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

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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.

530 Comments

  • TAYLOR B says:

    My Pontiac G6 check engine light came on, my car is shifting fine when accelerating (Sometimes it revs a little hard, but not always), when im going up hills I am losing a lot of power. Worse when the A/C is on. It stays at around a 2, when I am going up steep hills. Added 1.5 quarts of ATF. It now has no problem on flat or slightly elevated roads. Unsure of why im losing so much power going up hills and its not wanting to go up to 3 and shift..

    Thanks for your help!

  • Bigdrew says:

    i have a 2002 chevrolet tahoe and the shifts pull a bit everytime going into 1st 2nd and sometimes 3rd but drives fine and accelerates good also with everything else. i wanted to know if this is okay to leave as is or do i need a transmission flush or what would you recommend?

    • Martin Hand Martin Hand says:

      Changing the oil will probably not help at this point. If anything remove the pan and change the filter so you can inspect for signs of internal failure | metal, clutch material, etc… |

  • Barbara Horton says:

    I have a 2006 Nissan pathfinder. I pulled my 20 foot travel trailer with it and coming back home my transmission would not go into 4th gear. I had an auto mechanic check the fluid and it was a dark color and not red.
    The pathfinder has 126,000 miles on it. He said to do a transmission flush on it to see if it helps the problem. I read on one of your answers that doing a transmission flush is only for preventative maintenance only. Help me please,

  • juniva says:

    i have a 2002 mitsubishi montero sport, we have transmission issue, so we were considering on just buying a used transmission to hopefully fixing the problem but its quite hard to find one, we just got this car, so our second thought was so flush out the transmission about 1/4 out and putting some slip and stop in there. We dont wanna flush it out all the way only because there is a 50 50 chance something could go wrong. Now the real question is whats better to do?

    This is what happens to the car ill warm up the car for 20 mins when i start to drive off, if i press on the gas the first gear goes to quick when it hits about 5 to 10 mph then starts to slips and jerk, and only does it if i either go to slow or to fast but once it gets pass 2nd gear its okay, there are time where it doesnt do it at all once you drive it, only jerks if i stops but once i drive for a while nothing happens . it acts as if its okay. but heres another thing, when i barely press the gas the gear goes way to quick up then quickly drops down, but still drives fine. but yeah thank you for taking your time, please let me know if i could give you more info on the car on what else it does thank you .

  • Jan Broeks says:

    I have a Nissan Murano Z50 2006 with 80000km.
    When driving at city pace about 50km I feel some friction in switching the ‘virtual’ gears.
    Is that an indication to flush the transmission and what is the average mileage to do a flush for this type of Murano.

    Thanks in advance

  • Kishon Singh says:

    Hi,
    I have a Subaru Legacy 2008. Lately, the AT Temp light started flashing, and the transmission changes from 1st to 2nd and 3rd and then goes into limp mode, as though it’s on neutral. I ran a diagnostic and got the code P0753, “Shift A Electrical”.

    Would flushing and changing the oil possibly repair this problem? I have recently ourchased this vehicle, and I don’t know the previous owners maintenance schedule.
    Please advise?

  • Elaine says:

    Hi I hope this helps someone. I’m from London I have a beautiful Nissan Micra 2007 I had it serviced 2 months ago. Suddenly now there is a rumbling sound every time I put my foot on the break and put it in drive. The technician looked at the transmission dip stick and found silver shavings on it, hardly any and very tiny. He told me I need a new automatic gearbox but is going to do a transmission flush. Is that going to work?

  • james krzeminski says:

    Have a 2006 Subaru Outback with 73000 miles. I just checked my transmission fluid and it is real dirty and dark.
    Should I dump the drain on transmission pan and replace 4-5 quarts that come out and then a few weeks later do the same or will this new fluid cause me problems. Transmission works perfect right now.
    Confused on what is right thing to do.

  • Akash says:

    I have Lexus ES330 2006 automatic transmission. It has 67000 miles on it. I feel slight jerk sometimes when it shift from 2nd gear to 3rd gear. Shall I change the transmission oil or flush it? The transmission oil is still pink though.

    • Martin Hand Martin Hand says:

      I would remove the pan and clean/replace the filter. This way you can check for metal or clutch material in the transmission. Most likely a problem with the valve body or shift solenoid. Always check for codes even though it may not have any set yet.

  • […] are two types of machines that are usually being used. The pump inlet flush machine increases the transmission flush price because the machine operates with pumps to change the fluid […]

  • B Richards says:

    Hi there. I have a 2000 Nissan Frontier I got from my brother in law. It drives smooth, but when I first get in to drive it I have to either wait for it to “warm up” or drive it in 1st gear and then switch it over to D after driving in 1st for about 5mins or so. The transmission fluid is a light rust/orange color. Should I have the fluid replaced or will this cause more issues? Is this issue even related to the fluid?

    • Martin Hand Martin Hand says:

      Replacing the dirty transmission oil will not fix a up-shift problem the transmission has either an electrical or mechanical failure. Check the computer for codes first and remove the transmission pan to check for excessive metal +/or clutch material in pan and filter screen.

  • TEQUILA MCCALL says:

    My mechanic unknowly put the wrong transmission fluid in my cvt transmission it had a leak due to seals.What is the best way to handle this problem.

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