The screen in the Subaru AT filter bypasses the media, not unlike the bypass valve in many engine spin-on filters that opens when the media is clogged or engine flow demands exceed the ability of the oil to sufficiently pass through the media.The screens that are in the spin on filters for the AT are probably there to keep the filtering media from breaking off and getting into the valve body.
What makes the Subaru AT filter different is that whereas the bypass valve in a regular filter normally remains closed, and opens only when the pressure drop across the media reaches the valve's setting, the AT filter screen is open to flow at all times. Because the screen is in a bypass position, it can't and, therefore, isn't there to keep pieces of the filtering media from getting into the valve body.
Moreover, fluid that goes to the valve body is not sourced from the filter. The oil pump draws fluid from the pan, and sends it to pressure regulators (line, pilot), and from there it goes to the various solenoids and valves that direct it to control active clutches, brakes, etc. Much of this the drains back to the pan and is reused again.
The flow to the filter and cooler is from the torque converter hydraulic circuits. Most of the heat generated in an automatic transmission is in the torque converter, and primarily when it isn't locked. When not locked, the lock-up control valve directs ATF draining from the torque converter to the filter and cooler, after which it's returned, essentially, to the pan. When the converter is locked, the drain passage out of the converter is closed off.
Whether unlocked or locked, there is always some flow of ATF from the lock-up control valve to the filter and cooler. When the converter is unlocked, the draining hot fluid is mixed with this continuous flow on the way to the cooler.
Subaru introduced the spin-on filter in the change from the 4AT Phase I to the 4AT Phase II around 1999. It remained until, roughly, the end of MY2007 production.I know that the spin on filter is an option and from the standpoint it can be deleted. In other words it is for extra filtration. With that said it wouldn't make sense that it would affect pressures internally should it plug up and not bypass. In other words that filter shouldn't affect the transmission in any way that I can see.
For MY2008 there were some transmission units with filters that were carried-over and are found on early 2008 vehicles. Also, for new production 2008 4ATs, the filter and fitting was not installed; instead a cap was placed over the filter mount on the side of the transmission. This directed fluid to the tube going to the cooler (as if it had passed through the filter). As these older castings were used up, the flow pattern in the new ones were changed to direct the fluid straight to the outlet tube. There was no longer an add-on cap.
Because the filter is in the torque converter hydraulic circuits, if it were to block, it would not directly affect other parts of the transmission other than allowing the ATF temperature to rise. But this is where the screen bypass in the AT filter is relevant, and makes filter blockage less likely.
In a normal oil filter with a bypass valve, the valve remains closed most of the time, so that all the fluid has to go through the media.
However, in the Subaru AT filter, with the bypass screen, there's no similar closed/open valve. There's continuous flow in parallel through the media and through the screen. Because the screen is not as fine as the media, it would not tend to become blocked unless there was major failure in the transmission leading to a large volume of relatively larger particles, and, in that case, that failure would likely be apparent in itself. In other words, the likelihood of the filter totally preventing flow to the cooler is low. In my tear-downs of used Subaru AT filters, I didn't find any in which the screen was clogged.
Subaru has not regularly published the hydraulic circuit schematics for its automatic transmissions. However, its 2003 Legacy/Outback FSM has a related "Mechanism & Function" document with the schematics for the 4AT Phase 1 version 1 that was current at that time.
Unfortunately, I had stored the related photos in the earlier posts of this thread on Photobucket, and subsequently the rules relating to third party linking were changed. As we still cannot edit earlier posts, I am unable to replace the blocked photos in their proper posts. But here's a few that might help understand the uniqueness of the screen in the Subaru AT filter.
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