When to Change Your Car’s Transmission Fluid?

You may have heard or read somewhere that you only need to replace your transmission fluid every 100 000 miles or so, but the fact is that by doing so, you could damage your transmission simply because you do not remove contaminants regularly enough. Your transmission depends on clean, filtered, and uncontaminated fluid to work properly. Something else you may not know is that most automatic transmission failures occur because dirty, degraded, and contaminated transmission fluid prevents the smooth operation of a myriad small, but critically important moving parts.

Automatic transmissions briefly explained

Without getting too technical, it is fair to say that instead of using a lot of different gears that need to be locked together on a manual transmission to achieve a power output, an automatic transmission only uses a few gears that can be locked together in different ways to achieve a similar power output.

Nonetheless, the way in which an automatic transmission uses the same gears to arrive at different speeds is controlled by a hugely complicated hydraulic system that is in turn controlled by a complicated electronic control module. The whole system is supplied with pressurized fluid by a powerful pump that also serves to circulate the fluid around the transmission for lubrication purposes, and this is where the quality, as opposed to the quantity, of the fluid in the transmission plays a critical role to prevent problems with harsh or erratic shifting.

While it is true that the actual gear set can function with relatively dirty fluid, the same cannot be said for the hydraulic control system. This system is mostly located in a part of the transmission called the “valve body”, and it consists of an intricate system of small-diameter valves, check valves, and pressure regulators that are all interconnected via small-diameter passages that supply each part with pressurised fluid.
One more critical component of an automatic transmission to consider is the set of clutches that locks different parts of the gear set together in different ways to achieve gear ratios. In most transmissions, the clutches are controlled by hydraulic actuators that are in turn controlled by an electronic control module, and the quality of the transmission fluid plays a crucial role in how well, or reliably these clutches perform to prevent slipping, or hesitation when taking off.

How does it all work?

In full automatic mode, a transmission will not shift up or down unless at least half a dozen sensors and control circuits agree that a shift is required, which is when a signal, or impulse is generated by the transmission/engine management system(s) that tells the transmission what to do.

When the transmission receives the impulse, one or more valves in the valve body is instructed to divert pressure to a different part of the assembly. This causes the clutches to re-arrange the way the gear set is interlocked, and the transmission is said to have executed a gear shift. In this manner, the transmission will shift gears in accordance with instructions generated by the control system(s) without any input required by the driver.

However, when one or more valve shuttles sticks in position because of the presence of contaminants in the fluid the transmission cannot execute the shift, but if it does manage to shift, the operation may cause harsh, or jerky shifts- or it may “miss” a shift, and shift to the next higher or lower gear.

Similarly, if the transmission selector is moved from “P” or “N”, the valve body may not be able to divert pressure from the by-pass circuit to an active circuit, which means that the transmission is unable to engage the desired gear because the affected valve shuttle(s) cannot move due to the presence of dirt, or other contaminants in the fluid.

So what is automatic transmission fluid?  

Apart from its excellent flow properties, transmission fluid is similar to other lubricating oils in the sense that it also contains additives that clean, lubricate, and prevent the formation of corrosion inside the transmission.

However, transmission fluids are not created equal, and some formulations work better in specific applications, which is why it is critically important never to use a generic formulation. Moreover, some transmissions, such as those on pick-up trucks require a specific formulation because it may run at higher temperatures than say, a transmission in a normal family sedan, which is why you should only ever use the formulation recommended, or specified by your manufacturer.

Moreover, the ability of a specific formulation to withstand the effects of heat is just as important as that formulation’s ability to resist oxidation, which is aggravated by higher than normal operating temperatures. Oxidation is the single biggest factor in the degradation of automatic transmission fluid, and when that is coupled to the effect of heat, some of the additives start to form various gums, varnishes, and sludge that can literally “weld” moving parts together. Once this has happened, the transmission must be replaced since there is no known way to free the seized parts without ruining the transmission.

Below is a table that illustrates the life expectancy of transmission when the fluid exceeds acceptable operating temperatures. Note how small differences in temperature bring about a drastic reduction in the life expectancy of a transmission.

Transmission Fluid Temperature Chart

From the above it will be seen that overheating of the fluid must be avoided at all costs, and since most cars use the same radiator to cool the transmission fluid that cools the engine, any engine overheating will affect the life expectancy of the transmission as well, which is why the transmission fluid MUST be replaced if the engine has overheated.

One more thing to remember is that transmission fluid is translucent and bright red in color when it is fresh.  Fluid that is not translucent, dark in color, or has a “burnt” smell, is degraded to the point where it could start to affect the performance of the transmission, and it must be replaced, regardless of the distance traveled.

Many car manufacturers recommend fluid replacement at 30 000 miles or so, but factors such as towing, carrying heavy loads, or engine overheating can dramatically reduce this mileage, so whenever this has occurred, have the fluid checked and replaced if necessary.

Some symptoms of degraded transmission fluid

It is not always easy to diagnose automatic transmission problems because many electronic issues could mimic the symptoms of dirty or degraded fluid. However, whenever your car develops transmission related problems, always check the fluid level and condition first, since in some cases, the problem can be resolved merely by replacing the fluid.

Bear in mind though that a fluid replacement may be too little too late in many cases, especially where the transmission has become noisy at any speed, or in any gear. Nevertheless, below are listed some of the most common issues that could be directly related to dirty, contaminated, or degraded transmission fluid.

Noisy transmission:

Dirty fluid can mimic the symptoms of low fluid levels that can cause some transmissions to be noisy.  So whenever your transmission becomes noisy in any way, first check the fluid level to confirm that the transmission is not running dry. If the fluid is at the correct level, but appears dark, “tarry”, or has a “burnt” smell, the cause of the noise is likely to be related to an inadequate flow rate. If however, the fluid is at the correct level and appears clean, fresh, and translucent, the cause is more likely to be a purely mechanical malfunction.

Harsh, or erratic shifting:

In almost all cases, this is caused either by a low fluid level, or dirty/contaminated fluid. In some cases, the issue can be resolved by replacing the fluid, but do not bank on this- in some cases, the harsh or erratic shifting may damage internal parts that can only be repaired or resolved by professional servicing and repairs.

Slipping in gear, or hesitation on take-off:

This is almost always caused by insufficient fluid pressure, which in turn, can have many possible causes. However, if the fluid appears dark, “tarry”, or has a “burnt” smell but is at the correct level, it would be safe to assume that one or more internal fluid passages is partially, or even completely blocked by dirt, varnish, or sludge. In these cases, a fluid replacement is seldom the answer, and the issue must be seen to by professional personnel.

Unexplained surging:

In transmissions that have not had their fluid replaced at recommended intervals, it is possible for dirt and sludge to cause intermittent blockages. However, a simple fluid replacement may, or may not resolve the issue, and in these cases, it is recommended that the transmission be flushed by suitable equipment, and by personnel that are trained to operate such equipment.
Incorrect flushing procedures can cause permanent blockages that cannot be cleared by any means, but it is also important to remember that some manufacturers, and Honda in particular, strongly advise against any form of flushing on their transmissions, so before you decide to have your transmission flushed, contact the manufacturer or dealer for information.

There are off course other causes of transmission problems that have nothing whatever to do with the condition of the fluid. Defective, or flat batteries, malfunctioning electronics, or extreme temperatures can cause most, if not all of the problems associated with degraded transmission fluid, so before you replace the transmission just because a mechanic says you must, have the electrical system of your car checked out with suitable diagnostic equipment.

It could save you a ton of money!

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