[plain OM]

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 (http://www.subaruoutback.org/forums/showthread.php?threadid=18231).

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as the black Tokyo Riko filter that is used for the engine (or at least was available until last year) but has since been replaced by the blue Honeywell-made filter (at least in North America).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.

 

[miker104]

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!

 

[bikemobile]

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."

 

[TheFixer253]

Or you can just get one from www.subarupartsforyou.com for $25 plus shipping.

 

[ETC]

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 ?

 

[plain OM]

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.

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.

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.

 

[CNY_Dave]

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.

Dave

 

[plain OM]

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.

 

[ETC]

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 ?

 

[plain OM]

. . . .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.)

 

[plain OM]

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 ?

The transmission oil filter did go through a part number change (but, I believe, the same manufacturer) sometime after the filter was introduced on the 4EAT in 1999, but I'm not sure exactly when. (The earlier p/n is 38325KA000, now superceded by 38325AA032.) The AT filter I have is the earlier one. [Edit: for comment on the later one, see my post 03-19-2010 below.] My source for filters, which is my Subaru dealer's service dept., has so rarely changed them that I haven't yet received one of the later ones. (The parts department stocks the engine filters by the case; but they order in the AT filters a few at at time -- there's no need to keep them in large numbers!)

The price issue is interesting. It's not only the Subaru AT filter that is more expensive. Some aftermarket filters that are labelled and boxed as AT filters but are still made the same as the engine filters, that is, with a by-pass valve, are also more expensive.

 

[ETC]

That's what I'm getting at, seems like a scam.

So, lets say the screen material is more expensive and harder to manufacture .. add a couple bucks a filter and, they aren't ordered in as great a quantity, add another couple bucks. Now you've got a $10 filter that sells for $22 bucks.

Seems reminiscent of the BMW MAF for my wifes 740. Same exact Bosch part number for the exact same motor in a Land Rover ... BMW $300 ... Land Rover [from the dealer] $140 ...

I've had 0 issues with the NAPA filter so far but, I'm interested to see what's inside and why is was $22 bucks ;]

 

[ETC]

I just checked Napa

The list the FIL 1365 as both an oil and a trans filter but it's only $5.09. The oil filter for my 00' is listed as a FIL 1334.

I'm going to go check what's on my car now and like I said, I'm interested to see what's in it. I know it was $22 bucks when I bought it because I thought it was ridiculous then.

Just checked it

White can, no brand or anything stamped on the round surface. On the end in small letters it says "made in indonesia 18 05 06 p"

Guessing it's just some generic oil filter but, have to crack it open and find out. Might do it just for the **** of it ... have to order a "premium OEM" unit to replace it first ;]

 

[plain OM]

You probably wouldn't believe the list price of the AT filter here in Canada!

While the price difference (versus the engine oil filter) on either side of the border might not seem to be justified by the apparent physical difference, I can't discount the possibility that the mesh screen might be expensive to make due to it's fineness and perhaps use of some rarer metal (it's not magnetic; it is woven; could be stainless steel filaments), or if the media itself is significantly different.

Also, from the FHI and Tokyo Riko perspective, the filter is rarely if ever changed, so production of the filter is limited to original factory assemblies plus some for repairs and a maintenance buffs who insist on changing it, and all this applies only to a limited series, primarily 1999-2008. In the world of filter manufacturing, this is not a high production item, and that will impact on costs and prices.

Finally, from the user perspective, with what we know about the filter, there is a rationale for less frequent changes. And even for those who are more rigorous in their maintenance routines than is required, the higher cost of a (significantly different) Subaru AT filter spread over many miles and years of the car's life would represent a relatively small amount in the cost of maintenance.

Now, turning back to the examination of the filter itself, I spoke with two additional dealership service departments about the AT filter. Both said they "never" change it; in other words, my quest for a used later version to examine internally is frustrated. But I was able to examine a new AT filter (p/n 38325AA032). It is slightly different from a visual standpoint. The holes in the center support tube (that keeps the media from collapsing inward) are a bit smaller. But the more obvious difference is a metal cap on the end of the mesh "tower". (Newer version, unused on the left; older version, used, on the right; not taken at the same distance.)

