transmission repair

Transmission Problems – Automatic Transmission Repair

In Transmission by Magnus Sellén7 Comments

Are you feeling that your transmission is not changing the gears as it should? Maybe it is changing the gears too hard or not at all. If you are experiencing something strange with your transmission. You could check out this guide and I’ll teach you how to repair the most of all small transmission problems. As many of us know, automatic transmission problems can be very expensive to repair. In most of the cases, the cheapest thing is to replace the whole transmission with a new transmission gearbox if you already tried to save it but you realise that it has some internal damage.

But there are some easy fixes that can solve many problems with your transmission. I will write here how you diagnose the gearbox fault in some steps to make it as cheap as possible. It may not work but it can be worth a try and can fix some transmission problems. The diagnose moment could be a bit different depending on if its a new or old car, new direct shift transmission with a dry clutch etc. But you can follow this stages to see what you should do on the way.


1. Find out what transmission you have.


Many new cars use a direct shift gearbox as an automatic gearbox. They are a normal manual gearbox but with a robot that is doing the shifting and controls the clutch. They use a normal manual gearbox clutch and gears. Because of that, a fluid change won’t probably help on this kind of gearboxes because it’s just a normal manual gearbox inside. It could help to replace the fluid inside the mechatronic robot but I doubt that it will help. Often the mechatronic robots is impact and you often have to replace the whole robot. Another common fault on this type of gearbox is that the clutch is worn out. I will make a guide in the future of how to diagnose and fix these gearboxes too.


But enough of that, now we are going to fix your automatic transmission problems. The first step that you should take is to find out exactly what transmission you have. You have to find the transmission number. If you find it and want to answer further questions you can do it here on our question site. You can ask whatever question you want and we will try to answer your question as detailed as possible. If you are not really sure what transmission you have, you can check out this site. Just select what year, brand, model and engine and you should find it.

The link is here:  Transmission search

If you are now sure that you have a normal transmission and not a new gearbox with a mechatronic/robot,  we can continue this guide about how to fix your transmission. Let’s go to the first step.

2. Read the fault memory of TCM

The first step I always do is to read the fault memory of the car. If you have a very old car without a diagnostic function, this won’t be possible and you can go to the next step directly if you want. On some old cars, you can still read the fault memory with blink codes etc, but I won’t write about it here. To read the fault memory you need a diagnostic tool that works with your car. Here I will list some diagnostic tools for a good price. Because many fault diagnostic tools only can read from the ECM. Read the description of which cars you can read and which systems before you buy.

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If you want to buy a diagnostic tool to use at home. There are a lot of variants out there, but a tool I can recommend is the FOXWELL NT301 Car Obd2 Code Scanner. It is a really good DTC tool for the money and you can read and erase the DTC memory in the most types of cars.

If you read the fault code and you find a DTC code. You should do some research on exactly what that fault code is telling you. You can take the fault code number example: P0700 and search for the code either on google or here:  Trouble code search. If you cant find any information about the trouble code you can ask me and I’ll help you. It can be good to check if it’s an electrical transmission problem or not. If it’s an electrical problem, for an example a shift valve problem, then this guide will most likely not help you. This guide could help those problems too, but it will most likely not and I recommend to use another diagnostic guide.

If you did not found any DTC codes there you can continue to follow this guide.


3. Transmission Fluid & filter change

transmission fluid

In the most cases when you have transmission problems and I didn’t find any fault codes, I do a transmission fluid & filter change. Old and dirty transmission fluid can cause a lot of problems and it’s worth a try because it’s not that expensive. At least not as expensive as a new transmission gearbox. All cars are different and it’s hard to say exactly how to do it on your car. But my tip is to search on google, there is so much good information out there!

You should always read your repair service manual before doing any work to your transmission.
But In the most transmissions, you do it in this order.

  1. Remove the plug from the transmission oil sump (if you have one)
  2. Remove the transmission oil sump
  3. When you have removed the fluid sump, you will probably see the filter.
  4. Put everything back with the new filter, as it was from the beginning and make sure to make a good seal for the fluid sump.
  5. Fill up transmission fluid, a litre less than you were tapping out
  6. Start the engine and put the transmission in N
  7. Let the engine warm up and check with a computer till the transmission fluid gets a temperature of around 40 degrees celsius
  8. Go through all the gears P N 1 2 3 etc for 3 seconds each. Put the gear in Park mode
  9. Check the transmission fluid stick while the engine is running in P
  10. Fill up if necessary

This is just a general guide and you should read your repair service manual first to make sure that you are doing a proper job.

After you have replaced the transmission fluid. I can recommend you to remove the negative battery cable for a while. Make sure that you have one door open so the car is not locking itself with the car keys inside when you are putting the battery cable back on. You want to do this to reset the parameters in the transmission control unit. In many cars, the transmission control unit is storing a lot of parameters of the way you are driving the car etc, so after a transmission fluid change, it can be good to reset this parameter to make the control unit start from the beginning again.

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You can also reset the parameters with a diagnostic tool if you don’t want to remove the battery cable. Remember that removing the battery cable will reset the clocks inside the car, trip mileage and if you have a radio with a radio code you need access to the code to get the radio back up again. So it’s always better and easier to reset the tcm with a proper diagnostic tool if you have access to one.

