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.

MDH Motors logo

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has 13 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 & Web Programming 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 future plans of automotive diagnostic software development. Please feel free to Ask a Mechanic if you have not received a response for your comment within 72 hours, thank you.


  • Joseph Somma says:

    I purchased a 2001 Jeep Cherokee two years ago with 124000 miles on it. I nor have 150000 miles on it I was thinking of having the transmission flushed some people are saying to leave it alone it’s running well. Would you recommend I have it done.

    • Martin Hand says:

      On any vehicle with high mileage I would first ask if your have transmission shifting problems, if no then you should service the transmission. I would definitely remove the pan and change filter on top of a complete flush. When you remove the pan look for excessive brass, aluminum, steel or friction material and that can give you an idea of what’s going on, okay.

  • Kenney Johnson says:

    I have a 1999 Chevy Silverado 1500 front wheel drive/six cylinder with 160,000 miles the question I have is I bout this vehicle about a year ago and checked the transmission fluid at the time of the purchase and it seemed ok now it’s starting to grind and pull forward when in gear but stopped would a flush solve this issu? Looking for some sound advice can you help me out

    • Martin Hand says:

      No, I don’t ever recommend a transmission flush when your having transmission performance problems. That vehicle has a 4L60E I believe, there common for the sun gears to break.

  • CMansfield85 says:

    My transmission shifts into gears but I hear the same noise that a steering wheel makes when you need power steering fluid. It also won’t leave out of 1st gear. What could that be?

    • Martin Hand says:

      Sound like the transmission pump is making noise, same exact concept as a power steering pump just a bigger pump. Make, model or vehicle type would be helpful, thanks.

  • Robert Santos says:

    This is one of the better, explanations and recommendations I have seen regarding transmission flushes. However, your recommendation to not flush a transmission if, “the fluid smells burnt or there are lots of particles” visible in the fluid. Your reason being that your experience has been the transmission often times immediately breaks down and the flush is for not.
    Many automotive engineers cannot understand this theory. They do not understand how changing very old, very dirty fluid can in any way harm a transmission, especially if you remove the pan and change the filter and diligently clean the pan, with a follow up filter cleaning and pan oil change after two or three thousand miles.

    • Martin Hand says:

      I see your point. I always tell my customers that their best service option is to do and flush and remove the pan to clean it out then change the filter. I never recommend a flush if there experiencing transmission problems.

  • Boyce Sr. says:

    Just wanted to tell you that after searching for half an hour looking for some Trans Flush answers I ended up on your site and was VERY pleased. You answered most of my questions in terms I could understand. You never mentioned price letting me believe what you were telling me was for MY benefit rather than yours. All in all a very pleasant and worthwhile time spent.
    Thank you from Las Vegas!!

  • Jim S says:

    I’m trying to keep My 1995 156,000 Bronco On the road Its a one owner so I know its history pretty well and since cold weather set in, I am finding Start up and Go capability is lessoned, over the last few weeks I often hop in Warm it up “breifly” ie: enough to let the Motor oil get around… But usually Not for 30 minutes either I noticed I can drive out and get to say the grocery store which is less then a mile, and no reverse, seems like you pull up to back in a parking spot and The engine stalls, starts and No problem…backs in and everything is fine…. Last ni ght was Cold (getting Very cold over the next couple days) sub zero… and I left my drive got 70 feet down the road and it stalled in drive Loss of power put the 4 ways on, started it back up and sat for 15 and as The vehicle warmed up I began moving the slowly selector from Drive to Neutral.. to neutral.. to drive and after a bit all was good again…. Like everyone these days I would LIKE to keep the vehicle since It Physically fits me well and of course I am on a ixed Income… (I would consider replacing the the Trans you know? Way Cheaper then 50,000 g which keep me from Eating… Thanks for your Thoughts……

    • Martin Hand says:

      I believe that vehicle has an AODE or an AOD (the E is for electronic) transmission. These transmissions are common for valve body control issues. The valve bores wear causing fluid pressure blow by. I’m not sure if you repair AT transmissions but if you look up Sonnaxx they make o-ring end plug kits to fix this problem. Find out what type of transmission for sure and do a little research. You will be surprised on what you find when you know what transmission model your working with. Good Luck!