I managed to rig up a tiny mirror that I could pass between the wider center support tube and the side of tower and confirmed that the new cap is indeed on top of a 360 degree mesh tube. I couldn't see if there is the same open hole in the bottom but for the time being I presume that is the case. As in the earlier version, there's a metal ring around the base of the mesh tower. The ring sandwiches the end of the mesh (which is flared out) between it and the end cap, and is spot welded in four places, thereby sealing and holding the bottom of the mesh tower in place.

So as far as I can tell, this filter also differs from the typical engine-type oil filter in that there is a continuous flow through a mesh screen, perhaps partial or parallel flow through the media, and no by-pass valve or by-pass function.

(As a consequence of examining the AA032 filter, the appearance of the "cap" leaves me wondering if the folded mesh that is apparent at the top of the tower in the older filter was originally covered by a similar cap that came off at some time -- again, whether that is possible will have to wait until I can get a new version filter to open up and examine how the cap is attached.)

Which brand of AT filter should be used? As with the engine oil filters, based on comments here and in other Subaru forums, there are lots of Subaru engines fitted with aftermarket engine oil filters that don't have the 23 psi by-pass valve, and probably many 4EATs that have engine-type spin-on filters installed on them (or on the remote mount in the left front fender found on some models). My objective here is to identify differences between the Subaru product and typical aftermarket brands. The decision on which to use is left to the reader who I hope can now make a better-informed decision, whatever that will be.

 

[ron917]

Plain OM,

I have a used 38325AA032 that I took off of my '07 last summer. I saved it for future dissection, but haven't gotten around to it yet.

How did you disassemble the filter without completely destroying it? If I can do that, I'll get some pics. Mine is dented (very difficult to get off the car), so there is likely some damage to the media.

As an alternate plan, I can mail this one to you, if you want it. Not sure what Canadian customs would think of a used oil filter....

 

[plain OM]

Here's your opportunity!

I use a rotary ("Dremel") tool with a cutting disk. I hold the filter in a strap wrench/clamp. Works fine. (Attached pic shows a mock-up of how I do it. I normally work outside on the picnic table -- saves the workshop table and floor from dust and drips.)

Cut around the can about 1/4 inch from the mounting plate end (I follow the lower printed white line). Don't cut too deep. It's a slow process (to avoid breaking or wearing out the cutting disk). I rotate the filter periodically to keep the cutting area near the top. When the cut is complete the mounting plate will fall away, but the main filter assembly will usually remain inside.

When I first started with the engine filters, I used a hack saw, cutting around the same line; it worked okay, but has two disadavantages -- very difficult to hold the filter in place due to the teeth catching, and lots of metal bits inside. After the first one, I have used the rotary tool exclusively.

Because of the remaining oil inside, I start the cut, opening about one inch at the top, then turn the filter so the cut is at the bottom and it can drain out into a container. Takes some time to fully drain . . .

If the can won't let the internal filter assembly inside drop out after the mounting plate is removed, I think a narrow screwdriver can be used to pry it away, levering the screwdriver on the metal top plate. It should be solid enough.

I do appreciate the offer to mail it. However, packaging so that it won't leak and trigger alarms might be more of a challenge.

 

[plain OM]

. . . .(As a consequence of examining the AA032 filter, the appearance of the "cap" leaves me wondering if the folded mesh that is apparent at the top of the tower in the older filter was originally covered by a similar cap that came off at some time -- again, whether that is possible will have to wait until I can get a new version filter to open up and examine how the cap is attached.)

Which brand of AT filter should be used? As with the engine oil filters, based on comments here and in other Subaru forums, there are lots of Subaru engines fitted with aftermarket engine oil filters that don't have the 23 psi by-pass valve, and probably many 4EATs that have engine-type spin-on filters installed on them (or on the remote mount in the left front fender found on some models). My objective here is to identify differences between the Subaru product and typical aftermarket brands. The decision on which to use is left to the reader who I hope can now make a better-informed decision, whatever that will be.

