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Old 03-16-2010, 06:37 PM   #1 (permalink)
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Default Subaru transmission ATF filter IS different -- it's not the same as the engine filter

. . . and not only in the way it’s made. The way it works is different too.

Some time ago I opened, examined, and tested several engine oil filters to see what differences there were. My main focus was on the bypass valve because in a number of Subaru forums there were claims that Subaru used a higher bypass pressure specification. My tests confirmed both that this was true and that some commonly used aftermarket brands did not meet the same bypass specification. This was reported in the thread “Wisdom of Using OEM Oil Filters ONLY”, started by 96OBWin NY, in April 2009 (Wisdom of Using OEM Oil Filters ONLY).

In addition, there was, and continues to be questioning about the Subaru transmission spin-on oil filter – is it just an engine oil filter that fits, or is it really different? I finally got hold of a transmission oil filter and can say without doubt it is physically different, and based on this difference, it also functions differently. Here goes . . .

The filter I examined is an original black Tokyo Riko filter that is marked for use on the Automatic Transmission. This one had been removed from a 1999 Outback 4EAT that had delayed shifting problems.

Externally, and for the most part, internally, it looks the same as the black Tokyo Riko filter that is used for the engine (or at least was available until last year) but has since been replaced by the blue Honeywell-made filter (at least in North America).

Transmission filter -- looks pretty standard:


Even the inside, initially is much the same. Here's both with mounting caps off (engine filter on the left)


However, there is a very noticeable difference when looking inside. The engine filter has a visible metal “tower” in the middle which encloses the bypass valve spring. The transmission filter, instead, has a mesh screen (engine filter on left, tranny filter on right):


Furthermore, with the filter medium removed from the can, on the bottom of the engine filter one can see the plate that forms the valve over the bypass hole. On the transmission filter there is no blocking plate. The hole is open right through to the mesh. This difference is very significant.


Here I have a light on the other end of the filter showing that the hole is open to the mesh and the filter outlet pipe:


I could find no other apparent distinction between the two. The filter media is held in the same metal end caps; both have what look like identical anti-drainback rubber seals; both have the same six inlet holes; both have the same spring in the bottom of the can holding the media and anti-drainback valve against the mounting plate; and, both have the same mounting thread. (I have no way of telling if the filter media itself is in any way different – visually it isn’t.)

What makes the tranny filter so different from the engine oil filter (regardless of brand) is the fact that the hole leading to the mesh is not closed by a valve, as in the engine filter. This means that there is no restriction to the flow of ATF, other than the size of the hole, through the mesh. There is a constant flow of ATF through the hole and the mesh, not through the filter media! It might even be that it’s only when the flow increases to beyond that allowed by the hole and mesh, or the mesh itself becomes blocked, that the bulk of ATF “bypasses” through the media. This is the reverse of the bypass function of the valve in the engine oil filter.

This further suggests that the use of a non-Subaru filter based on an engine filter, that has a bypass valve or that is full flow through the media alone without a bypass could excessively limit ATF flow to the transmission under normal operation. It also might explain why the ATF filter is changed only when needed, such as a transmission internal breakdown when actual pieces might block the mesh. Barring this, the filter should pass ATF through with little or no restriction for a long time.

Of course, this doesn’t preclude the possibility of changing the ATF filter more frequently, but the transmission filter is less likely to present an impediment to ATF flow even over a lot of miles, unlike an engine oil filter which will tend to clog progressively due to the combustion by-products that can contaminate the engine oil during use, and build up on the filter media, restricting flow and causing more (and undesirable) “bypass” operation.

Well, that's the way I see it.
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Old 03-16-2010, 07:58 PM   #2 (permalink)
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pOM,

Thanks for another informative and interesting filter posting!

I guess from what you have posted, there isn't a need to change the transmission filter often. In the event one does change it at the cost of a transmission repair/rebuild, it would make sense to spend the extra few dollars to get a Subaru branded AT Filter instead of saving $10 to only spend $1000's down the road.

Thanks again!
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Old 03-17-2010, 12:00 AM   #3 (permalink)
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So you are saying do not put an engine oil filter in place of the trans filter?

