How To Rebuild Transmissions: Here’s How You Can Do It To Your Car

A transmission rebuild can cost an excessive amount at a car dealership or shop, but with a little know-how. you can save hundreds by doing the rebuild yourself.

A transmission rebuild is when an old. worn-out transmission is taken apart and all of the parts are replaced until the entire unit looks and works new. Some shops offer a flush service where they remove old fluid from inside the car by driving it through a machine, but this only prolongs the time before another rebuild needs to be done.

Learning how to rebuild a transmission is not overly complicated, but it must be done correctly. The transmission is a complicated device that has a lot of small parts and a few different types of fluids. If it isn’t done correctly, there’s no guarantee the new transmission will work any better than the old one did.

Before attempting any auto repair job, be sure that you have the right tools and are familiar with the process.

rebuild transmission

What is the importance of a transmission to a vehicle?

A car’s transmission is essentially the ‘heart” of the vehicle. It is rigged with all sorts of gears and shafts which are controlled by hydraulic fluid. The driver inputs power through the gas pedal, indirect power to the wheels via different gear ratios, thus transmitting gear rotation to move the vehicle along.

The transmission is connected to the engine, and it helps to transfer power from the car’s engine to its wheels. The spinning of one or more gears inside the transmission changes how much torque is used to turn the wheels. A low gear makes it easier for a vehicle to start moving because less effort is needed by the engine than if the car was in high gear.

What types of transmissions are there?

There are primarily two different types of automatic transmissions: the standard 4-speed and the more advanced continuously variable transmission (or CVT). A 4-speed will usually have four forward gears, but may also be equipped with an overdrive gear that functions as a fifth speed.

A continuously variable transmission does not have preset gear ratios but instead has a pulley system that can alter the gear ratios infinitely. This means less shifting for the driver and better fuel economy. A car equipped with this type of transmission may be considered “smooth” or “seamless” when in motion.

What are the parts of a transmission?

The main parts of a standard 4-speed automatic transmission are the input shaft, the countershaft, the layshaft. and the output shaft.

  • The input shaft is connected to the engine crankshaft by a clutch system. It spins at full speed whenever the vehicle is in motion.
  • The countershaft is an internal gear that the driver cannot see. but is connected to the engine crankshaft. When power is applied, it will spin at 1/2 speed.
  • The layshaft is another internal cog that spins independently from the other two gears. It functions as a counter for the output shaft gear.
  • The output shaft is attached to the rear axle and is what changes speed as a result of the input from the engine.
parts of car transmission

The transmission transfers power from gear to gear by changes in the rotation speed of internal shafts. In a manual vehicle, this is done by mechanical levers that push on hydraulic fluid which then moves different gears on transmissions.

The end result of all this is the ability to move your car along. The same applies to vehicles with an automatic transmission (known as a “semi­automatic”). but there are other hydraulic systems that make the process much more complicated.

How do you know that your Transmission Needs a Rebuild?

Technician installing transmission pump
Technician installing transmission pump.

Every vehicle owner has to deal with this problem. It is almost inevitable, but the solution to this dreaded situation can be daunting if you are not sure where to start or how to do it

You realize that your transmission is deteriorating when there are problems shifting gears. There may also be an increase in engine noise and a decrease in acceleration due to the lack of power. A car that is having trouble shifting into first gear is on its way to a total breakdown, if not immediately then eventually.

This is caused by small metal particles breaking off from inside the transmission and contaminating the hydraulic fluid. This rust-like substance will then clog up the system and cause a buildup of pressure, resulting in failure.

Expert Tip: Reasons for this include excessive wear and tear from age, transmission fluid leakage, low fluid levels, engine overheating, and a lack of regular oil changes.

This problem cannot be treated with a simple fluid change. A transmission rebuild is necessary to ensure the proper operation of the vehicle. Repairing the system will usually include replacing worn-out parts, flushing all contaminated oil. and adjusting or even replacing valve bodies.

However, rebuilding your transmission is always preferable to having it repaired piece by piece. One small mishap during this process could lead to complete failure of the system, which would be much more costly.

