Rick's Free Auto Repair Advice

Transmission Flush

Should I get a transmission flush?

All the quick oil change places push transmission flush services. But are they a good idea?


GM recently issued a technical service bulletin to their dealers regarding flushing services. I’ve posted it here. But if you want the short version, GM does NOT recommend engine flushing, nor does it recommend TRANSMISSION flushing. Ford, Chrysler, Honda, and most other car makers has issued the same warnings.

Here it is:

TSB #04-06-01-029G
September 27, 2012

“..many of your service departments merchandise supplemental services and fluids to customers indicating that these “flushes” have a positive effect on durability of the component part. It should be noted that analyses of returned parts that have been exposed to these aftermarket “flushes” show indications of potential damaged and shortened durability.

General Motors does not endorse or recommend engine crankcase flushing for any of its gasoline engines.

Flushing of air conditioning lines, radiators, transmission coolers, and power steering systems are recognized practices to be performed after catastrophic failures or extreme corrosion when encountered in radiators.

The use of external transmission fluid exchange or flush machines is not recommended for the automatic or manual transmission. Use of external machines to replace the fluid may affect the operation and durability of the transmission. Transmission fluid should only be replaced by draining and refilling following procedures in Service Information.”

Every time you take your vehicle in for service, the oil change place will recommend a transmission flush. Don’t agree to that service until you’ve read this article and consulted your owner’s manual. Many manufacturers warn against doing a transmission flush. Even if you decide to do it, you must make sure it’s down with the correct fluid.

Clean transmission fluid is vital to your transmission’s performance and, well, its life. Transmission fluid lubricates metal parts, carries heat out of the transmission, and contains friction additives to make sure your shifts are smooth and on-time. Just as important as the friction additives, transmission fluid also contains detergent additives that prevent deposits from forming on critical metal parts and carries.

But transmission fluid has a limited life. Over time, the fluid begins to break down and lose its ability to lubricate. When that happens, metal parts heat up and the heat scorches the fluid—just like scorched butter in a frying pan. That scorched fluid is referred to as varnish because of its brownish appearance.

For decades, the proper way to change transmission fluid was to remove the pan, drain the fluid, and replace the filter. This procedure only removed about one-third to one-half of the total fluid in the transmission. The new fluid would mix with the old and the detergents in the new fluid would dissolve any varnish deposits in the transmission. Those deposits would be trapped by the transmission filter.

Today, dozens of equipment companies make transmission flush machines that force fresh fluid into one of the transmission cooler lines and flow the old fluid out the return line. The equipment companies refer to this procedure a “fluid exchange.” That term is correct in theory. In practice, however, it’s a bit misleading.

To understand why, picture a swimming pool filled with dark blue water. Then imagine what would happen if you took a garden hose and began filling the pool with clear water. The clear water would start to displace and dilute the dark blue water. But it would take a LOT of clear water to replace all the blue water.

Get the picture? When these fluid flushing machines inject fresh fluid, it isn’t like a rigid wall of fluid pushes all the old fluid out. Instead, it dilutes the old fluid. The average automatic transmission holds 10-13 quarts of fluid.

Reputable shops that use the fluid flush machines know that it takes at least 16 quarts of fluid to get close to that 100% target. So, when you get the quote for a transmission fluid flush, ask how much fluid they plan to use.

Transmission fluids are not all the same

Each car manufacturer has different specifications for their transmission fluid. Use the wrong fluid in your transmission and you can cause harsh shifting, late shifting, or the early demise of your transmission. In fact, GM, Chrysler, Ford, Toyota, and many other car makers have recently introduced synthetic transmission fluids that outperform the older fluids by a long shot.

Unfortunately, these new synthetic fluids are fairly expensive—usually double-to-triple the cost of the older fluids. That’s an important factor if your shop is quoting you on a fluid flush. Why? Because many fluid flush machines use an inexpensive generic Dexron fluid as the flush fluid. Once the flush is completed, the shop adds a bottle of “fluid converter” to modify the friction characteristics of the fluid to match the car manufacturer’s recommendations.

