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How to Rebuild a Transmission – What You Should Know!

How to Rebuild a Transmission – What You Should Know!

Knowledge on how to rebuild a transmission can be every car owner’s advantage. Whether you are doing it yourself or asking help from a professional mechanic it is very important that you know the mechanism behind it to have an idea of what is happening and how much it will cost. A vehicle’s transmission, being a major part of the vehicle’s operation, constantly needs to be in a good running condition. So if you’re seeing signs of a failing transmission better read up and act now. 

Auto Repairs Are EXPENSIVE


Driving around with bad gears is inviting trouble for other parts of your car. In some cases, a rebuild is needed, which means repairing and replacing parts of transmission for it to be able to operate as new again. In some cases, a major overhaul is needed but there are also times when only a simple process is needed with just a few replacements and some cleaning.


You may ask how hard it is to rebuild a transmission. Rebuilding a transmission is definitely not as easy as changing tires, but it can be something fun and challenging for a DIY person. But take note that how to rebuild a transmission can on your own is difficult and there's a chance you could end up damaging it beyond repair if you don't know what you're doing


How to Rebuild a Transmission: What is transmission in a car?


First of all, you must understand the basics of a transmission system. The transmission system is found on a vehicle’s engine which is responsible for changing gears. It converts the engine's combustion power to the momentum that turns the wheels. In short, the transmission system enables the vehicle to move forward.

How to Rebuild a Transmission: Automatic vs. Manual


There are two types of Transmission: automatic transmission and manual transmission.


For a vehicle with automatic transmission, the vehicle shifts the gears on its own. It doesn’t need input from the driver.  In short, the driver does not change the gears of the car.


In a car with a manual transmission, however, the clutch bridges the engine and transmission, so the driver can change gears when they press the clutch pedal. So it is the driver that shifts the gears of the vehicle with the help of a transmission stick and the clutch pedal.


Manual or not the vehicle will not be able to move from point A to point B without the Transmission. This is how important a transmission system is in any type of automobile. This knowledge is significant in learning how to build a transmission. 

How to Rebuild a Transmission: Parts of a Transmission System


You may already know the difference between an automatic transmission and a manual transmission, now you must also familiarize yourself with the various parts of a transmission system. Here they are and their functions: 


A car’s transmission understandably has a lot of parts and may not be easy to fully understand at first. But just to familiarize yourself, take note of its main components that include: planetary gearset, the torque converter, pump, clutches, bands, sensors, valve body, and the transmission fluid, also known as ATF. This will help you better grasp the process of how to rebuild a transmission.


All these parts are crucial. Their functions work together to deliver torque to the wheels so your vehicle gets to move forward. It’s a complex piece of machinery, and to better understand how your car works through its transmission we’ll explain all the roles of the parts of a transmission:


  • Planetary gear set – One of the most important parts of your vehicle, that all other transmission parts also depend on is the planetary gear set. It consists of planet gears, sun gear and ring gear. This part is responsible for all of the gear ratios that the vehicle has. 


The planetary gear set works with the ring gear connecting to the planet gears, with the sun gear at its very center. How the planet gears are locked or unlocked determines the gear ratio. The planet gears revolve around the sun gear and the rotational force produced is sent to the output shaft that is connected to the wheels.


  • Torque converter – a hydraulic fluid coupling connecting the engine to the transmission. It consists of a stator, impeller, and turbine, which combined forces produce torque that the transmission uses by way of fluid pressure. The produced torque allows the vehicle to accelerate after stopping, but also allows the transmission to stay in gear while the vehicle is stationary. This is made possible with the way the torque converter connects to the engine and transmission. 


The torque converter can spin however fast the engine runs as it also spins independently from the transmission. The stator part of the torque converter will then allow you to accelerate again after a complete stop. But if the torque converter fails, you will notice shuddering and slipping in your transmission.


  • Pump – Found at the center of the torque converter and the planetary gear set, it is the pump that draws transmission fluid in and pressurizes it for the torque converter and transmission. The fluid pressure is what an automatic transmission heavily relies on for its components to function. The pump serves as the heart of the transmission, which is responsible for all the necessary fluid to do its purpose.


  • Clutches & Bands – These parts go hand in hand to help the transmission shift gears by making it possible for gears to rotate, engage, or disengage. Pressurized fluid is applied to the pack which causes the pistons to engage by way of the clutch, and the power produced is then transferred to the wheels. On the other hand, the bands that are wrapped around the gear train, and they will tighten up or loosen depending on gears having to be engaged or disengaged.


  • Sensors – Serves are the control units of the transmission. They calculate the speed of the engine and wheels to determine which gear should be used. There’s an input speed sensor and an output speed sensor as well as park and neutral switch for safety.