It is also necessary to replace the metal particles with new ones in order to avoid future problems. The best kind of replacement parts are made from actual steel, not aluminum or other cheaper metals that will cause problems when used without proper lubrication.

Some of the tell-tale signs that indicate your vehicle needs a transmission rebuild are:

  • The gears don’t change properly or there seems to be no power when accelerating.
  • There is an increase in engine noise, caused by loose parts inside the transmission hitting each other.
  • The car won’t go into the higher gears.
  • The car occasionally falls out of gear when idling at a stoplight or when driving slowly.
  • The car jerks when accelerating.

Although you can still drive your vehicle in such conditions, it is not advised to do so. Drive normally and get your transmission serviced ASAP before any more damage occurs.

How do you Rebuild the Transmission? (Step By Step)

A mechanic rebuilding a transmission installing a clutch
A mechanic rebuilding a transmission and installing a clutch.

Rebuilding a transmission isn’t rocket science, but it is certainly an advanced job that requires plenty of training and experience.

The best place to start when you’re looking for instructions on how to rebuild your transmission is by consulting the service manual provided by the manufacturer. This will tell you all there is to know about rebuilding your vehicle’s automatic transmission, including how to remove components, reassembly procedure, and torque specifications.

Before you start, it’s wise to have a friend help you. It is difficult to hold one end of the transmission while the other end needs to be taken apart. You will need three people when removing the rear housing cover because two people can’t possibly lift it off by themselves.

Expert Tip: If you are using a transmission jack, you can have one person operate it while another person assists with removing the parts. You will also need an extra hand to hold open the filter when you are removing it.

When assembling a transmission, keep in mind the order in which you disassemble and reassemble each piece. This is important because if you do not put something back exactly where it came from, the gear shifter may be difficult to move or gears may not shift correctly.

Here is a step-by-step procedure that provides an overview of the rebuilding procedure. This is just meant to be a general guide for informational purposes only and you should not attempt it without consulting your service manual.

  • Raise the vehicle on a lift so that all four wheels are off the ground
  • Drain the transmission fluid from its sump by opening a drain plug located near it. This is usually under one of the front seats, in the center console, or behind a plastic panel somewhere on the floor. You can also take out a bolt inside a pan that will drain all fluid at once.
  • Remove the transmission oil pan. This is a large wide panel that is located at the bottom of your car’s engine bay. By removing it you will gain access to all internal components and can thus remove them with ease.
  • Drain partially the torque converter from its sump by opening a drain plug at its base. You can also remove the transmission filter which is located at the bottom of the oil pan.
  • Remove center support mounting bolts and then disconnect all electrical connectors attached to it under the hood. This includes speed sensors, throttle cable, etc.
  • Disconnect all cross members that secure the automatic transmission to the car’s frame. This includes the cross member that is located behind the front engine mount as well as those holding it from below.
  • Remove all bolts that secure automatic transmission to the engine and take off both units with a helping hand lifting them from beneath if necessary. Make sure you have a drain pan under them to collect any residual fluid which might come out.
  • Use a screwdriver to carefully pry the gasket that seals the transmission from behind or just cut it off with tin snips. Remove any other gaskets attached to your automatic transmission and set them aside. Underneath you should find a large gear housing cover, remove it as well.
  • Inside the transmission, you should find a very large planetary gear set. remove it by unbolting all retaining bolts and pulling it out slowly to avoid damaging other internal components.
  • Separate each of the gears that are inside your manual transmission case one-by-one using a soft mallet This includes the large internal gear attached to the main shaft, the sun gears, and other planetary gears.
  • Unbolt all bolts that secure manual transmission case halves together. This is best done using a portable drill with the right-sized bit head to avoid over-tightening them later. You should also remove shafts protruding from each side.
  • Remove shift forks and shift rails using a screwdriver or scriber to pry them out carefully. Make sure you note which gear they belong in before removing them, otherwise you might mix things up later on.
  • Find the bearing caps that hold brand new bearings in place and unbolt them from the case. Make sure you note whether they are marked on top or bottom. Unbolt all bolts holding the old bearings in place and then put the new ones in position using a press if necessary. Coat everything with bearing grease before starting to reassemble things later on.
  • Use files, sandpaper, and cleaning tools to remove corrosion and buildup from inside the manual transmission case where all gears will be moving around.
  • Reassemble everything in reverse order of how things were removed. Put everything firmly in place without over-tightening bolts as you need some room for adjustment once your car is running again.
  • Replace gaskets as needed and make sure everything is firmly in place before replacing the manual transmission case halves together.
  • Add fresh new fluid to your automatic transmission if you have removed the oil pan. The type of fluid will vary depending on what is required by your car’s manufacturer, just check a service manual for more information.
  • Reinstall the torque converter and fill it with new oil. Inspect the automatic transmission filter once more for any signs of damage or leaks before reconnecting it back to your car’s frame using a fresh new gasket. Then reinstall center support mounting bolts, connectors, cross members, and finally the entire assembly back under your vehicle.
  • Connect the transmission to the flywheel and fit it carefully back into place. Bolt it firmly in place using a torque wrench and then connect all other components back into place. These include speed sensors, throttle cable, etc. Finally, reconnect your vehicle’s battery and make sure you test drive it thoroughly before getting rid of any fluid that might be left inside.
Step by step process of rebuilding the most feared part of any car restoration… the automatic transmission.

