Transmission Service - Changing Fluid - Drains and Fill vs flush

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If the transmission has been serviced before and/or has relatively low miles (under 100k), I would drop the pan, clean it out and replace the filter & gasket. Top off with with fresh fluid and then just do drain & fills every 30k going forward. IMO flushing the converter is unnecessary in most cases, as is replacing 100% of the fluid. A good freshen-up every so often will keep it going a long, long time. But if the transmission has never been serviced and it's a high mileage unit, I would just maintain the fluid level until it starts to give warning signs.
 

Fitz98

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All good advice.
I am leaning towards getting new spring clamp and seal and try the cooler line removal idea.
1 problem I have always had is when I install a hose over a flared fitting and clamp it behind the flare, it always leaks.
Any advice how to compensate for this? Maybe clamp the hose on the flare instead of behind it?
Does it matter which line I take off and is it better to remove from the cooler or the radiator?
Not sure how to identify which is the pressure side and which is return. I assume I want return.

As a counter argument, I am embarrassed to say the transmission has never been serviced and has a ton of miles, which makes me think the multiple drain and fill approach could help ease into a fresh fluid exchange.
For whatever reason, I have heard that a "complete" change on un-serviced high mileage vehicles can create problems.
Fluid is reddish brown and does have a "used" smell to it, not burnt, but not fresh.

I also don't want to waste a bunch of money trying to dilute multiple changes until it is fresh and then there would be a concern about compatibility between new fluid and existing.

FWIW I have a drain nut and deep pan.
 
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If it has a factory auxiliary cooler, that will help you determine flow direction. Trans>Radator>Cooler>back to Trans. Without looking it up, I would only be guessing about the in/out lines if it doesn't have a cooler.
 

CleanSump

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I also don't want to waste a bunch of money trying to dilute multiple changes until it is fresh and then there would be a concern about compatibility between new fluid and existing.

FWIW I have a drain nut and deep pan.
The trans came with Dexron III in it. Any Dexron III or VI will be compatible with any other Dexron III or VI. No worry about mixing them. GM back specked Dexron III vehicles to Dexron VI, so no worry there either. Maxlife is compatible.

Since you have a drain plug, you could do a filter change first, then just do drain/fills going forward without dropping the pan.
 

ls1mike

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I have done fluid exchanges using the cooler line method, not a flush so detergents get run through the system.
 

Fitz98

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If it has a factory auxiliary cooler, that will help you determine flow direction. Trans>Radator>Cooler>back to Trans. Without looking it up, I would only be guessing about the in/out lines if it doesn't have a cooler.
It does have a factory trans cooler, so this helps!
Thanks
 

Fitz98

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How about tips for not having leaks when clamping the hose over a flare on a tube for pumping into a bucket?
I am using a clear tube that has the same ID as the OD of the flare and using a hose clamp behind the flare, but it always leaks. I just did this with my power steering and it leaked too. Had same problem last time I flushed my Expedition tranny.
I imagine this will be under pressure when pumping new fluid through using the engine to power the pump.
Would the pressure of the fluid coming out be too much that would cause a splash out of fluid if I put the hose in an empty bucket and start the vehicle, or should I start with a gallon of old fluid in the bucket and bury the hose in the fluid first?
I want this to be a clean process... on my Expedition it was a nightmare of a mess all over my driveway.
 
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I’m old. So here goes….the last cooler line trans fluid empty process I did was in the early 80s. Did this to ensure converter and most of the trans was drained for trans swap. We kept rebuilt units ready for swap out. It was very messy because of tremendous fluid pressure. Hose in a 5 gallon bucket with a lid and a mist of fluid would still cover the bay floor. Has line pressure for trans coolers been stepped down? I know my truck (2017 GMC) has a thermostat that controls fluid temp in the cooler lines. I think that thermostat is a ****ing stupid idea…..
 

Fitz98

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Just "finished" this job, at least the hard part.
Let me tell you, for anyone looking to do this flush without a mess, look into this adapter, not sure what other vehicles it fits, but it fit mine perfectly.
Cooler Line Adapter
Couldn't figure out why they sent such big hose clamps with it...made me laugh.
I removed the transmission line from the transmission cooler (since it was easily accessible by taking the grill off and it is after the radiator, so this allowed for a flush of the cooler and the radiator portion as well).
I hooked up a clear tube to this adapter and plugged this right into the fitting of the trans cooler using the original clip to hold it in place.
That drain hose went into a clear 1 gallon bottle to capture the fluid in 3 quart increments. Fill pan, start car to flush, stop car, refill and repeat till it came out nice and red. No problems, no leaks, no mess, controlled and contained.
I think this adapter was the key to making this so clean and simple.
When I was done, remove the clip, unplug the fitting, replace the clip, plug in the return fitting, fill and done.
Piece of cake. Just need to top off and do a test run during daylight.
Thanks for all the tips.
 
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I was never scared to have the Chevy or Ford dealers exchange the trans fluid using the machine. I did that every spring on my work trucks so the transmissions would have fresh oil ready for the 3 months of hi temps. I never had a trans failure in any of my commercial trucks doing this every year since the mileage was 20k in 12 months

I also had "machine flushes" done to my private autos but every 3 years. People that do flush and fills are just gluttons for pain🙂
 
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Since you have a drain plug, you could do a filter change first, then just do drain/fills going forward without dropping the pan.
This is exactly what I'm doing with the Tacoma. Dropped the pan and changed the filter & gasket at 120k, did a drain & fill at 150k, another at 180k, still going strong.
 

JHZR2

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I like to do a suction out and fill, then drive a bit, then do a pan drop, filter replacement, etc. This gets some new clean fluid in there, but without a huge shock of all new fluid, then get to the point where somewhat fresher fluid is left behind.

Obviously if you can drain the TC, do so, but this approach makes it less of a concern, and a more gradual change.
 
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I did it on my Kias with no clamps, just a clear tube from Home Depot. Clear tube goes on ATF cooler 'out' line and the other end goes into a bucket. Start the car, drain about 1qt, stop the engine, top up with same volume, repeat till you see clean fresh ATF coming out. Use graduated pitcher or so to measure ATF out and refill with same volume. I did drain and fill first before going with cooler line process, cleaned drain bolt off metal shavings. Kias' ATs have to be opened up to replace filter, I didn't bother, too much work to remove AT and open it.
 
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