Automatic Transmission

Transmission Problems – Automatic Transmission Repair

In Transmission by 9 Comments

Do you have the feeling that your transmission does not change gears as it should? Maybe it changes gears too much or not at all. If you experience something strange with your transmission, take a look at this guide and I’ll show you how to fix the majority of your transmission problems. As many of us know, repairing automatic transmission problems can be very expensive. In most cases, the cheapest way is to replace the entire transmission with a new one if you have already tried to save it, but you find that it has internal damage.

But there are some simple corrections that can solve many problems with your transmission. I will write here how to diagnose the gearbox faults in a few steps to make it as cheap as possible. It may not work, but it can be worth a try, and your efforts may fix some transmission problems without the burden of paying a professional. Here’s how you can try to fix your automatic transmission issues without seeing a mechanic.

1. Find out what transmission you have.

Many new cars use a direct shift gearbox as an automatic transmission. This is a normal manual transmission, but with a robot that does the shifting and controls the clutch. They use a normal manual gearbox with clutch and gears. For this reason, a fluid change probably won’t help with this type of transmission, because inside it is just a normal manual gearbox. It might help to replace the fluid inside the mechatronic robot, but it’s unlikely. Often the mechatronic robot has been damaged by an impact, and you often have to replace the whole robot. Another common defect with this type of gearbox is that the clutch is worn out. In the future, I will create a guide on how to diagnose and repair these gearboxes as well.

But enough of that, now we’ll fix your problems with the automatic transmission. The first step you should take is to find out exactly what transmission you have. You have to find out the transmission number. If you can find it and want to answer more questions, you can do so here on our questions page. You can ask any questions you want and we will try to answer it as detailed as possible. If you are not really sure which gearbox type you have, you can visit the page below. Just select the year, make, 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 take is to read the fault memory of the car. If you have a very old car without a diagnostic function, this will not be possible and you can go directly to the next step. With some old cars, you can still read the fault memory with blink codes, etc., but I will not write about that here. To read the fault memory you need a diagnostic tool that works with your car. Here I will list some diagnostic tools at a good price. Make sure you check that the tool you’re buying is compatible with your vehicle.

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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 price, and you can read and erase the DTC memory in most types of cars.

If you read the fault code and find a DTC code, you should do some research on what exactly this fault code is telling you. Take the example of the fault code number P0700 and search for the code either on Google or here: Fault code search. If you can’t find any information about the fault code, you can ask me and I will help you. It may be good to check if it is an electrical transmission problem or not. If it is an electrical problem, for example, a shift valve problem, then this guide will most likely not help you. This guide could also help with these problems, but it will most likely not, and I recommend using a different diagnostic guide.

If you have not found any DTC codes, you can follow this guide further.

3. Transmission Fluid & Filter Change

Transmission Fluid

In most cases, if you have transmission problems and have not found any fault codes, it’s best to change the transmission fluid and filter. Old and dirty transmission fluid can cause a lot of problems, and it is worth a try to change it because it is not that expensive to buy – at least, not as expensive as a new gearbox! If you’re not sure how to change the fluid, try a Google search!

You should always read your repair service manual before performing any work on your gear unit.

But for 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 the transmission fluid, a liter 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 until the transmission fluid reaches a temperature of around 40 degrees celsius
  8. Go through all the gears P N 1 2 3 etc for 3 seconds each. Then put the gear in Park mode.
  9. Check the transmission fluid stick while the engine is running in P.
  10. Fill up if necessary.
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This is only a general guide, and you should first read your repair service manual to ensure that you are doing the steps correctly for your car.

After you have replaced the transmission fluid, I recommend you remove the negative battery cable for a while. Make sure you have a door open so that the car does not lock with the car keys inside when you put 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 stores many parameters of the way you drive the car, etc. Therefore, after changing the transmission fluid, it may be good to reset this parameter so that the transmission control unit can start again from the beginning.

You can also reset the parameters with a diagnostic tool if you do not want to remove the battery cable. Remember that removing the battery cable resets the clocks in the car, resets the mileage, and if you have a radio with a radio code, you need access to the code to get the radio working again. So it is always better and easier to reset the TCM with a suitable diagnostic device if you have access to one.

Once you have replaced the fluid and filter, you should take your vehicle for a long ride and see if the problem still occurs. If you feel that the problem still exists, we can move on to the next step in our automatic transmission repair guide.


4. Transmission Flush

Transmission Fluid

Now it gets worse. In many cases, it will not be enough to simply change the fluid and the filter, because the oil was so old and a lot of dirt got stuck in the gearbox. Therefore it can be a good idea to do a transmission flush and then fill it with new fluid again. In many cases, it can be sufficient to do two fluid and filter changes in a row, but I recommend doing a flush between them. A transmission flush is not that expensive, depending on where you live; about $100-250 in a mechanic’s workshop.

I do not recommend that you perform a transmission flush yourself at home. In most cases, you will need a special machine to clean the transmission. The machine pumps new oil around so that small dirt particles can come out. But if you really want to flush your transmission at home and don’t have a machine for the job, you could try building one of your own. There are many different ways you could try, but here is one way that I thought was very clever.

Check out this video.

After you have performed a transmission flush, it is time to top up the new oil. You do this in the same way as above. Take the car out again for a long drive and drive as usual to relearn the parameters in the TCM. If the transmission still does not work properly, go to the next step and I will explain what you should do now.

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5. Big Operations

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If your gearbox still does not work, you will need to contact some real gearbox repair experts. Either find a local expert for transmission problems, or ask us and we will try to help you. Back up any DTC codes you have previously acquired, and explain in detail what work you have already tried. Explain the problems as best you can and it will be easier for someone to help you.

I always recommend to let an expert try to find the problem before 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 transfer control module. And in this case, replacing the transmission does not help you and you are wasting your money.

But in the worst case, the car might have internal damaged parts of the gearbox that you cannot reach without disassembling the whole gearbox.

If you feel that you have enough knowledge, you can try to take the gearbox apart and repair it yourself. But I’m just saying… There are a lot of parts in a gearbox. You can search online for transmission repair manuals. There is a lot of advanced information on the internet if you really want to try to repair it yourself. But make sure you have the right transmission code number and that you find the right manual.

If your mechanic finds out that it is an internal problem and you decide to replace the entire transmission box, I recommend that you search for used gearboxes before buying a new one. There is a risk that the used gearbox is also damaged, but it might be worth a try. It depends a lot on where you live, but a good place to start is the local car scrapyards.

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 live, you can search online. eBay is a site I can recommend: it is a very big site that has a lot of used car parts worldwide, or you can use Google.


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 have 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.

Need more car repair help?

If you need more help with your car repair, you can ask your car questions in our community for free. You can find our community here.

9 thoughts on “ Transmission Problems – Automatic Transmission Repair ”

  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. Allison 1000 gen 3 2004 allison shop says solenoid D causing problem PO846 and PO848 to replace this solenoid do I need to remove the valve body or can solenoid D be replaced insutu obviously got to drain and remove pan. I am in Australia very expensive in the shop

  7. I drive a VW Passat 2006. I recently changed the transmission fluid and after the change, I notice a loud bang noise when I put the gear on D and R. I also noticed that when the gear is downshifting from 5-3, it stutters. What could be the cause of this and a possible solution to this problem
    Thank you.

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