. Rest assured, there’s not much to worry about most of the time. It’s only your vehicle’s way of telling you that something is not working at 100%. Most yellow warning lights relate to minor or intermittent glitches in the system.
Red TCM warning lights are more disturbing. You should stop driving your car as soon as the TCM light starts flashing. All red or flashing dashboard lights mean that the engine should be turned off as soon as possible to prevent any damage. These are usually related to more severe troubles like dangerously low fluid level or severe malfunction of one of the main components.
In the case of a yellow light problem, start by scanning the TCM for codes and write it down. Get the troubleshooting chart for that code and see if it’s something you can fix yourself. Most external sensor problems can be fixed at home with basic tools. Simply locate the faulty sensor and replace it.
Flashing warning light troubles are harder to fix by yourself. Data and communication problems often require the use of a professional scan tool as well as programming skills. Since the car shouldn’t be driven anyway, you might as well have it towed to the nearest transmission specialist.
Rough gear changes
Aging transmissions are often prone to rough gear changes. Sometimes, fluid may be contaminated by dirt and debris, and replacing it will fix everything. Other times, it’s only because the car is getting quite old and the transmission is not in top shape anymore.
When trying to fix rough or jerky gear changes, the first thing to inspect is the transmission fluid. Make sure it’s bright red and free of any metal shavings and debris. If it’s dark, muddy, or has a strong burnt smell, replace the fluid and the transmission filter right away. When in doubt, it’s always better to start by replacing the fluid since it’s the cause of most automatic transmission problems
If it didn’t do it or your fluid looks fine, scan the TCM for codes. You may be lucky and find a code telling you what’s wrong right away. Take a look at the live data and perform live tests to ensure all the valves are doing their job.
A neat trick is also to erase the learning data of the module. New transmissions learn from your driving habits, and corruption of the data may occur with time. Resetting it could fix the problem.
If everything looks fine and you still have jerky gear shifts, worn internal components may be the answer. In that case, there’s not much you can do, and you might have to buy a replacement tranny altogether.
It’s worth mentioning that specialized fluid additives can help with jerky shifting. While these products might temporarily help, you should probably seek the help of a mechanic anyway. Try one of these when everything else fails.
Transmission fluid tends to overheat when the car is driven under load. To solve this problem, car manufacturers install transmission coolers and lines on their vehicles. When this system comes into failure, the fluid temperature will suddenly rise and lead to overheating.
Signs to look out for
Whenever a transmission is overheating, the first warning sign is a burnt tranny fluid smell. A red warning light usually comes next, soon followed by sluggish or slow gear shifts. In the worst-case scenario, the transmission will start slipping until the wheels stop moving forward.
The first thing to inspect when suspecting an overheating condition is the transmission coolant system. Look out for dents in the transmission cooler usually located in front of the radiator. Make sure all the cooling fins are straight and that air can easily go through. If there are dirt and debris lodged in the cooler, use a garden hose or a pressurized washer to clean it.
Kinks in the cooler lines can also reduce the flow to a point where it’s not circulating fast enough to cool down efficiently. Rust can also build up inside the lines leading to the same result. Replace all rusted and bent lines with new ones.
If a visual inspection is not enough, try removing the cooler and make sure the inner passages are free of obstructions. Using a water hose to push water on one side can sometimes help to clear out the debris.
Always remember to replace the fluid completely after facing an overheating condition. Failing to do so may cause other problems in the future.
The car doesn’t move
The absolute worst situation is when a driver puts the car in D, and it isn’t moving forward at all. Numerous failing components may cause such a condition. On modern vehicles, any loss of data from critical sensors may trigger the LIMP mode. When it happens, the car will stop moving or will limit the speed under 30km/h for your safety.
Among other things, a malfunctioning module or a loss of communication between the TCM and the PCM is sure to bring the car to a halt.
On a simpler note, a broken shifter cable or faulty shift lock will also prevent the car from shifting from P to D.
The absolute first thing to do here is to make sure that the shifter is moving the switch on the transmission. Open the hood and ask an assistant to move the shift lever. Inspect the switch and confirm that it’s moving along with the shifter. If it’s not, the cable is broken and will need to be replaced.
Every time a car enters the LIMP mode, a code is set in the TCM. Take out your scan tool and look out for transmission-related DTCs.
Follow the designated troubleshooting chart to find the faulty component.
If there are no recorded codes, the problem may come from the transmission switch itself. Using a multimeter, test the switch for continuity and verify any unwanted short-to-ground condition. Look out for blown fuses and cracked wires.
Once you think you have found your problem, replace the part, reconnect and reset the TCM. If the vehicle is still not moving or stuck on LIMP mode, have your vehicle towed to a transmission specialist.
How does a transmission code reader help to diagnose transmission problems?
A transmission code reader is every mechanic’s best friend. While most basic transmission repairs can be done without one, DTC codes give clear directions of where to look first. The major part of the job is still yours to do, but having access to codes will speed up the whole process.
When do you need a transmission code reader?
With today’s vehicles, a code reader is essential in troubleshooting any serious transmission problem. A code reader will help you visualize what’s working and what’s not inside of your tranny.
A live data-enabled OBD2 scan tool is also required to monitor inputs like the transmission temperature, output and input shaft speed, and other sensors.
Nowadays, TCM modules are so advanced that a full-fledged laptop is often required to read and modify their programming. Performing re-flashes and computer updates also necessitate the help of a professional scan tool.
How to prevent future transmission problems
No matter the transmission type, they all require the same general maintenance. Taking care of your car’s drivetrain can extend its lifespan and ensure it stays in good working condition.
As with any other automotive system, the drivetrain needs to be regularly examined by a certified mechanic. Replace the transmission fluid and the filter at fixed intervals. Inspect the casing for leaking gaskets and seals. Make sure the cooler is free of debris at all times, and watch out for cracked and rusted hoses.
Checking fluid level
If your vehicle is equipped with a transmission dipstick, check and adjust the levels at every oil change. Vehicles exempt from this feature usually require less frequent inspection.
Using the correct grade
A mistake that many vehicle owners often make is to use an incorrect oil grade when replacing their transmission fluid. In the past, it was common to fix a rough or slow shifting problem by using a different oil grade than recommended. Newer transmissions don’t work that way, though.
Using the wrong grade can lead to high-pressure problems and seriously damage a transmission’s inner components. Look for the correct grade to use in your owner manual and stick with it to prevent any unwanted failures.
This one is pretty much self-explanatory. High-speed driving will stress the various components and reduce the efficiency of the drivetrain in the long term. Driving safely and carefully is the easiest and the least expensive way of making sure your transmission lasts.
Once you know how an automatic transmission works, it’s easy to have a good idea of where troubles can occur. Of course, understanding the basic principles won’t turn you into a specialist in a day. Becoming a mechanic takes time and hard work. But, with some effort and perseverance, everybody should be able to troubleshoot transmission problems in no time.