ATF flush techniques

[elektrinis]

I have an Impreza 2.0L 4AT 2008 at 100k mark and the gearbox is acting up: slow to shift, rough shift, vibrations on D when stopped.

Inspected the ATF, level is good, but color is weird: clear, slight yellowish, almost just like fresh engine oil. Likely wrong oil was replaced by someone before me. Also manual states that vibration can be caused by higher viscosity ATF (dexron III instead of VI). The manual also list possible mechanical issues causing the delays, but let's try flush first.

 

I know there are different opinions, flush vs. drain, let's not discuss that here. I decided to do a full flush.

Now I was searching the internetz how that could be DIY'ed... And oh man, some people are brave. In other words, I did not find a safe way this could be DIY'ed without fancy flush machine that only large specialized shops have.

 

Did I miss something? Was even thinking about making a special pump myself.

Edited December 22, 2016 by elektrinis

[DaveT]

A couple of ideas.

Multiple drain and fill with low cost atf, then use good stuff for the last 2.

 

Drop the pan, find the pickup tube, adapt a hose to it, to a tank (jug) of new fluid.

 

With a big pan under everything, cycle through the gears.

[elektrinis]

Hmm.. Heard of this method, sounds bit time consuming.

I was thinking of some kind of bidirectional pump... Outgoing ATF drives a hydraulic motor, and the motor drives a pump and pumps exactly the same amount of fluid. So ultimately you would maintain constant level of fluid during the procedure. For example:

 

Such pump should be fairly easy to do...

Procedure for full 100% flush goes like this:

1. Drain the pan
2. Change filter(s) if you like.
3. Add the same volume of new ATF as drained.
4. Disconnect radiator return line from the transmission and connect it
to OLD ATF IN port of the device. Connect a hose from OLD ATF OUT port
to an empty bucket.
5. Connect NEW ATF IN port of device to a tank with fresh ATF, and the
NEW ATF OUT port of the device to return port of transmission.
6. Run the engine and watch. Slowly go through gears in meantime. Stop
the engine when you see bright red (fresh) ATF going out, or when your
tank of fresh ATF runs out.

 

Would this work?? I have access to some machining toys... Could do a prototype in a few days.

[idosubaru]

I would just drain and fill and see if it gets better.

 

I've done this before: 

 

1. fill ATF

2. pull ATF output hose and aim it towards a container

3. turn key temporarily so ATF pumps out

4. turn key off

5. refill ATF

6. repeat steps 3-5 until new fluid is coming out

 

i'm not saying that's a good idea and i'm not sure anyone else has done it, i could see people saying it's risky but i've done it. 

[Numbchux]

Are you sure your checking the transmission fluid and not the front diff? The transmission has 2 dip sticks, longer one on the left is for ATF, short one on the right side is for the front diff. Differential uses gear oil, which would be exactly as you describe.

 

 

My other thought, is ATF turns clear when it boils. So maybe you have a transmission cooler issue that's cooked the fluid.

 

 

 

And yes, a conventional transmission flush is done through the transmission cooler lines. Could easily be done without any device, just put the return line in an empty bucket, and put the supply line to a full container of new fluid.

[elektrinis]

Yes, I am sure I am checking correct dipstick... Oil in front diff is much thicker. Will drive around with OBD connected to check temperatures. That is alarming though...

Just contacted previous owner and he says he never did anything to the AT (no oil changes).

 

If it's really cooked, but gears are not slipping, so likely it was towed incorrectly and that lead to overheat?

 

About the return line. Isn't it going directly from radiator to pan? In this case I guess it will not suck any fluid back, unless you make sure dipstick (and breather) is completely plugged to create vacuum?

[DaveT]

Your fancy pump idea would work as far as keeping the system volume constant. But would be the same as multiple drain aND fills, since the only some of the atf is sent to the cooler lines by the pump. As much goes to the clutches, bearings, gears, etc. Multiple return paths to the pan. This is why you need a big pan under the transmission when you do my drop the pan method. With the fancy pump, you still need 3 to 4 times the transmission oil capacity to get rid of most of the old stuff .

