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Flushing your Transmission

How the professionals flush your car's transmission

By Michael Knowling

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At a glance...

  • Step-by-step automatic transmission flushing
  • Comparing different flushing techniques
  • Benefits and pricing
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This article was first published in 2006.

Maintaining your car’s automatic transmission is probably one of the most cost-effective preventative maintenance steps you can take. For the less than AUD$200 you can have a specialist flush the trannie fluid and install a new filter to greatly reduce the potential for damage. And if you need just a little bit more motivation, you only need to find out how much it costs for a complete transmission rebuild – you might be shocked!

So what’s involved in a professional transmission flush? Let’s find out...

Flushing Process

We took our ’97 Mitsubishi Verada to one of South Australia’s longest established auto transmission specialists, Bruce Cussans Automatics. Bruce started his business in 1984 and has vast knowledge and experience in the field – he’s the perfect person to give us ‘the good oil’ on flushing.

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The first step in the transmission service/flushing process is to take the vehicle for a test drive. This is an important step to identify whether there are any faults that the owner might not be aware of. In this case, our Verada had developed a thump when shifting into third gear – more on that later.

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Next, the transmission fluid level is checked to help identify whether there are any leaks. While under the bonnet, Bruce also removed our Verada’s externally mounted transmission fluid filter. Note that many cars don’t employ a filter of this type – instead, a strainer or filter element is mounted inside the transmission and is accessible by removing of the transmission pan. Removal of our Verada’s transmission filter required prior removal of the airbox snorkel.

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Once removed, the original fluid filter is examined for metal particles. Depending on the amount of particles, this might indicate serious damage to the transmission.

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The car is now raised on a hoist to make the task of fluid drainage easier. Our Verada has three small bolts along the bottom of the transmission case which are part of the valve body circuit. These bolts were removed (allowing a small amount of fluid to drain) and a high-pressure air gun was used blow out any remaining fluid and sludge. This process helps ensure the transmission valve body circuit is clean and shifts are precise. The three bolts are now refitted.

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The majority of transmission fluid is drained by removing a drain plug on the side of our Verada’s transmission case. As seen here, Bruce uses a sheet of cardboard to guide the fluid into an oil pan positioned below. At this point, we should point out that doing this yourself at home can be an incredibly messy job!

Note that Bruce is keen to remove the transmission oil pan wherever possible. This enables him to clean the sludge that’s settled in the bottom of the transmission and allows visual inspection of the transmission internals. Unfortunately, our Verada doesn’t have a transmission pan so this was impossible.

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After a few minutes with the drain plug removed, fluid will stop pouring from the transmission and it’s a good idea to inspect what’s been caught. Transmission fluid that’s in good condition is clean and has a red transparency while old fluid has a black colour and a thicker consistency. The darker and thicker it is, the more urgent the need for fluid replacement. The fluid should also be free of metal particles and shouldn’t have a burnt smell – this is indicative of a slipping transmission. Bruce says it’s also wise to look for coolant mixed with the transmission fluid. Some vehicles are prone to transmission cooler failure which can lead to coolant entering the transmission. This contaminates the bonding materials used on the bands and clutch lining and it’s likely a rebuild will soon be required.

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At the same time, the removed drain plug is inspected. In most cases, the drain plug is magnetically charged to attract any metal particles that are suspended in the transmission fluid. Thankfully, our drain plug was free of those metal particles...

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The transmission drain plug is now cleaned and refitted, a new fluid filter is installed (making sure the mating surfaces are clean) and the transmission is filled with around four litres of flushing fluid. In this scenario, flushing fluid is nothing special – it’s merely the name given to conventional transmission fluid that’s used for the purpose of flushing. Fluid is added to the transmission with the selector in Park or Neutral.

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The old fluid still lurking in the torque converter, oil cooler and associated lines is pumped into the transmission case by starting the engine and letting it run for a few seconds. Shifting the transmission through all gears also expels the old fluid in each individual gear circuit.

