Monday, July 22, 2024 Detailed Auto Topics
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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!

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