 

Regarding the other post - the atf cooler line that returns fluid to the transmission simply drops into the pan, no suction there.

[elektrinis]

Okay, multiple returns is an issue...

I did not take such gearbox apart yet, so no idea what the actual path of ATF is. And if it is more or less similar on most ATs.

Was under impression it goes something like this:

Pan -> pump -> torque converter -> valves and everything else -> cooling -> pan.

 

Saw some videos of people attempting DIY flushes and dumping the fluid from cooling line. The flow seemed quite near to what one would expect from integrated displacement pump running at idle RPM though. Let's check:

 

A volume in one tooth is roughly 2ml, times 9 teeth, times 700 RPM = 0.2 liter per second = 1 liter in 5 seconds, or full 10 liters in 50 seconds. This is roughly the figure various youtubers are getting to dump all of old ATF. So an assumption can be done that near to 100% of pumped oil is going a full circle through radiator.

 

Would be great if someone could correct me on this.

What's the actual internal path of ATF?

 

 

EDIT:

This is interesting:

http://www.sonnax.com/articles/124-anatomy-of-a-transmission-oil-flow-in-the-pump-pr-converter-cooler-lube-circuits

 

So it seems I did not get it totally right, ATF from torque converter goes to cooler and only then to gear train. I can't believe the cooler is under such high pressure (required for gear train). If it is as shown however, I wonder how much ATF is trapped in gear train when in neutral?

Edited December 22, 2016 by elektrinis

[DaveT]

The transmissions I know best are the 3AT. The path is pump to regulator. Iirc, Regulator overflow goes to converter and then cooler. The regulated pressure goes to the control and lube systems. All of the lube system "return" is just what flows out from the bearings and all. The cooler is not under any pressure, other than what it takes to flow the oil back through the return line.

 

The tourque converter doesn't need high pressure, just enough flow for cooling the heat it puts into the oil. It's kind of a big turbo pump loop. A big fan pushes fluid through another fan, getting the rotational energy through. The cooler helps get rid of the heat of moving all that energy.

[elektrinis]

OK, that is interesting and sounds reasonable. I knew there is something wrong with that diagram.

Then the question is... How much flow do we have on regulated line when no shifting is happening? I guess it's static more or less, just as much as leaks out through various clearances, which should not be too much? Can we pin a number on that, like 2%? And when shifting, it empties the cartridges and valves by simply dumping the oil in to pan?

Edited December 22, 2016 by elektrinis

[DaveT]

Not sure what the flow rate is or the bearings and what gets around the seals, but it is not trivial. The shaft seals that make the passages through the various shafts to get pressure to the clutch packs are similar to piston rings. I know of no reason they would not just let the valves that control the clutches and all just dump back to the pan via the shortest rout when they release pressure.

[DaveT]

Factory service manuals usually have great diagrams of the hydraulic system in the transmission, and descriptions of how things work.

[elektrinis]

Not in this case, at least not in car's service manual. Maybe separate manual is available for the 4EAT? Anyone have one available?

[CNY_Dave]

I had a detailed pic of the 4EAT system (it even showed the screen before the Duty C solenoid valve), the cooler feed is the return (dump) line from one specific circuit, and if the line is plugged that circuit will do weird stuff.

 

file name is

MSA5T0200A30120.pdf

 

(you can google and find)

 

32 pages of MPT and VTD fluid circuits for each gear selection.

Edited December 23, 2016 by CNY_Dave

[keith3267]

Some aftermarket CVT fluids are yellow.  Subaru's CVT fluid is blue.

[elektrinis]

Thanks Dave, this document is very detailed.

I see there are multiple sump dumps all around (marked with X), however it is not clear to me how is pressure regulation and torque converter arranged. I don't see the overflow from pressure regulator going to torque converter, the return is directly to sump. And torque converter is being fed by line pressure?? Lock up valve looks quite complex and difficult to understand. Does subaru 4EAT have a lock up torque converter?

What are the differences of MPT and VTD (I googled the definitions, but no idea), and how do I know which one is mine?

