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.

This post was written by: Martin Hand


If you find this information helpful please consider a donation. These articles, questions and comments are very time consuming so even a small donation gives me motivation to keep educating automotive owners. Donations will allow us to continue open questioning/comments, automotive education and repair tutorials in the future as the business grows. All proceeds go to the expansion and maintenance Thank You

Martin Hand

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.


  • caesar says:

    2004 mitsu endeavor AWD, 4speed trans right should you feel the transmission shift 3rd-4th gear or feel only the 1st to 2nd gear confuse trans has no shrudder or noise please advise

    • Martin Hand Martin Hand says:

      Yes you should be able to feel all shifts. 1-2 upshift, 2-3 upshift, 3-4 upshift and depending on the transmission you can sometimes feel the torque converter clutch when applied, would feel like another gear. I believe TCC is only applied for 3rd and 4th gears.

    • Martin Hand Martin Hand says:

      Yes you should be able to feel all shifts. 1-2 upshift, 2-3 upshift, 3-4 upshift and depending on the transmission you can sometimes feel the torque converter clutch when applied, would feel like another gear. I believe TCC is only applied for 3rd and 4th gears.

  • Jenny says:

    I have a 2012 Hyundai Sonata recently the check engine light when on, I had it checked & it said it was the transmission. Car works perfectly fine no problem except for a couple days ago it smelled like burnt, what would be recommended in this situation

    • Martin Hand Martin Hand says:

      You need to follow the diagnostic procedures for the diagnostic trouble codes set for the transmission first. Also clear code, test drive to see if the code comes back.

  • Kenneth says:

    I have a 2001 Nissan Pathfinder SE. I has a problem kicking down when trying to pass a car and it goes to neutral and the engine revs at high RPM. Also if I turn off the Over Drive button, the transmission goes into neutral and the engine revs high RPM. Do you think a Transmission flush could fix this issue (maybe a stuck valve) or does it sound more mechanical like a Transmission Temperature Sensor or Solenoid Pack? The transmission was rebuilt 3 years ago. It had the same behavior before the rebuild. It was working OK for about 5 months then suddenly started acting up again.

    • Martin Hand Martin Hand says:

      Some type of failure, changing the fluid will not help. Scan for codes and possibly remove the pan to inspect for friction material or metal, this would give you an indication of possible burnt clutches or hard part damage.

  • Webrad51 says:

    I have a 2010 Chev Malibu it was running fine then all at once it started to not out. When I would start driving it would just stay stuck when I took my foot off the gas it would drive fine. The guy add 3 quarters of transmission fluid. The next day and since then has driven better than ever. Do I need to have my transmission fluid changed or flushed. The car has over a hundred thousand miles on it.

  • Blanca says:

    The transmission on my 2010 nissan rogue has been replaced twice but still doesnt accelerate after driving it for an hour..I dont know if the transmission cooler was you think flushing it would resolve the problem?

  • […] a policy of refusing to flush and fill a transmission once the fluid is dark, burnt, or worse. The best practice here is to check your transmission fluid regularly, keep it topped off, and get it changed at the […]

  • […] a policy of refusing to flush and fill a transmission once the fluid is dark, burnt, or worse. The best practice here is to check your transmission fluid regularly, keep it topped off, and get it changed at the […]

  • Alex says:

    Is aliment necessary after transmission replacement ?
    again, thank you.

  • Alex says:

    dose transmission replacement required transmission flush service?
    Thank you!.

  • Jennifer says:

    two questions:
    is it odd that
    nissan dealership #009 did an AT fluid replacement in March that required 9 quarts
    nissan dealership #6969 did an AT flush in April that used 6 quarts?

    2nd question
    Taking the technicians advice from dealership #6969 who recommends a repair for a DTC U1000 to replace the valve body assy and they also did an AT Flush. ($2140) Two weeks later THE EXACT SAME VEHICLE ISSUES, no better or worse than before along with the same MIL again – U1000 appeared (lack of acceleration, requiring you to turn off the engine wait and then turn it on again, to fix the problem for another few miles and then repeat), SO

    Does this indicate 1. they did not actually do the repair? or 2. if they actually did replace the valve body assy and nothing changed was it an unnecessary repair?

    the first time i was told to pay the $130 for the DTC Scan Code for the repair advice…now same issue, same MIL and instead of running a scan for the DTC – they simply said they pulled the pan, found metal fragments and now it needs to have a replacement of the transmission. ($4350).

    • Martin Hand Martin Hand says:

      The valve body should have never been replaced if the transmission was having internal problems causing metal to get into the transmission. This would just contaminate and cause the new valve body to fail

  • Melvin says:

    I have a 2009 ford expedition it has 127k on it. It runs and shifts good but i wanna know is it safe to do a transmission flush or should i just do a transmission service replaced the filter n gasket n refill????

    • Martin Hand Martin Hand says:

      Start by removing the pan and replacing the filter and gasket. If the fluid is still dirty and the transmission is operating normal then you can do a complete transmission fluid exchange using the transmission cooler lines to pump out the old ATF.

