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#4441083 - 06/25/17 12:44 PM The cooler line trans flush
Brad_C Offline


Registered: 07/27/12
Posts: 617
Loc: Perth, Western Australia
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?

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#4441090 - 06/25/17 12:51 PM Re: The cooler line trans flush [Re: Brad_C]
eljefino Offline


Registered: 06/15/03
Posts: 32629
Loc: ME
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.

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#4441113 - 06/25/17 01:10 PM Re: The cooler line trans flush [Re: Brad_C]
Jetronic Offline


Registered: 02/27/09
Posts: 3653
Loc: down in the park
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.
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#4441116 - 06/25/17 01:12 PM Re: The cooler line trans flush [Re: eljefino]
Kira Offline


Registered: 08/19/10
Posts: 5203
Loc: Champlain/Hudson Valley
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.

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#4441151 - 06/25/17 01:54 PM Re: The cooler line trans flush [Re: Brad_C]
Linctex Offline


Registered: 12/31/16
Posts: 6148
Loc: Waco, TX
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.
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#4441168 - 06/25/17 02:12 PM Re: The cooler line trans flush [Re: Brad_C]
WillsYoda Offline


Registered: 10/17/15
Posts: 471
Loc: MN
I just to the trans cooler line flush and then add a magnefine. No pan drop.

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#4441170 - 06/25/17 02:15 PM Re: The cooler line trans flush [Re: WillsYoda]
4WD Online   confused


Registered: 09/21/10
Posts: 6351
Loc: Texas
... and your pan filter just became a rock catcher (catastrophic failure screen) when the inline filter went in ...

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#4441226 - 06/25/17 03:30 PM Re: The cooler line trans flush [Re: Brad_C]
Donald Offline


Registered: 03/21/04
Posts: 20446
Loc: Upstate NY
Your not in a sterile operating room. So exchanging "pretty much all" is the best you can do. When the ATF coming out looks like new, you are done.
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#4441404 - 06/25/17 07:02 PM Re: The cooler line trans flush [Re: Brad_C]
Kestas Offline



Registered: 06/04/02
Posts: 12321
Loc: The Motor City
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.

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#4441700 - 06/26/17 07:14 AM Re: The cooler line trans flush [Re: Brad_C]
Brad_C Offline


Registered: 07/27/12
Posts: 617
Loc: Perth, Western Australia
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.

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#4441709 - 06/26/17 07:28 AM Re: The cooler line trans flush [Re: Brad_C]
Sam_Julier Offline


Registered: 05/22/12
Posts: 428
Loc: Connecticut
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.
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1993 Volvo 244 Delo 10W30, 152k


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#4441815 - 06/26/17 09:58 AM Re: The cooler line trans flush [Re: Brad_C]
KrisZ Offline


Registered: 12/23/06
Posts: 7456
Loc: Toronto, Canada
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%.
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#4441874 - 06/26/17 11:07 AM Re: The cooler line trans flush [Re: Brad_C]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 18947
Loc: Sunny Florida
^^^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...
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#4443437 - 06/28/17 05:58 AM Re: The cooler line trans flush [Re: Brad_C]
Kestas Offline



Registered: 06/04/02
Posts: 12321
Loc: The Motor City
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.

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#4444270 - 06/28/17 08:23 PM Re: The cooler line trans flush [Re: Brad_C]
Coprolite Offline


Registered: 07/03/11
Posts: 1224
Loc: Houston, TX
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

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