What You Should Know About Transmission Flushing

Although automatic transmissions have come a long way in terms of reliability and sophistication during the past few decades, there is as yet no such thing as a perfect transmission, and there will likely never be. For all their sophistication, modern transmissions still fail for much the same reasons they failed thirty years ago; manufacturing defects, defective or malfunctioning  electronics, and very often, because of lubrication related issues caused by substandard, or even unsuitable  transmission fluids.

On the other hand though, many transmission problems such as harsh/erratic shifting, hesitation, or a failure to engage any gear can often be fixed with a simple fluid replacement. However, other serious issues that involve grinding, whining, or rumbling noises are usually indicative of severe mechanical damage, and in these cases, a fluid replacement won’t cure the problem.

How do you know if a problem can be cured by a transmission flush, but more to the point- what is a transmission flush, and how can you tell if your transmission needs one? You may well ask, since transmission flushes can cause more problems than they cure, so read on, and we will explain the process.

Transmission flushes explained

Before the advent of transmission flushing equipment, the only way to replace transmission fluid required removal of the oil pan. This allowed the fluid to drain, but it did not remove all of the old fluid- the fluid in the pump, valve body, torque converter, and fluid cooling system remained behind.

Flushing equipment on the other hand, replaces all the fluid. In simple terms, the flushing machine is connected to the cooling lines, the engine is started, and the transmission’s own pump is used to expel the old fluid into the machine, while the machine delivers new fluid to the transmission at the same time. This process is continued until the entire fluid volume has been replaced. In practice, the old fluid is flushed out, but some of the new fluid is expelled as well, which means that it can take several times the required volume of fluid until all the old fluid is replaced.

Why flush the transmission at all?

Like engine oil, transmission fluid consists of a base oil and several types of additives to reduce friction, prevent corrosion and foaming, but also to prevent the formation of sludge and other harmful deposits. However, heat and contact with oxygen cause these additives to break down and degrade over time, which is why the fluid needs to be replaced at regular intervals.

Dirty and degraded fluid can cause all manner of problems, such as harsh or erratic shifting, hesitation, or even a refusal to shift at all. Severely contaminated fluid can also cause accelerated mechanical wear of the hundreds of moving parts in a transmission, but more commonly, dirty fluid causes small moving parts in the system that controls shifting to stick.

This system is commonly known as the valve body, and it works by hydraulic pressure that opens and closes various circuits that in their turn, control the configuration of the planetary gears that gives a transmission different ratios, or speeds.

Thus, when dirty oil prevents the correct functioning of the valve body, the transmission may refuse to shift, or if it does, it may do so erratically or harshly, and this is where transmission flushing comes into the picture.

The problem with transmission flushing

Transmission flushing is not recommended, but in cases where problems with shifting are not yet serious, it might be beneficial, although there are no guarantees that the flushing process will not cause additional problems. However, in many cases, some mechanics insist that the transmission be flushed with a solvent, which could be very bad indeed, and here is why.

Not all shifting issues are caused by dirty fluid- defective electronics and control modules cause a significant percentage of shifting issues, and a transmission flush will not correct this type of problem. All transmission problems must be diagnosed with suitable diagnostic equipment before a flush is considered, but back to solvents.

Almost all transmissions that are a few years old have deposits of sludge in them, but not only that- almost all transmissions also have areas where various waxes, varnishes, and gums have formed. While these deposits are usually not a problem when left undisturbed, they will almost certainly become a problem when they are attacked by powerful solvents.

No mechanic can ever be sure that all deposits have been fully dissolved and expelled, which means that some deposits may only have been dislodged, and are now floating around in the transmission, ready to plug small diameter oil passages, or more likely, to cause valve shuttles in the valve body to stick.

