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 a cleaning solvent is forced back into the transmission, removing deposits of old transmission fluid from parts and components, or the technician performing the flush may run a solvent through the vehicle’s 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?

Is Transmission Flush Good or Bad?

I constantly hear consumers, customers, and technicians debate whether transmission flush procedures can harm a vehicle and whether a transmission flush is 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 after over thirteen years of being in the automotive repair industry, I am still waiting to see a machine that flushes the fluid in reverse. Most machines just pump the new fluid in through the transmission 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 will help prevent contaminants from 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

  1. In the past, every vehicle had a second dipstick, other than 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 is 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, and I will be happy to give you some factory procedures.
  2. Many new cars do not have dipsticks. In 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 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. If 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 filler tubes 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.
  3. Most cars also require that the engine and transmission be warmed up in order to get the most accurate reading. This is because 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 between the two is found on the dipstick. If the level is below the lower mark, then some fluid must be added, but 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 definitely burnt smell, and fluid from a transmission that has completely failed smells downright disgusting.

Is a Transmission Flush Necessary?

  1. 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, not all manufacturers indeed recommend this service at frequent intervals. It is not uncommon for flushes to be performed only every 100,000 miles. There are two main types of transmission flush machines, and I will explain them below.
  2. Pump inlet flush machine:
  3. The first type of transmission flush machine, I will explain, attaches to the pump intake after the pan and filter are 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 are GONE and replaced with fresh new fluid. After the service, a new filter is installed, the pan is 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 of 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.
  4. Cooler line flush machine:
  5. The second type of flush machine connects to 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 machine requires 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 causes no pressure, and all fluid transfer is done by the transmission’s own pump.
  6. 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, operates the pistons and controls gear shifting. The other 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 into 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 draining and filling.
  7. 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 be sucked up into the filter, which 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

  1. Before draining or flushing, you should pull the dipstick and look at the fluid. If it is dark and burnt smelling, and you see little flakes or specks in it, DO NOT FLUSH IT. The fluid and transmission possibly have hard part damage, but the transmission just has not figured out if 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.
  2. 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 call 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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