Transmission Flush, When and when not to flush transmission

How a Transmission Flush Works

A transmission flush procedure is generally performed using equipment that either runs off the transmission pump or has its own pump built into the machine. The machines are connected to the transmission cooler lines, and while the old fluid is pumped out the new fluid is delivered at the same quantity and time. Sometimes there is a cleaning solvent that is forced back into the transmission removing deposits of old transmission fluid from parts and components or the technician performing the fush may run a solvent through the vehicles transmission before performing the flush. The cooling lines, cooler, converter and other parts are all thoroughly cleaned. One of the benefits of this procedure is that all of the transmission fluid is removed and replaced. This is different from a fluid change in which only some of the fluid is replaced. A simple fluid change cannot remove all of the built up deposits that accumulate over time as is done during a flush. Also, some of the fluid is usually stored in the torque converter potentially contaminating the new fluid that is put in. What is the point of that?

Transmission Flush Good or Bad?

I constantly hear consumer, customers and technicians debate as to whether transmission flush procedures can be harmful to a vehicle and is a transmission flush good or bad for your automobile. While many mechanics and experts agree that having a clean transmission will extend the life of a transmission, it is thought by some that the flushing procedure may not be the best way to achieve a clean transmission. One common thought is that the process, which forces liquid back into the transmission in the opposite way of the normal fluid flow, could potentially damage components or block tight passageways. When fluid is forcefully pushed back into the transmission, chunks of debris can be dislodged and possibly block narrow channels or one-way valves. When new fluid is put back in, these blockages can inhibit the normal flow of fluid through the transmission causing lubrication issues. Nonetheless, many car manufacturers and dealerships contend that these procedures are not harmful at all and help revitalize auto transmissions. MDH Motors does not use a reverse flush machine and over the thirteen years of being in the automotive repair industry I still yet to have seen a machine that flushes the fluid in reverse. Most machines just pump the new fluid in through the transmsiion cooler lines while containing the old fluid in a separate tank. The best transmission flush operation procedure is to:

  1. Perform the transmission flush
  2. Remove the pan and change the transmission filter
  3. Doing the flush in this order while help prevent the possibility of contaminates going through your transmission and potentially causing a problem by, let’s say hanging up a valve in the valve body.

    Checking your transmission fluid

    In the past every vehicle had a second dipstick, other that the engine oil dipstick, which was used for checking the level of the ATF. For vehicles that are still equipped with such, checking the ATF is very easy. Most cars require that the engine be running with the transmission in park. Some require that the transmission be in neutral. Honda trucks and cars with automatics require that the engine be off. If you are not sure what your vehicle requires, you can sometimes find directions on the dipstick itself. If that doesn’t work then consult the owner’s manual or Contact Me and I will be happy to give you some factory procedures.

    Many new cars do not have dipsticks. On these vehicles the fluid must be checked by climbing underneath the car and removing some kind of plug from the side of the transmission in order to see the fluid level. Some of these newer cars will still have the dipstick tube but no dipstick in it. On top of the tube you will find a plug that says in order to check the fluid level you have to take to take the car to the dealership service department. Once there, the technicians can check it with a special tool that looks just like a dipstick. This seems silly and it probably is. The reason for no dipstick is that the car builders want you to believe that you don’t need to check or maintain the fluid. Many of them actually say that the fluid they use is good for the life of the vehicle. This is not exactly true but with modern synthetic fluids, the fluid is at least good for the warranty period and that’s good enough for them. Some cars have a sensor in the transmission that will monitor fluid, and the level can be checked via the information computer located in the instrument cluster. The level is between the marks, then that is satisfactory, and no more fluid is required. If fluid needs to be added then usually it must be poured down the dipstick tube. These dipstick tubes that double as a filler tube are usually wide enough to put the end of a funnel into them. If the dipstick tube is too narrow to fit a normal sized funnel into the end of it, then there is likely a filler plug somewhere else.

    Most cars also require that the engine and transmission be warmed up in order to get the most accurate reading. The reason for this is that ATF expands quite a bit as it warms up. One might believe the fluid level to be low when in reality the fluid is just cold. Many manufacturers put separate marks on the dipstick that are used if the fluid is cold, but what if the fluid is somewhere between cold and warm? This is why it’s just best to check it with the fluid warmed up. The goal when adding or checking fluid is to make sure that the fluid level is between the two are found on the dipstick. If the level is below the lower mark then some fluid must be added, but if besides looking at the level the condition of the fluid can also be examined. If the fluid contains very tiny black particles that rub off on your oil rag or paper towel this is normal, but can it can indicate that the fluid needs to be serviced. These small black particles are bits of clutch pack material that are suspended in the fluid. This is a sign of normal wear and tear but if the particles become excessive, or if the particles are metallic looking; this could indicate some major problems. The last thing you can do that can help determine fluid condition is give it a sniff. Worn out fluid will have a definite burnt smell to it and fluid from a transmission that has completely failed smells downright disgusting.

