How to Rebuild a Transmission Yourself

Rebuild your transmission using a rebuild kit containing the necessary tools and parts.
Written by John Davis
Reviewed by Melanie Reiff
Rebuilding a transmission is not a job for inexperienced or novice mechanics; however, if you're up for a challenge, the easiest way to rebuild a transmission is using a rebuild kit designed specifically for your vehicle. You'll need to remove the transmission from the car, disassemble it, clean the parts you're going to save, and replace the old ones with the new components from the kit.
The transmissions allows your vehicle to change gear ratios depending on the speed you want to travel. It's an integral part of your car's drivetrain: if it fails, you could experience various issues. While transmission work is generally one of the more complicated areas of automotive repair, you can rebuild your transmission at home with the right know-howHere with all the tips you need is
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What is a transmission?

The transmission is responsible for converting engine power to the force necessary to move your car's wheels. It works by using a torque converter, a system of gears, and hydraulic pressure.  
The torque converter turns engine power into hydraulic fluid using a pump. The highly-pressurized fluid then rotates the turbine inside the torque converter. The resulting vortex power rotation sends power down the transmission's central shaft and activates the first set of gears. 

Why is a transmission important?

The transmission is crucial to your car's function because the wheels would not turn without it. You can build your engine as powerful as you like. But you won't be traveling anywhere without a transmission to convert and carry that power to the wheels. 
Yes, you could build a car without a transmission. However, since you would be unable to change gear ratios, your vehicle would not start or stop properly, and you would be limited to one speed. This means you might end up with a car that could cruise down the highway at 70 mph, but you'd struggle to reach that speed starting out, and slowing down or climbing hills would be a nightmare. 

What are the types of transmissions?

There are two main types of transmission in use today: manual and automatic transmissions. Manual transmissions require the driver to physically change the gear ratio, while automatic transmissions shift gears without the driver's input. 
Manual transmissions are typically found on older vehicles, sports cars, and large commercial trucks. You will usually find automatic transmissions on newer cars. 
There are also continuously variable transmissions (CVTs), a newer type of automatic transmission. Unlike conventional automatic transmissions, which have a fixed number of gears, CVTs offer an infinite number of gear ratios. These transmissions use pulleys and a belt instead of a gearbox, allowing the vehicle to maintain peak efficiency. 

What are the parts of a transmission?

Transmissions are composed of many moving parts that work harmoniously to move your wheels. Transmission parts include the torque converter, a hydraulic pump, planetary gears, sensors, clutches, and brakes
The torque converter takes the energy from the engine and sends it to the pump. The pump converts that power into transmission fluid and sends it to the first set of planetary gears. This whole process is governed by the transmission control module (TCM), which uses valves to control the clutches and brakes to swap between different gear ratios. 

Signs you need a transmission rebuild

No one likes to discover that an integral part of their car is failing. It's even worse to find out your vehicle needs a repair after it's left you stranded on the side of the road. You can avoid these issues by learning to spot the tell-tale signs of a faulty transmission.
Let's look at some of the most common signs you need a transmission rebuild.

Trouble shifting gears

If you have a bad transmission, your car may struggle to shift gears or fall out of gear. In vehicles with automatic transmissions, this typically feels like a lag or a lurching sensation when shifting from park to drive. In manual transmissions, it may feel as though your car is unresponsive after you shift gears.
In either case, the speed will not match your vehicle's RPMs, and it will feel like you're accelerating much slower than you should. This is often referred to as "slipping gears."

Inability to enter gear

If your car won't go into gear at all, there's a problem with the vehicle's transmission. Transmission problems, from dirty fluid or a worn-out clutch linkage to faulty sensors, internal mechanical failures, and bad solenoids, can cause your vehicle to refuse to enter gear. 

Unpleasant engine noise

Strange noises are a common symptom of a bad transmission. Automotive transmissions may make a whining, buzzing, humming, or clunking sound, while faulty manual transmissions usually produce scarier noises, like loud clunking. 

The vehicle jerks when accelerating

A jerking sensation while accelerating is a classic sign of a faulty transmission. You might feel your car lurch into gear while speeding up or experience a delay between gear shifts, followed by a jerk as the transmission finally finds the correct gear. 

Grinding gears

Grinding gears is a common problem in manual transmissions. It typically feels like resistance when attempting to shift gears along with a loud grinding noise. 

A burning smell

If your car has a burning smell without any noticeable fluid leakage, it could be caused by the transmission overheating due to broken metal parts and grime or clutch debris. Transmission fluid has a distinct and pungent smell when it burns, more like burning rubber than oil. 

Fluid leakage

Puddles of bright red fluid beneath your car can be a sign that your transmission is suffering from a malfunction. This issue could be caused by bad gaskets or physical damage. If the fluid is dark brown or black, the transmission is probably already in very poor condition and may require extensive repairs. 

