Transmission Flush vs Fluid Change: Which is Better?

It depends on what you want to accomplish. But, first of all, check your owner’s manual to see if your vehicle manufacturer recommends one instead of the other.

If you want to ensure removal of nearly all the old transmission fluid, then have your transmission flushed.

Here’s what we’ll cover:

What is a Transmission Flush?

The benefits are self-evident: all the old, dirty fluid is replaced with fresh, high-quality fluid. (And, as you can see in the image, new transmission fluid is preferable to old fluid). As a result, your transmission should run cooler and receive maximum protection against wear to clutches, gears and bearings. It should also shift consistently and crisply since the new fluid will provide the correct frictional properties (old fluid loses its frictional properties over time).

Not only that, but performing a flush helps clean the transmission. Sludge and other contaminants can accumulate in the fluid due to extreme heat breaking down the fluid. These contaminants circulate throughout the transmission before lodging in the filter. Before the filter can safely capture the contaminants, however, they can lodge in the narrow fluid passages inside the valve body, leading to poor shift quality.

Transmission Valve Body
The narrow fluid passages in the transmission valve body can easily clog with debris, reducing shift quality.

Performing a flush also allows you to use a flush additive to help clean the transmission and more effectively remove accumulated sludge and other contaminants.

Downsides of a Transmission Flush

For one, it’s more expensive. And some people warn against performing a flush on a transmission using old, dirty fluid. The flushing procedure may direct the fluid in the opposite direction of normal flow, which may increase the risk of dislodging debris and causing it to settle somewhere it shouldn’t. Since the way each shop performs a flushing procedure varies, you can’t know for certain.

What is a Transmission Flushing Machine?

A typical flushing machine uses hoses that connect into the transmission cooling lines. It drains the old fluid and holds it inside the machine while replenishing the transmission with new fluid. Unlike a simple pan drop, a flushing machine removes just about all the old fluid, including the fluid inside the torque converter.

Since the procedure uses new fluid to perform the flush, it requires several quarts of new fluid beyond the transmission’s final capacity. Those extra quarts are where most of the added cost lies.

(Find out how an automatic transmission works.)

Pan-Drop Transmission Flush

If you have reservations about a flush, go with a pan-drop instead. While it reduces the risk associated with flushing old, dirty fluid through the transmission, a pan-drop also has downsides.

  • Removes only about a third of the fluid
  • Can be a mess
  • Can be a pain on some vehicles

I can tell you from experience that a pan-drop can be a bigger job than you think. You may have to remove plastic splash guards or metal skid plates to access the transmission pan. In case you haven’t been under your vehicle in a while, plan on encountering rusted, stuck bolts if you drive in wet, snowy conditions. Don’t be surprised if you crack a splash guard in one or two places as you try to remove/reinstall it.

Tips for Performing a Pan-Drop:

  • Have a large catch pan handy. Otherwise, once you loosen the pan bolts, fluid is going to ooze from the pan/transmission interface and end up all over the floor.
  • If you’re crafty, you can back out the pan bolts in one corner further than the surrounding bolts, effectively tilting the pan so the fluid drains from a single corner instead of overflowing the entire pan. This reduces mess quite a bit.
  • Wear safety glasses and gloves.
  • Don’t forget the new filter and pan gasket.
  • It’s a good idea to know the torque specs on the pan bolts and use a torque wrench to reinstall them. Otherwise you risk overtightening and ruining the gasket.

You might get lucky, though

Some vehicle manufacturers install a drain plug on the transmission, similar to a motor oil drain plug. This allows you to easily and cleanly drain some fluid from the transmission minus the hassle of removing the pan. Again, though, you only get a third to half the fluid out.

You can then drive the vehicle for a while, then drain the fluid and change it again. Do this 2-3 times and you’ll remove nearly all the old fluid and perform a sort of poor-man’s transmission fluid flush.

Bottom line: Visit a pro and have the transmission flushed for best results and least hassle (unless your vehicle manufacturer specifically warns against it in the owner’s manual). But, if you have reservations about dislodging debris due to old, dirty fluid, a series of pan drops works just as well.

FIND AMSOIL TRANSMISSION FLUID FOR MY VEHICLE

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Comments

  1. I used to have a trans shop that offered Amsoil and would do a complete drain of the torque converter on my old Caravan and use the pan removal process. The shop closed due to the owner’s health. With my ’06 Volvo V70R there is no service requirement, but I bought it used and have a Volvo independent shop that has to use the cooler lines with the transmission operating to flush old fluid with new until it runs clean. This is the only way to refresh the fluid. Took 16 quarts for best result. It is a very specific process. My previous Volvo required a similar process, but it had a dipstick that made level checking simple. Both these transmissions responded well to fluid changes.

  2. If the fluid is red or brown and it doesn’t smell burnt, I’d say drop the pan, replace the filter. Remove that bolt from the torque converter and drain it as well. Keep in mind how much you drain out after because a pint of transmission fluid will be lodged in the valve body. I’m assuming this is the 4r70w in this vehicle.

    If you go the flush route, you can remove the lines from the transmission, but remember, 8.5 quarts will have to come out of that torque converter if you don’t want to remove that bolt but the benefit of this route is it’ll flush your valve body and push it through the cooler lines. You can use the cooler line removal to drain your transmission, but have someone stop the vehicle once the flow starts to slow. This makes it a much less of a messy job.

    Approx 14 quarts to complete the service.

    1. Hi Brad,

      The advantages to the flush are the use of a cleaning additive to help remove contaminants from the tranny. That’ll help give it a fresh start prior to new fluid. The other obvious benefit is that the shop will do the work for you.

      If you’re a DIYer and want to save some money, a pan drop should work nearly as effectively. Since you can drain the torque converter, you should only need one change as opposed to 2-3.

      Thanks,

      John

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