    Is a Transmission Flush Necessary?

    Ultimately it is wise to consult the manufacturer owner’s manual and follow the guidelines outlined within. If the manufacturer recommends a transmission flush, it is probably advisable to have the service performed. However, it is true that not all manufacturers recommend this service at frequent intervals. It is not uncommon for flushes to be performed only every 100,000 miles. There are two main type of transmission flush machines and I will explain them below.

    Pump inlet flush machine:

    The first type of transmission flush machine I will explain attaches to the pump intake after the pan and filter is removed. This machine only supplies fresh new fluid to the pump intake and as the fluid passes through the transmission it dumps out to a collection tray and never goes back through for a second pass. All of the old fluid and crud is GONE and replace with fresh new fluid. After the service a new filter is installed, the pan replaced and then it is topped off with new fluid to the proper level on the dipstick. This process takes a total of 20 quarts of fluid to flush out 15 quarts of old fluid, replaces the fluid, and gives the mechanic the opportunity to look in the pan for anything unusual that would indicate a pending failure. Everyone should have this type of service done every 30,000 miles, but definitely before your truck goes out of warranty. By looking in the pan you may get an indication that you are about to have transmission trouble that might show up right after you get out of warranty.I will tell you that this type a flush does take more effort and makes more of a mess, costs a little more, but I think it is worth it. The extra charge will be for 1 hour labor and additional parts and fluid.

    Cooler line flush machine:

    The second type of flush machine connects into the transmission cooler lines. BG makes this kind and here is how it works. This type of transmission flush machine is more common in quick lube places where low level lube techs can operate the machine without any problems. They don’t have to operate any electronics or remove the transmission pan. They simply hook up the cooler line and start the vehicle. The other type of flush machines require you to maintain pump pressure and know the correct transmission cooler line flow. The line going from the transmission to the transmission cooler is disconnected and connected to the machine line in. The line out from the machine carrying new fluid is connected to the line going to the cooler. There is a chamber on the machine that has a diaphragm in it. The top part of the chamber above the diaphragm is filled with new fluid. The engine is started which turns the torque converter and the input shaft on the transmission. The input shaft turns the transmission pump and it makes hydraulic pressure. This causes fluid to flow through the cooler line. As fluid leaves the cooler line it enters the chamber on the flush machine. As the old fluid side of the diaphragm fills it pushes the diaphragm up and forces new fresh fluid into the transmission. After a while the old fluid is collected in the machine and it is replaced by new fluid. Now the transmission has been flushed. Really pretty simple. As you can see the machine cause no pressure and all fluid transfer is done by the transmissions own pump.

    Here is what I do not like about this type of machine. When the fluid leaves the transmission pump it passes to two different pressure regulators. One regulator supplies fluid at one pressure to the transmission itself that operates the pistons and controls gear shifting. The other is supplies the torque converter and the transmission cooler. So you can see that all the fluid leaving the pump does not go to the cooler. A bunch of it is cycled through the transmission and dumped back to the pan without going through the cooler. This type of flush machine does not remove all the old fluid, but it continuously dilutes it down with new fluid. It never really removes all of the old fluid, but is far superior to just drain and fill.

    The other thing I do not like about this type of flush is that they sell the supposed benefit that they do not have to drop the pan and change the filter like that is a benefit. Dropping the pan is very important. Looking in the pan is a fantastic diagnostic tool that can tell you if something is going wrong in your transmission. Now let’s say some crud is flushed out of the transmission with this flush method. Where does it go? It can go into the pan, and then sucked up into the filter that may clog the filter causing the pump to starve for fluid and a pressure loss. On the engine the filter is after the pump and if the filter gets clogged there is a bypass valve that opens and oil bypass the clogged filter so the engine is still supplied with oil. Unlike the engine oil pump and filter the filter is on the intake side of the pump. If it gets clogged, that is it, it is clogged and stuff does not get lubricated and the clutches do not get enough clamping pressure and they slip and burn up. In just a fraction of a second you just bought a new transmission if the filter clogs.

    When not to flush your transmission

    Before draining or flushing you should pull the dip stick and look at the fluid. If it is dark, burnt smelling, and you see little flakes or speck in it, DO NOT FLUSH IT. The fluid and transmission possibly has hard part damage, but the transmission just has not figured out it should die yet. If you flush a transmission in this condition it could fail right away. Real strange, but that is what seems to happen. If your transmission is in this condition just drive it while you save for a replacement transmission. There is no way of telling when it will fail. It might be today, next week, or next year, but it is doomed.