I have the advanced auto parts trans filter on mine. It was noticeably heavier than a similar looking oil filter.

Seems to be fine so far. At my 90k trans drain I will get the subaru brand filter. Its worth it to me. When I tried to buy it at the dealer they didnt have any and didnt want to order it because they "never change them."
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Old 03-17-2010, 12:10 AM   #4 (permalink)
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Or you can just get one from www.subarupartsforyou.com for $25 plus shipping.
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Old 03-17-2010, 01:34 AM   #5 (permalink)
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Thanks, that's ... enlightening sort of ...

Unless the hole is designed to create enough pressure to force fluid through the filter media, the media itself is essentially pointless.

Seems like it would make more sense to have a series of different size mesh to catch progressively small particles.

It's difficult to say whether the flow is restricted enough to cause an issue without flow testing an auto filter at various stages of "dirtyness".

Does the fluid flow through the filter then to the pan ? if so there would be a "safety net" of fluid to draw from and the danger would be in backing up of the oil from the source which in theory would be taken care of by the bypass valve [if the pressure was low enough not to cause issues.

If it goes from the pan through the filter, to the pump, that could be a problem.

I'll have to remember to crack open the NAPA trans filter I have on mine [$22] to see how it's made.

Anyone got a blue one they can open up to see if it's the same as this one ?
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Old 03-17-2010, 09:01 AM   #6 (permalink)
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I wish I knew where the ATF actually goes. Other automotive FSMs often show the hydraulic flow circuits, but the Subaru manual, and the 4EAT training manuals, do not, at least not fully. I was under the impression, from comments here, that the spin-on filter (there's several other mesh filters inside the transmission) is somehow in the line going to the ATF cooler, but I haven't been able to confirm this, or where the cooler lines are in relation to the pump and the rest of the transmission.
Quote:
So you are saying do not put an engine oil filter in place of the trans filter?
bikemobile: Well, sort of. There's a distinct difference between the engine oil filters (various brands) and the Subaru tranny filter. I have to think there's a reason for this, otherwise Subaru would have simply used the same filter on both -- less expensive in manufacturing and less inventory to maintain.

An engine oil filter that is also listed for a Suburu transmission will likely have a by-pass valve that is normally closed except when the pressure drop across the media exceeds the by-pass valve opening pressure point. This is not all that bad in the first instance because it means that more of the ATF will be going through the media, getting nice and clean. But, we don't know the normal flow rate range of the ATF pump (it varies with rpm) or what is required through the filter. So there is a possibility that not enough ATF will pass through the engine-type filter, and perhaps (and I admit that at this stage this is only a hypothesis) starve some parts of the transmission. This also depends on where the filter is in the hydraulic circuit, and whether or not there is a bypass route elsewhere in the transmission.

Quote:
Anyone got a blue one they can open up to see if it's the same as this one ?
ETC: When I did the tests on the engine oil filters (linked above), I disassembled each one of them in order to remove and measure the by-pass valve opening pressures. I still have one of each, including the "new" blue Subaru filter. However, I don't want to get into an engine oil filter dialogue on the difference between the original black, the new blue, the WIX, the Purolator etc. They are different in construction brand to brand (e.g. end caps, anti-backflow valve, by-pass valve etc) and this has been dealt with here and in other forums. (All the engine filters are like the black engine filter in the above pics with a closed bypass valve at one end or the other.) The key point here is that all the engine filters are clearly not the same as the Subaru tranny filter in design or, it appears, the way it functions. The engine filter is intended to trap the smallest contaminants (along, of course, with larger pieces), whereas the tranny filter seems to be less concerned with the smallest of particles (I'm speaking of the micron-type specs for engine filters), at least in regard to the apparently unrestricted flow through the mesh.