[subnz]

I would just drain and fill and see if it gets better.

 

I've done this before: 

 

1. fill ATF

2. pull ATF output hose and aim it towards a containerw fluid

3. turn key temporarily so ATF pumps out

4. turn key off

5. refill ATF

6. repeat steps 3-5 until new fluid is coming out

 

i'm not saying that's a good idea and i'm not sure anyone else has done it, i could see people saying it's risky but i've done it. 

Am a strong believer in doing jobs properly and not 1/2 assed.

That's my opinion / belief  for partial replacement of just what comes out of  trans pan.

 

Its similar to draining 1.5 litres of old oil out of engine  then banging in the drain plug to hold the remaining 2.5 litres of old oil in there and topping up with 1.5 L ( removed) with new oil.  (ie for 2.5)

 

Agree did it this way for my 3.0 5EAT recently in August  (my 1st vehicle with an AT  in 41 years of vehicle ownership and maintenance.)

ie  drain auto pan  (got about 2.5 - 3 litres out of the 9.5 litres in trans / torque converter.   Replace with new fluid of similar amount. (via funnel into trans dipstick tube)

 

Pull off and put trans cooler return hose (connected anextra length of clear plastic hose) and into drain pan.

Run engine briefly to remove / replace in  about  1 litre increments -  until fluid looks new looking.

The brief start / stop -  drain / refill is to take care not to run trans to low in fluid in this flushing process.

I also moved the trans selector through RND and back while engine running briefly each time to aid in the trans flushing process.

Put 10 Litres through / into mine.

 

Its a bit of  a fiddle / effort required to do this but knowing have done a better job.

 

There's enough stuff on how to do this ie on youtube. etc and use of google.

http://www.subaruoutback.org/forums/4328281-post414.html

Edited December 23, 2016 by subnz

[lmdew]

Yep, works great.

I've done this numerous times.

I end up using about 3 gallons of fluid.

Edited December 23, 2016 by lmdew

[elektrinis]

After some thinking and playing around on CAD, this is what I came up with:

 

This is a dual lobe pump. One pump gets old ATF from cooling return line and dumps it in to an empty bucket. This pump works as a hydraulic motor, driving second pump. Now that second pump sucks on fresh ATF from another tank and feeds it back in to return line of AT.

The beauty of it is that constant oil volume is maintained during flush and the process is not interrupted by stopping refilling quart by quart (which leads to mixing internally).

 

Would this work reasonably well? I have access to some machinery, could throw a prototype rather quickly.

[DaveT]

If you were to drain and fill the pan, then use that pump, it should significantly reduce the old oil left in. Since the old stuff would only be what's in the valve body, channels, clutches, etc. The new oil in the pan would be pushing all the old stuff out, which will drop into the pan still. But that's a lot less than what's in the converter. It will still take a good amount extra to fully change the converter, since there is a lot of mixing circulation going on in there.

[subnz]

Hmm.. Heard of this method, sounds bit time consuming.

I was thinking of some kind of bidirectional pump... Outgoing ATF drives a hydraulic motor, and the motor drives a pump and pumps exactly the same amount of fluid. So ultimately you would maintain constant level of fluid during the procedure. For example:

 

Such pump should be fairly easy to do...

Procedure for full 100% flush goes like this:

 

1. Drain the pan

2. Change filter(s) if you like.

3. Add the same volume of new ATF as drained.

4. Disconnect radiator return line from the transmission and connect it

to OLD ATF IN port of the device. Connect a hose from OLD ATF OUT port

to an empty bucket.

5. Connect NEW ATF IN port of device to a tank with fresh ATF, and the

NEW ATF OUT port of the device to return port of transmission.

6. Run the engine and watch. Slowly go through gears in meantime. Stop

the engine when you see bright red (fresh) ATF going out, or when your

tank of fresh ATF runs out.

 

Would this work?? I have access to some machining toys... Could do a prototype in a few days.

This would definitely better and do a better job with running continuously than the start stop / drain - refill process in quart / litre increments

and  agree there would be less mixing of old / new fluid  with continuous running.