    The goal here is to flush the transmission BEFORE the fluid gets contaminated. What you are taking out should look like what you are putting in. Do not wait for a color or smell change. Most manufacturers calls for transmission fluid change at 30,000 mile intervals. The industry standard is two years or 24,000 miles whichever comes first. It is your vehicle, you decide what is best for you.

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About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development. MDH Motors offers moderated guest posts! Click Here To Submit Your Article For Review Please feel free to Ask a Mechanic if you have not received a response for your comment within 72 hours, thank you.


  • Juan Ayala says:

    Hi, I’ve recently checked my Honda civic trans oil and it is a light brown color. Also, I was driving the other day and my car shut off while I was on freeway. I put in neutral and started right back up. Then it started to jerk after shifting out of it’s first and second gear. Is there any info you can share with me about this issue?

    Kind regards

  • Kathie says:

    Have 2000 lx470 248,000. Truck runs like new just had flush first time. No prior trans problems. Was flushed with t tech. If filter is changed and pan was dropped do you think there will be trans problems later?

  • Peace says:

    Hello, I recently become the third owner of a 2010 Mazda 6, with 108k on it. At first it served in a rental company and the second owner also drived it a a lot. Seems that he knows little about taking care about the transmission, because the ATF apparently has been dark. But the car still runs good as I feel, not obvious issue during shifting. I want to keep this car for long so I’m planning change the filter and fluid in transmission, using gravity drain, not machine. I want to know if this is necessary, and if there will be any harm?Thanks a lot!

    • Martin Hand says:

      no harm should be done your just exchanging the fluid, you can just pump the fluid out through the cooler lines just make sure not to run the transmission dry

  • hi says:

    ….also,my 2011 camry 4 cyl manual states…tranny oil capa city is 6.9 quarts….why does repair shop want 17 quarts to flush it?

    • Martin Hand says:

      There is additional fluid in the converter and cooler and cooler lines that should be cleaned out. I would say 17qts is excessive though. I usually do a 10qt flush and they’re fine.

  • hi says:

    Hi there, I have 2011 camry 4 cyl. The tranny is sealed unit with no dipstip. I have 70,000 original miles with no service yet….it runs fine but I plan to keep the car for long time….is there a way to check fluid level n condition without doing flush or drain…I don’t trust any repair shop even dealers…. can I do it myself? ThanK you.

    • Martin Hand says:

      The transmission is not a sealed unit, it just doesn’t have a dipstick. The fluid level indicator is a plug on the bottom of the transmission, only check while running at operating temp otherwise fluid will start running out.

  • Ameena says:

    Hi, I bought a 2006 Ford Fusion with 87k miles in March 2015. To date it has 101k miles (mostly city miles). I have been having shifting issues (no long after I bought it), even upon take off. Until I reach about 45-50mph (without any deceleration) my car struggles to shift to next gear. My trans fluid is black. I’ve only had 2 oil changes since purchasing the car. I hope this is enough info to advise me on what trans service is needed for my car. Thank you!

    • Martin Hand says:

      If the transmission is having shifting performance problems I recommend to take it to a repair shop and have them inspect it for that. A transmission service or flush is for preventative maintenance only and will not correct any problems your having.

  • Danielle says:

    I have a 2008 Nissan so I have to check transmission fluid if not low n I used seaform threw my gas so I think I need to do transmission flush.

  • ChrisRab says:

    I just brought my 2007 ford fusion SE 2.3L i4 auto to Sears for a oil change and transmission drain and refill via drawing fluid from dipstick tube and refilling….

    After driving home and noticing no difference, i pulled the dip stick to find that it is still gray and smells awful. What should i do? is this normal to have poor fluid in the tube even after a replacement of fluid?

    • Martin Hand says:

      Drawing from the dipstick tube will mix the old fluid with new. It will eventually clean the fluid it just takes more if the transmission fluid is really contaminated. Your probably fine for now but next time I would recommend a complete fluid exchange (minimum of 10qts) using a cooler line exchange machine.

  • swetul says:

    hii i have a 2005 hyundai accent and i got a transmission flush as the previous owner didnt do an oil change for god knows how many years and before i did the flush my gears shifted at 3k rpm and now after the flush they still shift at 3k RPM but the issue is not that the issue is shifting upwards or downwards on the automatic transmission makes the car feel like its accelerating even my foot is not on the gas and i have to hit the brakes more harder as the gears shift downwards i dont know why .can it be a posibility the mechanic didnt flush it out properly because honestly i didnt have this problem before the flush and now sometimes the RPM has to be over 3k to shift nd sometimes it does it at 2k. any ideas why i am facing such problems

    • Martin Hand says:

      The flush most likely increased the symptoms. I never recommend a transmission flush when your having transmission performance problems. I would remove the pan and change the filter. Inspect for excessive metal or friction material in the pan/filter.