  • Daniel Turner says:

    I have a 2003 Subaru Baja, This last weekend the car would be very slow to get upto speed while feeling like it was stuck in N, however I have a friend telling me that I need to change the filter and run some Lucas Transmission oil in it for a bit and drain it again and refill it and it should help, however as optimistic as it sounds I have an appointment at AAMCO tomorrow and Im not quite sure what to do.

    • Martin Hand Martin Hand says:

      If you’re having transmission performance or shifting problems then it is most likely too late to correct the problem by servicing the fluid. I would remove the pan and inspect for excessive friction material or metal.

  • […] a policy of refusing to flush and fill a transmission once the fluid is dark, burnt, or worse. The best practice here is to check your transmission fluid regularly, keep it topped off, and get it changed at the […]

  • […] a policy of refusing to flush and fill a transmission once the fluid is dark, burnt, or worse. The best practice here is to check your transmission fluid regularly, keep it topped off, and get it changed at the […]

  • Ivo Pranjic says:

    Hi, great article! I have a 2005 Mitsubishi Gallant GTS that is having some transmission issues. When i first start up the car there is a decent amount of resistance when I try to shift into drive or reverse. When i do there is a 2-3 second delay until the car actual shifts into that gear. It also jumps a bit when i go from a solid stop back into driving.

    The interesting part is that this all goes away after about 15-20 minutes of driving. After that i can easily change from park, reverse, and to drive without any resistance and the car does not jump when i stop at a light and press on the acceleration. Would a transmission flush be helpful here?

    • Martin Hand Martin Hand says:

      No there’s something wrong that would need repair before servicing the transmission. Transmission services should only be performed for preventative maintenance not for a repair attempt.

  • Ramulus says:

    I have a 2006 Nissan Xterra with 187k miles. Was at work one day and as I tried to put the the vehicle into reverse it did not engage. Turned the vehicle off for about a minute and then the engaged that time. Took it in for a flush only to find out that the radiator coolant was mixing in with the transmission fluid. The mechanic recommended replacing the transmission, radiator and lines which will cost more than the vehicles current value. I was told this was a common problem with the xterras and there was a recall in place.
    Question: will replacing the radiator and transmission lines plus flushing the transmission fluid rectify the issue? The vehicle drives but occasionally slips. Does this mean the tranny is completely gone?

    • Martin Hand Martin Hand says:

      You can flush the lines out and not replace your if they’re not leaking. The transmission will need repair and radiator replaced.

    • Jennifer says:

      This is very common. The norm really, with over 562 Nissan owners that tried and failed to get Nissan to own up to this problem. Find a cheap radiator (200 and under), and FLUSH that thing 2 times (200 +/-) and avoid driving in traffic or on the freeway, stick to back dirt roads or to 7-11 and back…hope you get it to last for another 10k miles…that’s the cheapest and easiest route before you sell it at pick a part for $300 bucks. If you did get cross contamination and didn’t catch it in time before it really got in to all the nooks and crannies then yeah, .your tranny is completely gone and rebuilt or new they are still in the range of 3k and up to replace. THANK YOU Nissan – You gave KIA a lot of business!

  • BC says:

    My 1998 Isuzu Trooper has not had it’s transmission fluid service done for 90,000 miles. I was driving it over to Phoenix from the Los Angeles area during the morning Heat about 3 weeks ago. I went up 1 of many long hills and then the Red Trans Check light came on along with the Yellow/Orange Check Engine light; the Owner’s Manual says that the Trans Check light means that you can drive it but you may have to manually shift the gears. I drove it back to LA going slower at 55 mph & the Trans Check light stayed on and so did the Check Engine light. The interesting thing is that I did not have to manually shift gears & it still has shifted by itself for the past week. Yesterday all of a sudden while driving it locally the Check Engine light and the Trans Check light went turned off! I forgot to indicate that when the lights originally came on, on my way over to Phoenix, the Trans Fluid Temperature light did NOT come on. Being that we do not use the vehicle as much anymore I had neglected to do a transmission service on it, which includes opening up the pan and I believe cleaning out the filter (I think it has a built-in filter maybe you can enlighten me on that issue). What would you recommend I do because my thought on the matter is that I should flush (& not a reverse flush) it and then have new transmission fluid put in. However, I have been told in the past that sometimes when you flush tiny particles that the transmission is used to having in there (not metal particles) if you flush everything out then what can happen is that your transmission does not last as long and it will go out on you in a relatively short period of time, assuming of course that the transmission is in good working condition. Please enlighten me before I go and waste money. My reason for taking it over to Phoenix was to offer it to my son-in-law who has another Trooper so that he could use the parts on this one or on the other one and make a better longer-lasting Trooper out of the two. I was going to take it to a transmission shop today whether to an independent or one of the several Aamco shops in the area. But after reading your many wise suggestions to other inquiring individuals I thought it best to consult you before going to any transmission shop.

    • Martin Hand Martin Hand says:

      I would start by checking the diagnostic trouble codes to see why the lites are coming on. After checking codes and performing diagnostics you can decide if it needs service or not. Most likely you’ll have to remove the pan for testing or repairs anyways.

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