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.

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. 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.


  • Kevin says:

    great article! i have a f150 that has been making noises like rattling and a deeper whine at low speeds, i think related to transmission, i slipped out of gears one day on the highway and seems the trans is shifting slowly or having some trouble. turning my overdrive off helps a little with gears but still struggles especially uphill. some people have written they had this problem and a flush helped what are your tbougbts?

    • Martin Hand Martin Hand says:

      I do not recommend a flush to improve any shifting problems. Transmission service or flush is for preventative maintenance only. You can always give it a try shouldn’t hurt. Also recommend to remove pan and change filter.

  • Jim says:


    I have a 2001 Honda CR-V with over 201,000 miles in her. I’ve been experiencing a bit of gear issues, as them sometimes taking a bit to change over to the next gear. Mostly when going uphill. I added a few bottles of ATF and it helped a little. I was told that a trannie flush would possibly kill it. Should i just replace the fluid without the flush? Planning on a small road trip and would hate to be stranded… Thanks!

    • Martin Hand Martin Hand says:

      Transmission flush or service should not kill the transmission that is very rare to happen. I wouldn’t expect it to improve the problem either. Good luck!

  • John says:

    I have a 2003 chevy blazer. I’ve been having trouble with the way it’s been shifting. Hard shifting, erratic shifting and it slipped out of gear the other night. I’m almost certain that it could be a bad solenoid. I won’t know for sure until I pull codes. Assuming this is the issue, I know the process isn’t extremely difficult. My main worry is that by doing this, I’ll kill the transmission. I’m sure it’s had a change in its lifetime but I don’t know when. I’ve had it 3 years and have never changed it. It’s got almost 170,000 miles on it.

    • Martin Hand Martin Hand says:

      You cannot hurt it by replacing a bad solenoid but driving the vehicle with a bad solenoid can cause other problems. Pull the codes, change the failing solenoid and re-check it. The worse you can do is waste time and money on the solenoid. Make sure to check the electrical connectors on pulse width modulated solenoids as they get melted from electrical heat.

  • Tom Trimmer says:

    I recently bought a 2002 Yukon. It shifts smoothly 1 -2 -3 and then 3rd to 4th, it just revs as high as you’ll take it without going into 4th. If I let off the gas and RPMs down to 2k or so, she shifts smoothly into 4th. Curious – should I flush and filter change? Any hope to fix it that way or think it’s something major internal? Or maybe a bad solenoid… Thanks!

    • Martin Hand Martin Hand says:

      A flush won’t fix anything that for preventative maintenance only. The problem is probably in the valve body, sensor or solenoid. The 4L60E has a manifold pressure switch that can cause shifting performance issues. Need to check for transmission codes or even history codes if not consistent problem.

  • Hec says:

    I have a 2001 lexus rx 300 awd . It only has 19,000 miles on it. The trans fluid looks dark red. What can you recommend? Drain and fill or should i have it flushed?

  • Eddie Ritchey says:

    hello, I have a 93 mercury villager with 204k miles bought it at 186k miles. The transmission has started slipping from 2nd to 3rd.. it’s not to bad yet… I checked the fluid it’s a little brown slightly burnt smell.. I’m not sure I should do a flush. Should I just do a filter change and do you think this will correct the slipping issue?

    • Martin Hand Martin Hand says:

      Too late to service it at this point there is mechanical failure causing the slipping, most likely valve body control issues but the clutches will be burned from slipping.

  • Yvette says:

    I have a 98 GU patrol 3 speed with over drive automatic, The fluid is starting to turn to a dark red color, It has 260,00k’s, Mechanics reccomend a flush and i should be good again, It has no troubles shifting or changing gear on it’s own.

    I should be ok right?

  • Sunny babbar says:

    hi, i have a 2008 honda accord with 140k miles on it. I dont think the transmission was ever flushed. the oil is black and smell burnt. Should i do a flush or let it be the way it is?