  • DebLein says:

    Thanks! This is exactly what I was looking for, especially the last section.

    I’m driving a 2001 Chrysler Town and Country that I found on the way to the junk yard for free. Changed out the radiator and water pump and turned it into a wee minivan camper to travel around to all the National Parks while I telecommute. I have driven it for 20,000 miles having gone from Shenadoah Valley, to the Everglades and now at Guadalupe Mtns. On the way to Guadalupe from Big Bend, my check engine light dinged at me, but I was traveling at about 65 mph with the closest town 50 miles out with no other occasion for believing anything was wrong.

    Once I stopped off at Van Horn, I checked the codes and got P1684 and P0700. Battery and transmission related. I have actually felt hiccups in the transmission down in the Everglades between 1st & 2nd. I had Walmart check the fluid when they changed out the oil and they said it all looked good. So I have continued on simply accelerating between 1st & 2nd at a more rapid pace to avoid the “hiccup” feel. (Denial is so helpful!)

    Well, four-wheelin’ (with only a 2wd!) in Big Bend probably didn’t help matters as I was mostly in 1st and 2nd. I have noticed a burn smell but my fluids are all good, though I do have to fill my radiator every once in a while (plus my temps are good). But my tranny fluid doesn’t look so good now, though it is at the right level. I’m at 205,000 miles right now (and I think it got a new transmission back in 2009 at 100k, if I remember what the fellow said).

    I was just thinking I should get the transmission flushed, when I saw your last section. This does seems to be the last leg of this old fellow! I’m hoping for the “year” option you list, or at least, get me to Phoenix!

  • wendy duerst says:

    i’m a manager of a small shop and i need some help i did a transmission service on a 2006 Hyundai Santa Fe and when it come in it was 3 qt. over full i told the customer and he told me to service it . i asked him if it had ever been done before and he told me yes 142000 miles on it and now it will not move after +60.Miles of leaving my shop. fluid is full and the rear end is locked up i don’t thank this could have caused this but i don’t know was told that the trans went out and put a strain the rear end please wright me back thank you

    • Martin Hand says:

      I highly doubt it could cause the problem your explaining. I would assume that the transmission or differential already had an existing problem. Did you remove the pan for the service (if it has a removable pan) and replace the filter? Always a good idea to do this along with a flush. This way you can inspect for excessive metal or friction material in the pan.

  • […] 30 Post(s) Thanks: 31 Thanked 10 Times in 10 Posts iTrader Score: 0 reviews FWIW….…-transmission/ 1999 Camry 3.0 319,000+ miles engine and tranny are original. Timing belt has at least […]

  • daniel says:

    i have a 2000 saturn the gentlman at pep boys suggested ihave the dip stick transmission flush and the the other way ?

    • Martin Hand says:

      I don’t believe the dip stick style transmission flush to be a good service. You just keep mixing clean fluid with old fluid and a lot of transmission fluid is wasted also.

  • don says:

    91 bmw 535i transmission flush now I have no reverse what happened what should I do

    • Martin Hand says:

      I would remove the pan and check if there is evidence of hard part damage (metal or clutch material in the pan). This will give you a better idea of whats going on. You can also perform line pressure tests to help diagnosis this type of problem.

  • rico riley says:

    Hey guys I have read your articular and I have a 93 Toyota Supra my transmission fluid looks lightly brown do you think I should change the fluid and what is the best way to change.Also I heard that people say when you change your fluid you will start having problems my transmission is shifting fine it is just the color of it thanks.

    • Martin Hand says:

      I would perform a complete transmission flush and remove the pan to change the filter (may just be a screen you can clean out). This would be the best service you could perform.

  • […] start slipping. i cant find anything right off the back but a quick google search brought me here Transmission Flush, When and when not to flush transmission | MDH MOTORS read the last section. __________________ my 06 scion tc 06 scion tc- injen CAI- exhaust- […]

    • michael says:

      I have a 2012 kia soul,it has 48000 miles.time for transmission service,dealer says it has no transmission filter so it will need to be flushed.does this sound right?

      • Martin Hand says:

        Yes, the Honda transmissions are that way also. This is why it is even more important to keep clean fluid in your transmission. There is a filter but it is not serviceable.

Leave a Reply

Your email address will not be published.

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>