The cooler line trans flush

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So I've seen it recommended frequently. I've done it a few times and the results have been universally good. I'm not questioning the validity, but I wonder about the absolute efficiency.

I can understand replacing the fluid in the pan. I can also understand replacing most of the fluid in the valve body, pump and cooler lines. That is directly fed from the pump, which is directly fed from the sump which is all clean and new. Over half my trans capacity is in the torque converter, which to me looks like a dirty great pudding basin. The fluid gets pumped in there, and the stuff that comes out goes out to the cooler. So I have a 8L capacity in the trans (~3 in the pan, ~1 around in general and), 4 of which is in the torque converter (which rotates and mixes like a washing machine).

So here's the question. When I pump 4L into that converter, how much of what comes out is old fluid? I'd *assume* that as the new fluid gets pumped in, it's constantly mixed with the existing fluid and it's that mix that comes out to the cooler, so short of pulling the converter and draining it there is no way to replace *all* the old fluid. Just a good proportion of it (in theory). The old proportional dilution conundrum, somewhat like the repeated drain and fill theory.

Am I missing something?
 
IMO what you get the most of is convenience and not constantly retorquing pan bolts which could dent the lips of the pan.

A dozen quarts of ATF is 90% new either way you do it, and 100% is unachievable.

I take it the TC drain plug is not present or easily reached.

If you do a pan drop and the shifting improves this might say, hey, do it again. If nothing changes it says, hey, you're right on schedule, good job.
 
Yes, it's usually a convenient way to flush the fluid. Go by colour, if the colour looks new after changing the amount the trans holds, it's time to quit. if it bothers you that you didn't get the last drop out, reduce the OCI slightly.
 
Hello Western Australia, Here's what's been missing all the while. Nobody will address the exit flow rate of a torque converter. I believe they do not empty that fast at all. I believe the pressure comes up when you start the car and the transmission's pump starts turning. Maybe an ounce or two fills any void in the TC.

I've seen TCs taken from operating trannies and handed about the room. Never is there fluid dripping out of them.
I liken them to a full-to-the-brim milk bottle. If you gently pour water from a shot glass onto the milk's meniscus-how long will it take to displace the milk so you end up with a bottle of clear water?

I do remember hearing something about older VW autos having TCs which empty down to the low point of the input shaft and upon startup they refill.

Also, I've seen TC drain plugs on my friend's Ford truck. That tells me that's "the only way out" for dirty fluid.

ERGO the best way to go about things is to displace dirt all the while. Do drain & fills from youth.
Ideally, don't let it get dirty in the first place. With drain & fills starting early on you'll always have clean fluid.

People will scream and shout at each other over this but no one seems to know about the TC.
 
I honestly (FWIW) think your obsessing about a few ounces of oil left behind - - in the "the real world", will not affect transmission life at all.

I only ever do "pan drops" (essentially, since I actually run out through the cooler line)... and 4 qts/liters every 30-40,000 miles (50,000-65,000 km) is all that's really needed for the life of the vehicle as you own it.

I also have an auxiliary cooler (I do a lot of towing) and a spin-on WIX filter in the cooler lines.

The transmission filter in the pan is typically very COARSE - some say no finer than 100 Microns.
 
... and your pan filter just became a rock catcher (catastrophic failure screen) when the inline filter went in ...
 
To put things in perspective, even an engine oil change doesn't get 100% of the fluid out. There's still old oil in the oil galleys and lifters.
 
Yeah, not disputing anything you all said. It was just something I've never seen raised. Everyone seems to imply that a cooler line flush completely replaces the fluid and I couldn't see how that could actually be true.

My trans is a Volvo AW70, which is basically a Toyota A43D. No drain on the converter and at rest the converter very slowly drains down to the bottom of the input shaft (so just over half). I've pulled the converter a number of times now, and it's only ever half full. That explains how the level on the dipstick is *so* much higher at rest.

I added a magnefine last time I did some work on it after I changed the internal screen and cleaned the pan. I must replace and inspect that, it has about 40,000km on the filter now. I don't bother with drain and fills, just do a cooler line flush every couple of years.
 
I have two AW70 transmissions both with inline filters. I do a cooler line exchange and filter change every 20,000 miles. It takes 10-12 quarts to get clean fluid.
 
When I do a cooler line exchange, I use clear vinyl tubing and I can see the dirty fluid coming out of the cooler line all the way until the last pan fill up, when the fluid starts to clear up. That tells me that there is minimal fluid mixing. And there shouldn't be a lot of mixing happening, because the majority of mixing happens in the pan itself, as the fluid that is already in the circulation can only be pumped one way, so naturally as the new fluid enters the system, it will push out the old fluid.

Pan drops or D&Fs require emptying the pan, refilling it and then running the tranny and going through the gear selector. This mixes the fluid 100%.
 
^^^Likewise. The fluid becomes clear quite suddenly. As close to 100% as is possible without disassembly. Nothing is perfect.

IMO you must start with a pan drop so you can change the filter or you're not doing it in the correct order. After some serious miles many slushboxes practically quit generating debris. But a driveway fluid exchange is a primary player in keeping a slushbox on the road...
 
When doing a cooler line flush, I'll shift through the gears during the flush. Sometimes I'll see a pulse of dirty fluid go through.
 
So this tranny holds 8 and takes 10-12 for it to come clean ? Is 125-150% a good ballpark for purchasing purposes? I am assuming it might take more if interval is extended or done for the first time on a used car with 80 instead of 40k miles.

Are these generalizations close enough? Thx
 
Mine holds 8L and I always use 12 becuase it comes in 4L bottles. Might as well use it all. The first time I did it I used 15 as I was using Maxlife and that came in 5L bottles. Now days I just use Castrol Dex-III, and at $30 for an extra bottle I figure why not.

That reminds me. I need to do the steering again this year too.
 
Return line flush is the most efficient and almost complete replacement.
Why do you care if it replaced 80-90% vs 100%, it is a mute point.
The very fact that 80-90% was replaced is must better than a D&F.
 
Originally Posted By: KrisZ
When I do a cooler line exchange, I use clear vinyl tubing and I can see the dirty fluid coming out of the cooler line all the way until the last pan fill up, when the fluid starts to clear up. That tells me that there is minimal fluid mixing. And there shouldn't be a lot of mixing happening, because the majority of mixing happens in the pan itself, as the fluid that is already in the circulation can only be pumped one way, so naturally as the new fluid enters the system, it will push out the old fluid.

Pan drops or D&Fs require emptying the pan, refilling it and then running the tranny and going through the gear selector. This mixes the fluid 100%.


I concur!

Did this method on my vehicle which holds 9 quarts.
Went through about 12ish quarts to get the same color oil as it was going in.

Can take pan off to check it out, maybe clean magnet and add inline filter (Like already mentioned by another poster)
 
Originally Posted By: KrisZ
When I do a cooler line exchange, I use clear vinyl tubing and I can see the dirty fluid coming out of the cooler line all the way until the last pan fill up, when the fluid starts to clear up. That tells me that there is minimal fluid mixing. And there shouldn't be a lot of mixing happening, because the majority of mixing happens in the pan itself, as the fluid that is already in the circulation can only be pumped one way, so naturally as the new fluid enters the system, it will push out the old fluid. Pan drops or D&Fs require emptying the pan, refilling it and then running the tranny and going through the gear selector. This mixes the fluid 100%.


That's why even when using an ATF exchange machine you must go through the gears and stop in each at least 2 seconds repeating again as you go back to park
 
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