Transmission Fluid Exchange Good Or Bad Idea?

By Author: JD Durham
Total Articles: 42
Comment this article

Manufacturers have always had a suggested service interval for changing fluids and that includes the transmission whether it is an automatic or manual shift. When automatic transmissions were first introduced the suggested service interval was about every three years or 50,000 miles. The transmission service consisted of draining the fluid from the pan and maybe the torque converter and replacing the filter. This was a realistic time frame interval and service method as the vehicles were very open and did not build up much heat.

Around 1980 the US auto manufacturers downsized the cars for fuel economy and mounted the power train transverse into the area previously reserved for the engine only. As you can imagine this increased the heat retention in the engine but especially so for the transmission. The demands placed on the transmission fluid have increased exponentially as a result. The unexpected warranty costs from this required a switch to synthetic fluids that can handle the heat better. Even the synthetic fluids and their additive packages fail over time, as evidenced by the number of valve body failures that ...
... cease working properly from gum and varnish deposits.

General Motors tried an experiment of sealing the transmission and stating that “it was good for lifetime service”, however that failed miserably. What happened was the fluid and additive package failed to keep gum and varnish off critical components like the valve body and clutch packs. These gum and varnish deposits stick to the valves in the valve body and on the clutch assemblies. The valves move sluggishly or become stuck completely resulting in improper operation or a burned up transmission. Clutch packs slip, hang or grab, which burns them up after some very bad shifting problems.

These problems can appear in any transmission some with really low mileage. Most auto manufacturers realized that a transmission fluid exchange was needed so they engineered a transmission flush machine for their own vehicles and made it an essential piece of equipment for their dealer network (meaning dealers are required to own it). Most transmission fluid exchange machines use a bladder type system to hold the new fluid (usually about 16 quarts), the old fluid flowing from the pump to the transmission cooler is interrupted and the equipment inserted into the loop. The old fluid supplies the hydraulic force to exchange the old fluid for the new. Sometimes they supplement the pressure with a low pressure pump. The new fluid is returned to the pan, where it flows through the filter first, then into the pump, valve body, etc.

The response to auto manufacturers equipment from the aftermarket equipment manufacturers has been excellent, with tools like the bulky T Tech II and others like Wynns, BG, and Mighty made by White Industries.

Transmission Flush Service

Most auto manufacturers suggest 100,000 miles for transmission service but that is too long. It better to be safe than sorry so it is still a good idea to perform a transmission flush service every 3 years or 50,000 miles. This could save you a tremendous amount of money and headaches in the long run. If a valve body fails from excessive gum and varnish deposits the expected cost can easily reach $1000.00, a transmission requiring replacement
can be even more.

The procedure is pretty much the same from one piece of equipment to another. A cleaner such as Wynns or Mighty that can remove gum and varnish deposits WITHOUT softening seals or friction disc materials is first added to a previously warmed transmission. The vehicle is then started and the transmission is shifted to all the different gear positions and allowed to run in each position for a time determined by the product manufacturer. Some suggest a short light duty 15 minute or under drive to clean the valve body better.

Then the equipment is inserted into the pressure line from the transmission to the cooler. The vehicle is started and the old fluid along with the transmission flush additive gets completely removed during the fluid exchange IF the mechanic does his job correctly.

This transmission fluid exchange should be interrupted part way through as the old fluid shows clearing. A new filter should be installed, then finish the flush and top up the transmission with new fluid. Follow the flush chemical manufacturers suggestions and add their required additives, but do not overfill the transmission.

As you can tell this is not a job for a DIY mechanic at home. If the shop you go to for this service does not have a transmission fluid exchange machine or only wants to perform a transmission flush without changing the filter, find a different shop.

Copyright © 2011 JD Durham

JD Durham is a World Class Technician, an Automotive Hall of Fame honoree with 45 years experience in the automotive service and repair industry and a staff writer at Straight Talk Automotive You are encouraged to visit for factory service manuals, plus the new members only area is FREE to join and has great resources for members only. Factory Transmission Service Training Manuals