  • Fisher says:

    Hi, i have a 2007 Mazda 6 and it’s done 124084km, it was brought at 103000km. Now the transmission fluid seems a bit dark and the gear switch to 5th too soon and it’s really hard to gain speed, thus I need to use triptronic to lower the gear manually.

    Now what should I do? Should I just drain and replace the fluid and the filter?


    • Martin Hand says:

      An earlier up-shift does not mean the transmission is failing. I would perform a service but include removing the pan and changing the filter. The up-shifts are electronically controlled and usually adapt to driving habits.

  • sharee says:

    I just bought a 2003 buick park ave. The transmission fluid is brown. While pulling off from a stop sign, It jerks sometimes. My mechanic said i only needed one motor mount and to not change the fluid or the trans would fail. I took it to another mechanic and they said it needed a flush and it might fail afterwards because its dirty. The third mechanic said it needed 2 motor mounts and a flush. Which mechanic should i go with and what should i do regarding this issue?

    • Martin Hand says:

      The transmission should be serviced if it’s working correctly. I also recommend to remove the pan, inspect for excessive contaminates such as metal and friction material then change the filter.

  • Naomi says:

    Thank you for sharing detailed user-friendly info. I just bought a used “salvaged” (that’s okay…only side damage…saw pics and had through inspection – all is well – 2011 Ford Escape XLT w/39k miles- no warranty. Transmission fluid is a medium tan color the tech said should have flush. The BG Protection Plan was brought up and owner said he swore by it and admitted his commission but he actually believes in it.

    I know what the plan is about…I”m just interested in your opinion as to whether it actually cleans things and prolongs things or if there is a video out there with someone showing how their products eat up the crud in the lines…pipes…etc.

    • Martin Hand says:

      There is most likely no crud or sludge in you cooler lines, automatic transmission fluid is a detergent so it cleans even as it’s dirty. The fluid usually gets burnt or contaminated with metal and/or friction material if there is a problem.

  • Sierra Morreale says:

    I have a 2006 Pontiac Torrent. I bought it around 140,000 miles and it now has near 160,000 miles. I’ve never done any transmission work and I don’t know if there was any done before me. When I begin driving and get around 30 ish mph it will shift but instead of a smooth shift it will kinda jerk. It kind of feels like it shifts like a manual car does. Anyway I was going to have it flushed this weekend but after seeing this it sounds like I shouldn’t…?

    • Martin Hand says:

      No, there is some type of failure that needs to be addressed. The service is for preventative maintenance only and will not expect it to improve the transmission performance.

  • Randy says:

    so i have a jeep cherokee XJ with 182k miles on it. the jeep runs great and i went through and did everything i could to recondition the engine, electrical etc. but nothing as far as a flush…. mainly because i hear two things, its so old that you shouldnt do it and you risk ruining your tranny, or it doesnt matter and go ahead and do it. any suggestions?

    • Martin Hand says:

      High mileage transmissions should have the pan removed and filter changed with the transmission flush. Make sure there is not a bunch of metal or friction material in the pan. This will give you an indication of the condition of your transmission also.

  • Camilo says:

    Hi, I have a Ford F-150 Ecoboost 40k miles. The dealer ask me to do the flush but I was not sure what benefit it would bring to my truck. After reading this article I assume i should do it, but my question is; Should i change at the same time the filter?
    Thanks for your article and support.

    • Martin Hand says:

      At 40K you should be good with just a flush but changing the filter also cannot hurt. This way your removing any contaminates that could have flushed into the filter during the service.

  • Georje says:

    I have a 1993 chrysler lebaron 87000 miles,68000 when I bought in 2012. Had Trans Flush at that time
    Mechanic showed me a leak from a hose so I made sure to top off fluid from time to time.
    Fluid is pink and does not smell Now.when car is warmed up transmission will not go out of 2nd gear. Rides Great and shifts fine till it warms up. Might a flush, flush and gasket (and hose repair from pan ) fix or help with the problem I’m having ?
    THANKS !!!!

  • lee says:

    HI, I have a 07 Honda Odyssey, it has 130k miles on it. It has never had a transmission fluid change. I am planning a long road trip next month, around 3k miles total. I was going to get my timing belt, water pump, and coolant change. Should I also flush out the transmission fluid or just change it? Or should I wait on that till I get back. The fluid is little dark, but not black. The only thing that I have had done constantly is the oil change, nothing else, what would you recommend. Thanks.

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