AGCO Automotive Repair Service - Baton Rouge, LA - Detailed Auto Topics

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Should I Have My Transmission Flushed

Automatic transmissions benefit from proper service. Unfortunately, many places push a highly profitable up-sell instead. The transmission flush does the vehicle little good and may cause harm.

Transmission flushing if very popular, because it is very profitable

With a proper transmission service, the vehicle is first test-driven. Next the transmission is inspected for leaks and external problems. Most transmissions have a removable pan and filter. On these units, the fluid is drained. The pan is removed, and the filter is replaced. Next the valve-body bolt torque is checked, and the transmission is reassembled, filled with the proper fluid and again test-driven.

Flushing a transmission is simply a matter of removing the cooler lines with the engine running. Fluid runs out and more is added. When a set amount has been added, the lines are reinstalled. No inspection of internal components are undertaken and what is more important; the filter is not addressed.

Transmission flushing is very popular because it is very profitable

A low-skilled technician can hook up the flush machine and do something else, while the transmission is being flushed. No inventory of filters or pan gaskets are necessary, and a universal fluid may be used, instead of the vehicle specific fluid.

Transmission failure is many times caused by slippage. Slippage is often caused by low pressure, such as from a restricted filter. The transmission flush does not replace the filter and may stir up debris that further restricts it. Transmission fluid causes very few problems, unless improper fluid is used.

Transmission fluid color is NOT a reliable indicator of condition

Fluid-color is often used to sell a transmission flush. This is very misleading, as fluid color does not show the fluid condition. Transmission fluid varies widely in color, even when new. Darkening of the fluid over time is perfectly normal.

Transmission fluid color is NOT an indication of a need for service

The exception would be transmission fluid that is black and burned. This shows a problem has already occurred within the transmission. With burnt fluid, it is too late for a service and repair will be needed.

Flushing will not replace 100% of transmission fluid

Proponents of transmission flushing often claim 100% of the fluid is replaced

This is untrue for several reasons. When the engine is running and the transmission is in park, only a part of the fluid is circulated. A good deal of fluid simply flows from the pump back to the pan, through the pressure relief valve. Several components in the pressure circuit do not receive flow at all.

The flush adds new fluid to a circulating volume of old fluid

New fluid is added to the old fluid as it circulates. For example, consider a transmission that holds 12 quarts of fluid. One quart of fluid runs out, as another quart is added. After the first quart, about 92% of the fluid is still old, 1/12 or eight percent has been replaced.

That quart is now mixed with the 92% old fluid and is circulated again. Another quart is added and now 8% of the 92% old fluid and 8% of the clean fluid is removed. Removing and adding another quart results in about 84% old fluid, not counting fluid that is not circulating.

Replacing 12 quarts, in this manner, will only remove part of the old fluid. Depending on the transmission and how much fluid is added, the amount of old fluid remaining may be as much as 50%. Adding new fluid to old fluid will never result in 100% replacement.

Another concern may be the waste of new fluid added. When continually diluting old fluid by adding fresh fluid, a good deal of the new fluid is also removed. This unused fluid goes into the waste along with the portion of old fluid removed. Perhaps up to 50% of the fluid flushed through the transmission may go to waste.

Transmission flushing will NOT clean the transmission

This filter is from a transmission flushed a week earlier

The transmission pump circulates fluid during the flush. This is no different from the normal pump operation. Any debris in the pan or transmission case would have to pass through the transmission filter to reach the pump. If this were possible, the filter would become even more restricted.

GM states their transmissions are NOT to be flushed

In the technical service bulletin 04-06-01-029E, GM warns dealerships against several flushing procedures. In regard to transmissions, GM has issued the following statement:

"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 or durability of the transmission. Transmission fluid should only be replaced by draining and refilling following directions in SI. Refer to Automatic/Manual Transmission Fluid and Filter Replacement."

Proper transmission service, by an expert like AGCO Automotive, can help extend the life of a transmission. Beware of the wallet flush. If someone tries to sell a transmission flush, just say NO and call AGCO. AGCO, it’s the way to go!

Post or Read Comments (1)

Sandeep - October 10, 2023
There are some assumptions here. The transmission fluid exchanges i have had, the dealership used twice the amount of fluid capacity, so that maximum amount of fluid is replaced. They dont charge much more than a drain and fill. Replacing the filter is a lot more expensive(about 650 vs 300 for a fluid exchange vs 220 for drain and fill) . Drain and fill only gets out about 25 percent for some transmissions which is not much. So i am not sure how the author can argue fluid exchange doesnt get all fluid out when drain and fill gets very little out . As far as i remember, they did change the gears so more fluid circulated. The dealers usually flush only when a transmission has been maintained, not when its neglected. So filter clogging is unlikely. I have done 3 flushes on the car and it is almost 200,000 miles. I will soon be doing a pan drop and filter change. One thing to note here is that a flush doesnt imply keeping the same filter. In best case scenario, dealer will drop pan , clean it, replace filter and fill the pan up. Then disconnect the hoses and attach the exchange machine. Now all the old fluid in torque converter will go to machine while new fluid from pan fills the converter. The machine replenishes the pan at the same rate. Then do a final level check. If you flush first. the new fluid will still mix with the grimy fluid in pan. Like a apple smoothie. There is still thicker fluid in bottom of pan. Fluid exchanges coupled with filter changes should keep the car running for a long time. There are definitely risks if the procedures are not followed but risks with drain and fill too. Many times people under fill or overfill them . Filters on newer cars are pretty sturdy and you dont have to change them on every flush if you flush every 50k miles. I change them at 100k miles. Most of the car manufacturers recommend drain and fill and no filter change. I did an oil analysis with blackstone and my results were exceptional compared to universal averages. The report mentioned that the wear rate on my transmission was less than half that of other cars. The comparison was not between a flush vs drain and fill. Rather it was flush vs (drain and fill + Flush averages). Which means the universal average score is imporoved (since they include flushes too) making my wear rate even better compared to a drain and fill. Keep in mind many car makers say- drain and fill after 100,000 miles. But then one asks the question. If its drain and fill after 100,000 miles then when is the nex drain and fill? The fluid will deteriotate much faster after 100,000 miles because even though we replenish it at 100,000 miles, it was not brand new like at 0 miles. So dont wait till 200,000 miles to drain and fill. It will probably need drain and fill every 30,000 miles . The question arises, why wait till 100,000? At that point you are running a suboptimal fluid. Lets assume the fluid is at 40 percent effective at 100,000. You drain and fill and its at 60 percent efficiency. Then at 130,000 it is back to 40 percent. So essentally you are operating between 60 to 40 percent. Now assume your car is at 30,000. The oil likely still is 80 percent effective. Now you drain fill and its at 90 percent effective. By the next drain and fill at 60,000 it is at 80 percent efficiency. So essentially you run the transmission at 80 to 90 percent effiency.

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