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Again, the trans drain plug is removed and the fluid is drained into an oil pan. As seen here, the fluid caught in the pan is much cleaner than previously. In most instances there’s no need for another flush.

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The drain plug is once again reinstalled and the transmission is filled to the appropriate level. The technique for checking the fluid level varies from car to car – Bruce suggests checking the owner’s manual for the correct procedure. In the case of the Verada, the level is checked with the transmission in Neutral. Bruce points out that it’s important to pour the trans fluid slowly – it’s much easier to keep adding small amounts of fluid than trying to drain a small amount of excess fluid... Of course, make absolutely certain that the suitable fluid is used for your particular transmission.

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Our Verada’s airbox snorkel is now refitted, any fluid dribbles are cleaned and the car is taken for a test drive. Job done.

Trans Flushing v Trans Flushing – the Debate Continues

There are numerous ways to flush a transmission.

According to Bruce, the technique employed on our Verada typically replaces more than 90 percent of fluid. If the fluid that’s drained after initial flush still looks dirty, he will usually repeat the procedure until the drained fluid looks clean.

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On the other hand, some workshops prefer to use a dedicated transmission flushing machine that forces the old fluid out under pressure while simultaneously introducing fresh fluid. This is a relatively quick and easy exercise.

The third approach is to remove the oil cooler line that returns to the transmission and start the engine to expel the old fluid. Using this process, it’s vital to ensure the transmission pump does not run dry.

So what’s the advantage/disadvantage of each?

Well, the flushing machine approach will typically achieve the most thorough flush. The biggest downside of simply hooking up a flush machine is there’s nothing learnt about the transmission – in many instances, the magnetic drain plug and filter aren’t replaced and, if there’s a serious trans problem, this could go unnoticed. The other disadvantage is, typically, the transmission pan is not removed so there may be a layer of sludge remaining in the bottom of the transmission. There’s also no visual inspection of the inside of the transmission.

The technique involving disconnecting the trans cooler lines achieves a similarly thorough flush. But, again, this is often performed without removing the drain plug, filter and pan. There’s also the added risk of transmission pump damage if the transmission is allowed to run dry with the engine running.

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Finally, the approach outlined here is the most hands-on and gives the best indication of transmission operation. This is especially the case where the transmission pan is removed (unfortunately, impossible in our Verada). The transmission flush is generally not quite as thorough, but it depends how many times the flushing procedure is performed. Maintaining a regular transmission flush schedule will keep the fluid fresh and minimize sludge.

So what’s our suggestion?

Undoubtedly, the dedicated flushing machine will give you the most thorough transmission flush. But make certain that, as part of the procedure, the drain plug is removed, the filter is replaced and the transmission pan is dropped allowing removal of sludge and visual inspection of the transmission internals.

Third Gear Thump

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A thump into third gear is apparently quite common in Mitsubishis of the late ‘90s. While the trans flush will almost certainly improve shift quality across the board, the real problem is with the Mitsubishi transmission electronics. The industry ‘fix’ is to disconnect the battery for a few seconds, reconnect it, start the engine and cycle the transmission from Neutral to Reverse and Neutral to Drive at least five times. This process serves to reset the ECU that controls the transmission and let it relearn the sensor signals for each gear position. Shift quality will continue to improve over the next few kilometres.

Results and Price

After a transmission flush and ECU reset, our Verada no longer thumps into third gear and overall shift quality is noticeably improved - but only just. And, now that we know the trans is in good condition and has fresh fluid, it’s important that we make an effort to keep it serviced regularly.

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Bruce Cussans typically charges around AUD$130 for a basic transmission service which comprises a fluid refill and, where appropriate, a new filter and removal of the transmission pan. This price varies depending on filter price, fluid price and labour. The flushing process, which will to achieve a more thorough fluid change, adds around AUD$30 depending on fluid cost. The ECU reset on our Verada typically adds another AUD$30.

For under AUD$200 it’s a wise investment.

Contact: Bruce Cussans Automatics +61 8 8298 7688

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