Warning lights on the dash

Dashboard warning lights can signify problems with your transmission. A Check Engine light might turn on due to malfunctioning transmission sensors or a faulty TCM. 
Additionally, many cars have transmission service lights. If these turn on, it's a sign that your transmission needs to be serviced, and you may need to take your car to a transmission shop to figure out exactly why the light is on. 
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How to do a transmission rebuild

Transmission repair is not your typical DIY job. The rebuild process is complex, and even basic mistakes can cause significant problems with your transmission. The project will be even more difficult if you're an inexperienced mechanic without access to a repair shop's lifts and tools. 
However, if you're confident in your abilities and have the proper guidance, you can overhaul your transmission at home. 
The best and safest way to do this is to purchase a transmission rebuild kit. These kits include everything you need to rebuild your transmission correctly, including special tools and parts designed specifically for your car's transmission. Look for a quality rebuild kit from a trusted manufacturer—ideally, it should come with a warranty. 
It's also wise to have a copy of your vehicle's auto repair manual on hand. These are excellent reference sources when working on your car. They contain valuable information, such as torque specs and diagrams, that can help you disassemble and reassemble intricate vehicle components.
While the rebuild process varies depending on your car's make and model, there is a general series of steps you must follow. When you have your rebuild kit, park your vehicle in a secure location—you won't be moving the car for a while once you start the rebuild, so make sure it's somewhere safe and out of the way. Then, follow the step-by-step guide below. 
Key Takeaway To rebuild your transmission, remove it from your car, disassemble it, and clean and inspect the parts before using a rebuild kit to replace the damaged and worn-out components. 

Remove the transmission from your car

The first step to rebuilding your transmission is removing it from the car. This is essential because you're going to be working with the internal parts, which are inaccessible while the transmission is attached to the vehicle. 
Pulling the transmission involves removing many bolts and other components. Some of the parts you'll probably need to remove include the driveshaft, axles, starter motor, and various brackets, hoses, and electrical connections. You must label these parts as you remove them and place them in a safe location so that you remember where they go during reassembly. 
Many of these parts—including the transmission itself—are heavy. Be extremely careful when removing the transmission so that you do not drop it on yourself or anyone helping you.

Disassemble the transmission

Next, you'll need to drain the transmission fluid and begin disassembly. Make sure to pour the transmission fluid into a container instead of letting it spill on the ground. 
While disassembling the transmission, be sure to keep track of the parts and label them accordingly. 

Clean and inspect the parts

Once the transmission is disassembled, you'll need to clean the parts you're planning to reuse, like the bell housing and the transmission case. But it's a good idea to wash all the parts, not just the ones you'll save. This allows you to inspect the components and identify any areas of excessive wear or damage.
Use a parts washer to clean the components and a file to remove old gaskets. If you don't have access to a parts washer, you can get by with a pressure washer or even your local car wash. 

Rebuild the transmission using a rebuild kit

Once you've cleaned your parts, it's time to start rebuilding the transmission. You need to be very careful during this stage—one small mistake can mean starting over from the beginning. Use the instructions that come with your kit to install the replacement parts into the transmission case. 
Do not force any parts together. Move slowly and double-check your work to ensure that you're putting everything in the proper location. Remember to reference your instructions and repair manual frequently. 

Reinstall the transmission

When you finish rebuilding the transmission using your rebuild kit, it's time to retrace your steps and reinstall it. This should be pretty straightforward as long as you accurately label your parts during disassembly and remember where everything goes. 
Remember to refer to your repair manual for torque specs when tightening connectors. Over or under-tightening bolts can lead to fluid leaks or broken components
Your last step after installing the transmission should be to perform a pressure test. You do this by locating the test port on your transmission case and attaching a pressure fitting. Your service manual will contain information regarding the location of the test ports as well as the proper pressure readings. 

How much does a rebuilt transmission cost?

The answer to this question will depend largely on your vehicle's make, model, and age. For example, if you drive an expensive car, the parts will be more costly. 
However, you can generally expect to pay between $200 and $500 for your rebuild kit. While this might seem expensive, it is significantly cheaper than taking your car to a transmission repair shop, where you'll likely spend anywhere from $1,000 to $3,000. 

How to dispose of old transmission fluid and parts

Disposing of your old fluid and parts might be the easiest part of the job.You can pour the old fluid into the new replacement fluid containers and return it to the auto parts store for proper recycling. And you can take the old parts to your local scrapyard or metal recycling center—you might even be able to get some cash back for this step! 

When to go to a mechanic 

As we mentioned previously, transmission repair work is complex and is not for novice mechanics. Mistakes can be costly to your wallet and the safety of yourself and others on the road. If you don't feel up to the task of rebuilding your transmission, don't attempt to do it
You'll save more money and time in the long run by taking your car to a professional and allowing them to safely and correctly do the work. 

Find cheap car insurance

Transmission repairs can be costly. And if you have a traditional automotive insurance policy, you'll probably have to pay for the work yourself. You can prepare for mechanical failures by adding
mechanical breakdown insurance
to your standard policy—but remember to shop around first to ensure you're getting the best rate.
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FAQs

You can rebuild your own transmission. However, it won't be easy without the right equipment and knowledge. The best thing to do is purchase a quality transmission rebuild kit that comes with the essential tools and instructions to guide you through the process.
It is cheaper to rebuild the transmission, especially if you do it yourself. Replacing the transmission is the most expensive option when it comes to transmission repairs.
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