Using a manual transmission is not difficult, just requires a bit more time and effort than an automatic. If you’re interested in giving your car more power and speed, you will certainly want to learn how to install a new clutch too. Just remember that it will take some time and patience to finish.

How Long Should it Take you to Rebuild a Transmission?

The amount of time it takes to rebuild a transmission will vary depending on what type of car you are working on. Most people would prefer doing this kind of work during the weekend when they have enough spare time available to get things done properly.

For small cars, it should not take you more than 6 hours, while larger vehicles may require double that amount of time. Just remember to take all your measurements beforehand so you know exactly what replacement parts to buy.

How Much Will it Cost Me to Rebuild a Transmission?

In most cases, the cost of rebuilding a transmission by yourself can be significantly lower than what you would pay a professional mechanic. There are several factors to consider in the cost of rebuilding a transmission by yourself:

transmission pump during the rebuilding process
Transmission pump during the rebuilding process

The cost of parts. This is obviously going to vary depending on where you are located and what type of car you are rebuilding.

Expert Tip: Keep in mind that it’s wise to order all your parts ahead of time and then use them during the rebuild. There’s no point in rushing things that could lead to problems later on. and you’ll save time and effort in the long run.

The cost of tools. This will vary greatly depending on how many tools you already have in your garage, plus the cost of any new ones that might be required for this job.

Removal and disposal fees for old transmission fluids

The cost of your spare time. In other words, is this a weekend project or something that you can do during your free weekday evenings? Shop rental costs. If you need a place where to work on your car’s engine safely, then you will have to pay for that service as well.


As you can see. rebuilding a transmission is not difficult to do if you have the right knowledge and tools available. Once your car or truck is running smoothly again, it will be time to think about doing some other upgrades at the same time. So make sure you consider adding new brakes, replacing worn suspension parts, improving fuel economy using engine tune-up procedures, and much more.

This article should give you the basic information you need to get started on rebuilding your car or truck transmission successfully. However, if you run into any problems along the way it’s always best to consult a professional mechanic for assistance. They will be happy to help you with all your concerns because that’s what they are paid to do.

I hope you find this information useful for rebuilding your car or truck transmission. Best of luck with your new project because it’s going to take a lot of hard work and dedication to finish successfully. Once you get the hang of it. there won’t be anything in this world that will stop you from completing a rebuild project quickly.

If you have any questions or comments, leave them below!

Avatar photo
About Matthew Webb

Hi, I am Matthew! I am a dedicated car nerd! During the day, I am a journalist, at night I enjoy working on my 2 project cars. I have been a car nerd all my life, and am excited to share my knowledge with you!