When you have replaced the fluid & filter you can take it for a long drive and see if the problem still occurs. If you are feeling that the problem is still there, we can go to the next step in out automatic transmission repair guide to try another thing.


4. Transmission Flush

transmission fluid

Now it’s getting worse. In many cases, it won’t be enough with just a fluid & filter replacement because the oil was so old and a lot of dirt got stuck inside the gearbox. Therefore it can be a good idea to do a transmission flush and after that put new fluid again. In many cases, it can be enough to do two fluid & filter replacements successively but I recommend to do a flush between them. A transmission flush is not that expensive, depending on where you live but around 100-250$ at a mechanic workshop.

I do not recommend to do a transmission flush yourself at home. In most cases need a special machine that is cleaning the transmission.
The machine is pumping around new oil to make any small specks of dirt to come out. But if you really want to do a transmission repair flush at home and not have a machine for the task, you could try to build your own. There are many different ways you could try, but here is one thing that I think was very clever.

Check out this video.


After you have made a transmission flush it is time to fill up the new oil again. You do it with the same procedure as I was talking about higher up in the blog post. You can do the same procedure as I told you before, reset the parameters in the TCM again. Take the car out for a long drive again and drive as you usually do, to learn in the parameters in the TCM again. If the transmission is still not working properly there are not much more to do. Go to the next step and I’ll explain more what you should do now.

5. Big operations

If your transmission is still not working you have to get in contact with some real transmission repair experts. Either you can find a local transmission problems expert or you can ask us and we’ll try to help you. Save all DTC codes you got in the trouble DTC memory before and explain in detail what work you have tried already, Explain the problems as good as possible and it’s easier to help you out. With some luck, it can help to change a shift solenoid or transmission control module etc or any wiring problem.

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I always recommend to let an expert try to find the problem before you are replacing the whole transmission box because it could be a wiring problem between the transmission control module and the transmission. It could also be an internal problem in the transmission control module. And in that case, a replacement of the transmission won’t help you and you will waste your money.

But in the worst case, it can be internal damaged parts of the transmission that you cant reach without taking the whole transmission apart.
If you feel that you got enough knowledge you can try to take the transmission apart and fix it yourself. But I’m just saying… It’s a lot of parts in a transmission. You can search online for any repair service manuals for the transmission. There are a lot of advanced information out there on the internet if you really want to try to fix it yourself. But make sure that you got the right transmission code number and that you find the right manual.

If your mechanic find out its an internal problem and you decide to replace the whole transmission box, I can recommend searching for used gearboxes. There is a risk that the used transmission is damaged too, but It could be worth a try.
It’s depending a lot where you live but a good place to start check is on local car graveyards.
Often if you have a common car and transmission it does not have to be very expensive.

If you cant find a transmission box where you can search online. eBay is a site I can recommend, it is a very big site that got a lot of used car parts worldwide, or you can use google it to find parts.



The most important notes of this article:

  1. If you do not have any DTC codes stored in the memory, always start by doing a transmission fluid & filter replacement.
  2. Reset your TCM parameters after you made a fluid & filter change
  3. Flush it or try to make a fluid change two times

I hope that this guide at least helped you out a bit and I really hope it fixed your transmission problems.
Remember that If you still got some questions or want us to add or change anything in this guide, you are always welcome to contact us to get your questions answered


Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.

  1. I think that there may be a problem with my car’s transmission, and it’s good to know that fluid changes don’t really have an effect on direct-shift gearboxes. My car is rather new, and I think that the direct-shift is the type that I have, but I’ll take your advice and check the transmission number, and, failing that, use the link you gave. It’s pretty cool that you can actually check for a fault code to see what problem your transmission is having, and I’ll definitely remember that and get help from a professional mechanic in diagnosing my car’s problem.

  2. I was told that my 2007 BMW X3 with a GM transmission..(GA6L45R) while showing a solenoid “B” as not functioning, I have been told that I need to replace, not just the full 6 piece solenoid pack but actually the entire valve body…at what I can say is well over $2000..
    On ebay, I can buy a valve body including the solenoids for about $400, but often the donor cars are BMW’s but different models. My question is in regard to coding the new valve body into my X3. Is it necessary to code the new part in, if the car is another BMW (even if different model), and if so, how difficult is it to do for a regular mechanic and is it possible to ask a transmission shop to do just that function?

  3. Thanks for your good advises.
    My car is Mazda premacy 2005. The problem is that when I engange reverse gear at abit slope place the car goes forward continuously as if it is at neutral.
    WHAT CAN BE A pROBLEM. Please help.

  4. I use a Toyota Corolla 2011 I observed it takes a delay to change from gear 2 to 3.i changed the gear oil still didnt stop and when diagnosed no erro came up.
    What can I do

  5. I have 4l30e on bmw 318i 1994 no code on board but i check main line pressure and still low( 50 psi )
    When i’m on the road at 2400 rpm some peak of pressure 75psi when shifting but trans is slipping when i’ts time to shift tcc lockup because the low pressure 55 psi
    All stall test is good 150psi in drive stall test
    200 psi in reverse like the spec
    Few test on trans and tcc solenoid change
    Pressure selenoid is in working conditions after few test amps

  6. Due to a very high demand and high ammount of comments, you have to wait for some time for your car questions to get answered. If you want to get fast answers from a certified master technician you can ask your questions here:
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