To flush an AT or not

I have a 1996 Volvo 850 wagon with 181k miles. I would like to change the automatic transmission fluid but have received conflicting opinions as to whether to FLUSH the AT as part of the process. Volvo in Maine says drain and fill, Click and Clack say flush. What to do?

Drain, take the pan off and replace the filter, refill. Unfortunately, this doesn’t change out 100% of the fluid. If you really want to do this well, flush it after you put in a new filter to change out all the fluid. Or drive it a little, then drain and refill again.

Drain replace filter and refill. Make sure you are using the right fluid. The small amount of fluid left will not be a problem.

A drain, filter change, and refill every 30K miles (using the fluid specified for your vehicle) should be fine.

I personally avoid flushes. Some members of this forum have no problem with them.

If the fluid hasn’t been touch in 181K miles, then definitely don’t flush it - as flushing will stir up the sediment, allowing it to get caught up in places where it will do harm.

This method of tranny fluid change only removes about 30% of the total volume of tranny fluid held in the tranny. That 70% that’s still in the tranny is held in the torque converter, valve body, and the tranny fluid cooler/lines.



When has that remaining 70% caused any problems? Drain/filter-change/refill has the procedure recommended by car manufacturers and used successfully for years.

Flushing and exchange machines are promoted heavily in the automotive trade rags as a way to increase profits. What technical problem are those machines solving that regular drain/filter-change/refill didn’t solve?

Are transmissions that get flushed/fluid-exchanged lasting any longer than the ones that get a drain/filter-change/refill every 30K?

(A different Joe).

Do not flush do not flush do not flush!!!

I’m not a professional mechanic by any stretch of the imagination, but I firmly believe that forcing fluid through a transmission is a terrible idea.

IF your “flush” involves disconnecting a fluid line and allowing the transmission to pull up new fluid from a container, then I have no problem with that technique - and that is the only way to guarantee a 100% fluid change.

Personally, though, I prefer to just pull as much fluid up the dipstick as I can every fall, and refill. Then every 3 years I drop the pan after pulling the fluid up (its cleaner this way) and clean out the pan and replace the filter.

I’ve NEVER had a problem doing it that way, and I’ve had some cars that didn’t have the best transmissions in the world.

When you do that sort of a change, your fluid will never be 100% new, but it won’t average very old, either, depending on how much fluid you get out the dipstick. On my Taurus, I manage to get about 60% of the fluid up the dipstick each time. If you can get that much out, then your fluid, on average, is not very old.

For me, that means that on average (by volume), my fluid is 0.667 years old right after a fluid change, 1.667 years old right before one, and 1.167 years old on average during the course of the year.

So while my fluid isn’t perfect after each fluid change, it is in spectacular condition compared to 90+% of all vehicles on the road. I’ve never had any fluid turn brown or burn. And long term, just 1% of my fluid is 5 years old or older. Only 6.4% is 3 years old or older just after a change…

Times have changed.

The auto industry now recommends that a fluid exchange be performed as normal maintenance on their transmissions. Why?

For one, the transmissions aren’t built to handle the heat like they used to be. In the past the transmission cases were made from cast iron/steel. The transmissions of today are smaller and casted from an aluminum/magnesium alloy with lighter components inside. So today the transmission fluid plays a bigger role in preventing damage/wear to transmission components.

In the past, transmission fluid comprised of whale oil. But this was banned in 1971. Whale oil is a much more stable oil under heat and pressure. So it was normal to drop the trans pan to do a fluid change because more than likely what was still in the tranny was in good condition. And that’s even if the fluid was ever changed. Todays transmission fluid is just a straight 10 weight oil with the proper additives. So just like the oil in your engine crankcase, the base oil in the transmission fluid breaks down, the additive packages break down from heat and friction, and the fluid becomes contaminated from clutch material and metal contaminates from normal wear. And if all this old fluid along with the contaminates aren’t removed, it accelerates transmission wear. And with the way transmission are built today, especially with all the electronics that are now inside the transmission, replacing all the transmission fluid has become more important. This is one of the reasons why vehicle manufacturers now recommend this service.

Now one of the misconceptions about transmission fluid exchanges is that it’s a money maker. Let’s look at this. You have a tranny that holds maybe 12-16 quarts. Depending on the type of tranny fluid, it could cost $4.00/ Qt. At 16 quarts that’s $64.00 just for the tranny fluid. Now you charge $125.00 for the fluid exchange service. WOW! You just made whole $60.00 in profit! Not exactly a get-rich-quick scheme.

I’ve had many people ask me if they should just do a pan drop and drain on their tranny. And I give them this warning. When you do this you only remove 30% of what’s in the tranny. When you add those 4-5 quarts of new tranny fluid, you also add the detergents that are included with the new fluid. These new detergents can cause the sludge and varnish inside the tranny to become dislodged and plug small orifices that are found in the valve body causing damage to the tranny. So be warned. And I’ve seen this happen many times.

So I guess some automotive techs on this board are keeping up with the times and what is now normal transmission service, and those who aren’t even automotive techs that are still living in the 1980’s.