I could also look at this differently. In the Subaru tranny filter the ATF flow through the hole and mesh could be similar, in terms of rate and back pressure, to the flow through the media itself. In other words, the mesh and the media are in parallel. At any one time, some ATF is going through the mesh and being filtered only for larger particles while some is passing through the media and being more finely filtered. (The mesh itself is quite fine as well, but is clearly less fine than the filter media). Again, the difference with the engine oil filter is clear. With the engine filter, all the oil is supposed to go through the media except when the by-pass valve is forced open due to excessive pressure drop across the media (e.g., blockage or thick cold oil or high engine rpm), at which time the unfiltered bypassed oil goes into the engine. In the case of the tranny filter, this doesn't happen. There is alway some degree of filtering. If an engine filter with a bypass valve is used in the tranny, it's possible that under certain conditions some ATF will go through the by-pass valve unfiltered. Perhaps this is a worse situation than might be for the engine (although even I don't find this convincing) and that is why the tranny filter is designed differently.
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Old 03-17-2010, 11:48 AM   #7 (permalink)
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I think the key was the insight by someone (thread by the guy that used the cooler line to change the fluid) that the cooler is pumped fluid that then goes back into the pan- it's not a 'supply side' cooler/filter.

On the filter-design side of things- Even if 90% of the fluid goes through the mesh, eventually all the fluid will be filtered through the media.

A 100% filtered 100% of the time solution seems to not be required because a properly functioning transmission simply does not produce crud or wear products at any kind of significant rate.

Also, does an auto trans have a single plain bearing, anywhere? That drastically changes the filtering requirements/crud tolerance.

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Old 03-17-2010, 02:51 PM   #8 (permalink)
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Just for further info, I disassembled the transmission filter so that you can see the mesh more clearly.

Here's the core part of the engine oil filter (left) and the tranny filter (right). The tranny filter media is dark because it is still saturated.



Here's a close up of the tranny filter interior with the closed top end of the mesh visible. Note that this is the outlet side of the spin-on filter:



Tranny filter with most of the paper filter media removed. After this I cut through the perforated metal tube at the center and the support on the side to separate the two end caps:



The mesh after the two caps are separated:



And finally, a close-up of the mesh. There are some specks visible trapped by the mesh:



Wix lists its 51365 filter for my 2007 4EAT automatic transmission. But the same list (see below) shows the same filter for the engine! The WIX spec sheet for the 51365 clearly indicates that the filter has a by-pass valve that operates at 8 - 11 psi -- not the same as the Subaru AT filter.

I also looked at a Pro-King 23013 that is listed for the Subaru automatic transmission and it too has a spring loaded by-pass valve rather than a continuous flow mesh screen.
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Subaru transmission filter IS different-wix-filters.jpg  
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Old 03-18-2010, 07:56 AM   #9 (permalink)
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About the blue filter, I must have misread something somewhere, I was under the impression they changed the trans filter too. Ignore that comment as the oil filter differences aren't super relevant here.

I under stand the rational of the screen and the theory that the fluid is eventually filtered through the media ... that makes sense.

Can someone explain to me the justification for the $19 difference in price when essentially they traded some mesh for a spring and plate ?
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Old 03-18-2010, 08:07 AM   #10 (permalink)
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Quote:
Originally posted by CNY_Dave
. . . .On the filter-design side of things- Even if 90% of the fluid goes through the mesh, eventually all the fluid will be filtered through the media.

A 100% filtered 100% of the time solution seems to not be required because a properly functioning transmission simply does not produce crud or wear products at any kind of significant rate.

Also, does an auto trans have a single plain bearing, anywhere? That drastically changes the filtering requirements/crud tolerance.

Dave
Indeed.

Perhaps it's also supported by the fact that prior to about 1999 the 4EAT did not have an external spin-on filter, and it was then removed sometime around MY2008. (Perhaps as much due to the confusion between the engine and AT filters by oil-change shops as actual need for fine filtering, but that's only speculation for now . . .)

Looking through Subaru training documentation I noticed (and mentioned above) that there are what I believe are simple mesh filters elsewhere in the transmission fluid lines, and I'm wondering if when the spin-on filter was eliminated in 2008, a simple mesh might have been substituted inside.

In any event, I'm now taking a closer look at the filter media itself. I have several samples from the different engine filters I tested last Spring, and will be comparing them as best I can to the filter media from the Tokyo Riko AT filter after I clean it. (It's amazing how much oil remains in the media -- it's like a sponge.) I'll report back, probably later today. (I'm trying to keep this to a comparison of the AT filter versus the engine filter, but there could be some overlap with the earlier engine oil filter discussion linked above.)
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