Another problem involves the transmission fluid filter. There is no point to flushing the transmission unless the transmission fluid filter is replaced as well. Besides, removing the oil pan to replace the filter is an excellent opportunity to inspect the filter for signs of metal wear particles. The size, and amount of wear particles allow  a competent mechanic to infer the overall condition of the transmission, which may be so worn that no amount of transmission flushing will ever get it working properly again.

The possible effects of transmission flushing explained

Nonetheless, assuming that the filter has been replaced,  the transmission is judged to be serviceable, the flushing equipment is hooked up correctly, and the solvents are “cleaning” the transmission, you are left hoping that everything turns out well, which it very often does not.

We have already mentioned the possibility of dislodged gunk plugging passages or causing valves to stick, but there are other possibilities. You may not believe this, but older transmissions sometimes only work because they are dirty. As moving parts wear, the clearances are often filled by varnishes and gums, which have the effect of preventing internal leaks. Thus, dissolving these gums and varnishes may actually cause a transmission to stop working altogether, since some internal clearances are now so big that the required hydraulic pressure cannot be achieved, or maintained.

In addition to this, some older transmissions are only quiet because the clearances in bearings and bushings have been filed by gums and varnishes. So if these deposits are removed, the bearings will run on significantly bigger clearances, which can cause whining, rumbling, or even knocking noises.

Finally, there is a more than even chance that you will receive fluid that is contaminated with fluid that may not be suitable for your transmission. Many car manufacturers specify specific fluid formulations for their transmissions for any number of very good reasons. Thus, if the flushing equipment is not emptied, and properly cleaned out before each flushing, there is a distinct possibility that the fluid you receive may not be compatible with your transmission. For instance, Ford has always required a specific formulation for their transmissions, while some German manufacturers, such as BMW, insist on a formulation that can only be obtained from the dealers.

What are the chances that the corner lube shop will send to the nearest dealer for the correct transmission fluid for your BMW or Ford? None, nada, zilch, so be aware of the fact that you may be receiving completely unsuitable fluid, contaminated fluid, or even a mixture of incompatible fluids. There is simply no single transmission fluid formulation that works for all applications, so avoid having your transmission flushed unless the lube shop can guarantee that you will receive the correct fluid, and that it won’t be contaminated. Good luck with that, but what other options do you have?

Stick to the maintenance schedule!

The best way to avoid all of the above is still to have the transmission fluid in your car replaced strictly in accordance with the prescribed maintenance schedule. However, if you do a lot of towing, the transmission works at substantially higher temperatures, which hastens the degradation process of the fluid. In these cases, it is a good idea to replace the fluid more regularly, like once a year, regardless of the distance travelled.

While oxidation of transmission fluid plays a major role in how long the fluid lasts, the single biggest factor that affects fluid life is excessive heat. Engine overheating has a direct bearing on the life of transmission fluid, so if you experience an episode of engine overheating be sure to replace the transmission fluid, since serious engine overheating can cause the transmission fluid to form a type of varnish that can literally “weld” together moving parts. There is no known cure for this, and a complete transmission replacement is the only thing that will get you back on the road.

So, to flush, or not to flush

We do not presume to tell you what to do with your car, but based on practical experience over many years, this writer (who owns and operates a repair shop), does not recommend flushing any transmission- ever. In fact, Honda Motors actively prohibits the flushing of their transmissions, and almost all other manufacturers only allow it under certain conditions, and then only with equipment that they specify.

The claimed advantages of transmission flushing should always be weighed against the very real risks of ending up with a transmission that fails within days (or sooner) of having been flushed. From a cost perspective, it is always better to have the fluid replaced regularly, and in the traditional way, than running the risk of having to replace a transmission at a price that may be higher than what your car is worth. Then again, it is your car, and you must decide what is best for it, but if it were our car, we would not have the transmission flushed.

Transmission Flushing

This article is provided by the staff writers of – the largest Auto Body Shop Directory in the US with over 200.000 verified listings. At our Auto Body Shop Blog you find more interesting articles about car maintenance and road safety.