Not a single car manufacturer recommends this procedure! In fact, most car makers now have issued service bulletins warning AGAINST this procedure. The bulletins warn that only certain flush machines can be used and only with the proper fluids. That’s right, they want you to use the real thing. If drive a late model Ford, and your manual calls for Mercon V, you had better put Mercon V back in it or you could void your warranty! Try telling the dealer that the “fluid converter” magically turned a generic fluid into Mercon V. It won’t work. The same holds true for GM’s new Dexron VI and Chrysler’s new ATF+4 fluids.

Change the transmission filter

Car manufacturers install a transmission filter for a reason—to trap any particulate matter and prevent it from clogging transmission valves. Most shops that offer a transmission fluid flush service do not change the filter. Here’s what the Filter Manufacturers Council has to say about that practice:

“There have been an increasing number of instances surfacing recently regarding transmission failures shortly after an evacuation service, without filter removal. At the time of a fluid evacuation service, there is no way to know the condition of the filter and how clogged it may be. The filters job is to collect and hold contaminants, (dirt, metal filings, friction particles, etc.), and prevent these particles from causing malfunction in such components as electronic force motors and solenoids.

Today’s transmissions are far more susceptible to malfunctions caused by fine dirt contamination. Without servicing the filter, there is no way to know if the filter is clean of debris or nearing capacity. If the filter is nearing capacity, transmission failure may not be far off. This is also a sign that there may be other internal problems in the transmission. Recognizing these warning signs could eliminate major service later.

Most of the transmission failures after an evacuation service have occurred primarily on relatively high mileage transmissions that have not been serviced in some time. One reason for this is that the sludge and dirt buildup within the transmission will not completely be removed during the service. When the new fluid (which has detergent properties) is placed in the transmission, over days and weeks, the internal components begin to wash the insides of the transmission.

This sludge does finally work loose and settles in the transmission filter, clogging it up even further than it may have been before service. In these extreme cases, where service has not been performed in some time, changing the filter may not completely fix the problem. Some mechanics recommend a second service a few weeks after the first, replacing the filter again, which may be partially clogged due to the cleaning process in the transmission.

Even if the fluid evacuation method is desired to remove the used transmission fluid, the pan should be removed also, and an inspection should be made of the pan contents, fluid, and filter to determine the condition of the transmission. Aluminum filings in the pan or iron filings on the pan magnet are signs of internal wear and may give light to potential problems in the transmission.

Transmission service is performed for preventative maintenance. Evaluating the overall condition of the transmission by removing the pan should be part of this preventative maintenance also.”

Source: Filter Manufacturers Council
Technical Service Bulletin 98-2
June 1998

Here’s what you need to know when it’s time to change fluid

If your shop offers a transmission fluid flush service, insist on these procedures:

1) The machine they used MUST be approved by the car maker. Some machines inject fluid as such a high pressure that it causes premature failure of the torque converter seal. That’s a MAJOR expense that the shop probably won’t pay for.

2) The flush must be done with the manufacturer’s recommended fluid—not a generic fluid and a magic “converter.”

2) Insist that the shop use 50% more flush fluid than the maximum capacity of your transmission. That will get you close to the 100% new fluid mark.

3) Insist that they drop the pan, examine any deposits, and change the filter.

Opt for the traditional approach of just dropping the pan, draining the fluid, and changing the filter. Just do this procedure more often than the manufacturer recommends since it doesn’t remove all the old fluid.

As with Engine Flushes, many shops now ask you to sign a waiver before they do a transmission flush. The waiver says that they are not responsible for any transmission repairs that may occur after the flush. NEVER sign that waiver!

Remember, car makers don’t want you to do a transmission flush, unless it’s with an approved machine and the approved fluids.


© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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