  • The valve body – serves as the hydraulic control center responsible for regulating incoming transmission fluid and uses it to control a network of check balls, spring-loaded valves, and servo pistons. This part of the transmission dictates what gear ratio should be used by sending fluid to the clutches and bands. 


The transmission fluid pressure determines which valves are to be opened or closed. Fluid pressure changes depending on the speed of the engine. Many modern automatic transmissions at present depend on the engine control unit (ECU) or the transmission control unit to regulate the valves.


  • Transmission fluid (ATF) – a very critical part of the transmission is the transmission fluid. It not only provides fluid pressure but also lubricates and cools transmission so it doesn’t overheat. ATF is composed of a variety of synthetic liquids and oils plus chemical properties such as detergents, lubricants, and rust preventatives.


The transmission fluid is the only part of the transmission that requires routine maintenance. The age of the ATF causes it to collect contaminants and with that, it also loses effectiveness. So best to check the fluid once a month and change it every 30,000 to 60,000 miles for best performance.

How to Rebuild a Transmission: Step by Step


Step 1: Identify which part you need and secure all parts and materials


Finding and ordering transmission parts is easy but identifying which parts you need to replace can be tricky. If you’re unsure which part you need to replace, consult with a trusted mechanic. Diagnosis of the transmission system error is the beginning of how to rebuild a transmission.


Step 2: Prepare work location


Preparing a  work location requires you to have a clean area that is free of dust and other particles. Cover the work surface in plastic trash bags to ensure a dust-free space and to also protect the area from fluids draining out of a disassembled transmission.


Step 2: Removing the Transmission and Engine Block


You will be able to work with ease if you remove the entire engine block from your car before removing the transmission from the engine. You will also have to remove the torque converter and the transmission oil pan.  As complicated as this may sound, a nonexpert mechanic with the right instructions and persistence will be able to do this.


Never just remove the transmission carelessly as transmissions have a variety of similar-looking parts. If you remove the parts haphazardly without paying attention, you can become easily confused with other parts. 


Step 3: Disassemble the transmission itself.


Disassemble the transmission itself and drain out the transmission fluid. Have a pan prepared to catch transmission fluid. Label every part of the transmission so you will not get confused when it’s time to reassemble the transmission.


Step 4: Clean and Replace Transmission Parts


Clean the transmission parts and replace the broken ones. Once you have disassembled the transmission, the transmission’s faulty parts should be obvious to someone even with a little training. Look for signs of broken parts, grime or synchronization problems that you can detect once the transmission has been fully disassembled.


Step 5: Reassemble the Transmission


Finally the most important aspect of rebuilding your transmission — the reassembly. Even the best quality and perfect parts won’t work if they are not put together correctly. So again, label your parts scrupulously. Have every tiny piece of the transmission organized in a specific spot, so you can also incorporate it into the transmission in the right order. 


And then another important part of how to rebuild a transmission is to make sure you are able to conduct all the proper pressure tests. Everything needs to be exactly where it should be for the transmission to work smoothly. If you are not a hundred percent confident with your rebuilt, best to consult an experienced transmission mechanic before having your vehicle back on the road.

How to Rebuild a Transmission Rebuild Kits


Another option you may consider in rebuilding a transmission is buying a transmission rebuild kit. The kit will have all the tools required to rebuild the transmission in one convenient package. While these kits make the whole process seem easier it is not necessarily the case. Some of them may give a false sense of ease especially for those who have little to no transmission experience. So you must look for a rebuild kit that has a detailed instruction manual, and secure all of the tools necessary to complete the rebuild process. 


How to Rebuild a Transmission: Other Frequently Asked Questions

Can I rebuild my own transmission?


If you choose to do your own transmission it can cost much less. All you will need is a rebuild kit as well as some automotive tools. Transmission Rebuild kits range from $50 to $300. The price depends on what parts are included and their quality.

How much does it cost to rebuild a transmission?


The Average Price for Rebuilding, Repairing, and Replacing Transmission are as follows:


The average cost of transmission replacement averages from $1800 to $3400, while a used/salvage transmission may cost from $800 to $1500. A rebuilt transmission, on the other hand, is from $1100 to $2800 and a remanufactured one from $1300 to $3400. (Source: Transmission Repair Cost Guide readers).


Now that you know how much the cost of rebuilding, repairing and replacing transmission you can now decide which option should you go for. Should you rebuild or replace your transmission?


Knowing how to rebuild a transmission and being able to rebuild a transmission will no doubt save you money, and can give you a sense of pride as an amateur mechanic or DIY person. The process however will seem like solving a complex puzzle. While it might be a worthwhile experience you should still be honest with yourself and decide if the needs of your transmission become too advanced compared to your skills and ability.


In that case, you will need to find a reliable mechanic who will do the job for you instead as you don’t want more damage or trouble for your car, leading you to even more costly repairs.