  • Justin says:

    Hi have a 88 dodge w150 with an auto trans. And it seems to shift pretty harshly make a chunking noise and the whole truck kind of bounces and lurches forward or back when I shift into reverse or drive but not when shifting between gears is this just how older autos engagement is? Also the fluid has a decent amount of black particles but no metallic flskes should I flush and fill or just change the fluid .

  • Hey i have a 92 ford f150 that was sitting up for years… Only has 80,000 original miles on it and it feels as if the transmission is pulling. Do i need to flush the transmission and put a new filter in it??? Need help guys

  • Juan says:

    Had a 2001 suburban with over 200,000 miles first time customer had a transmission service(flush) few hours after customer picking up his car transmission wouldn’t shift pass 2nd gear or into reverse.. What do you think happend ?

  • Vern Lousignont says:

    I have a 1089 Dodge 3500 ram van 15 passenger 360 V8, 3 speed automatic trans 104,000 miles. Tranny works ok after its driven a few miles and is warmed up. When first starting the engine and putting in 1st or reverse you almost have to put the gas pedal to the floor to get the van to move. The fluid looks fine. I have no idea if the tranny has ever been serviced, i bought it with 98k miles and it worked ok then. would the flush and filter change help with this problem?

  • Stoil says:

    Hi, a while ago I bought a used car with 100k mines on the clock. It has Aisin Warner TS-80SC automatic transmission. The only complains I have is a bit harsh downshifting (3->2; 2->1) and upshifting 5->6 only. These symptoms are fading a bit when the fluid gets warm. I’ve replaced the fluid using cooler line flush machine shortly after I got the car and the mechanic recommended to do a flush with transmission cleaning additive and then add a transmission additive to the new transmission fluid, but I refused since I was afraid that could get the things worse. Ever since I’ve replaced the fluid – about 10k miles now – this issue is still present but not deteriorating. Besides I don’t observe any weird noises or other shifting problems.

    Some guys from the auto club experiencing similar symptoms (same car, same transmission) replaced the valve body with a brand new one, which fixed the problem permanently. Some of them already have 20-30k miles reporting no issues at all.

    Would you recommend flushing the transmission with solvent/cleaning additive before replacing the valve body, or just fluid change would be fine? What would be the best approach in that case?
    Do you have experience with Aisin Warner TS-80SC in particular? Any other recommendations or tips?

    As far as I know, the transmission has to be taken apart to replace the oil filter, by removing the pan you can replace the valve body only.

    P.S. Dealer claims the transmission fluid is lifetime.

    • Stoil says:

      TF-80SC is the correct one. Sorry for the typo.

    • Martin Hand Martin Hand says:

      Yes, the valve body’s are very problematic. Sound like high line pressure which is controlled by the valve body and the TCM via a pressure control solenoid. Replacing the valve body would probably correct the problem but need to make sure there is no sensor failure first (check for TCM codes).

  • Joshua R says:

    I have a 2010 Subaru Legacy. It has about 65,000 miles on it, so I am about to take it in for a 60,000 mile service. The dealership list of services ( says they will do things like inspect and Top Off Differential (front & rear)
    Inspect and Top Off Transmission Fluid. It seems like these fluids should be changed, not topped off. Am I missing something?

  • NicVanFleet says:

    I am looking to buy a 1972 GMC 2500 truck with an automatic transmission, and as you’ve probably guessed, its been sitting for many years. Before I try to put it in gear and drive off, What should I do to make sure I’m not causing damage to the transmission that could have been prevented beforehand?
    Thanks. Nic

  • Chester Borland says:

    I have a 88 bronco 2 with a mazda 5 speed in it. I put the transmission in it about 5 years ago & now I’m not sure which fluid I put in it. Any suggestions?

  • says:

    To use or not use a cleaner before drain and flush?

  • Mike says:

    Hello, I have a 2011 Kia Sorento. I recently had a transmission flush down at 85k not knowing it should have been done more. When we got home the last few times I noticed there is a large wet spot under the driver side whenever the car is parked and off and the front of the car has almost a burnt smell… Is that normal? There appears to be no problem when my wife drives it around town.


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