I finally got a later version of the filter, the one that is now supplied with AA032 at the end of the part number. It's the same Tokyo Roki black case, and for the most part is identical inside as well. (I can't assess the filter media as I have no way to accurately check fibre types etc). There is only a difference in the mesh tower, as noted earlier.

First, the top of the mesh tower on the older version had the screening folded over and spot welded. This is clear in the pic on 03-20-2010. The newer version has a cap. There is no folding of the screening at the top; instead, the screen material is welded around the circumference to the cap.

Similarly, at the base, the screening is welded to the ring, and the ring is then spot welded to the bottom cap with a sealer in between. (Note: the mesh tower was perfectly straight, but became a bit twisted during removal.)

Here we see the reddish colored sealing material around the outer edge of the metal ring, as well as the spot weld marks where the ring is attached to the larger bottom cap of the filter

So, once again, it's apparent that the Subaru AT filter is notably different from the typical spin-on oil filter used for the engine and some aftermarket filters listed for the transmission. The distinction is that although latter two have pleated fabric filter media as in the Subaru AT filter, they also typically have a by-pass valve that opens when the pressure drop across the media is above the valve's threshold. The Subaru AT filter, in contrast, has an opening in the bottom cap into a mesh screen tube. The flow through the mesh is obviously relatively unrestricted.

My next step is to find out exactly where in the hydraulic circuit the filter is located and how a restriction in the filter would affect fluid flow to the rest of the transmission.

 

[Sir Sawgrass]

When I had my 2001 OB I tried a few time to take it to other places to get an oil change other than the dealer. I would always point out that, what looks like an oil filter was a transmission filter and to leave it alone. Even in doing so one garage grabbed it with a pair channel locks and put a pin hole in it. (Young kid) left it in place for me to drive home with transmission oil covering the underside of the car. So, I took it back in which the Y. Kid said he didn't do it. The owner of the shop agrees to replace it and did. Turn out to be an aftermarket brand, in which I replace the next day with a Subaru Transmission filter. I just did not know for sure if there was any different. After this I drove the 200 mile round trip to the dealer to do an oil change. I am unable myself to do oil change.

Thanks for taking the pictures and posting. Learn a lot on this site.

 

[CNY_Dave]

I'm wondering if there is no bypass valve because the pump in question is not designed to provide much if any pressure.

Anyone have a manual pic of where the fluid comes from and goes, and what the pump is?

Maybe it's actually the return from some hydraulic circuit in the trans that can't tolerate any backpressure at all.


Dave

 

[plain OM]

I'm wondering if there is no bypass valve because the pump in question is not designed to provide much if any pressure.

There's only one pump in the transmission, and it is capable of pressures well over 200 psi. (The higher pressures are required by the clutches used throughout the tranny, including the AWD.)

Maybe it's actually the return from some hydraulic circuit in the trans that can't tolerate any backpressure at all.

That's probably closer. Some models have the spin-on filter mounted in the left front fender where it's quite obvious the filter is in the oil lines going to the ATF cooler in the radiator. But even in this case, the question remains as to where the cooler lines originate in the transmission, and what impact a restriction could have.

Neither the filter nor the ATF cooler circuit could likely handle the peak line pressures used by the transmission. So, yes, it's more likely the filter is in a pumped, but relatively lower pressure circuit.

If the filter is in series with the ATF cooler circuit, the source might be the area that generates the most heat, which I believe is the torque converter.

Anyone have a manual pic of where the fluid comes from and goes, and what the pump is?

Been through a number of FSMs from mid-09s to 07 and haven't found it. Also reviewed the 4EAT training modules -- they have some hydraulic circuit diagrams, but none including the location of the filter in the system.

The FSM Mechanism and Function section has this description of the pump:

"The pump draws automatic transmission fluid (ATF) from the oil pan through the oil strainer located under the control valve assembly. The ATF then flows through a passage in the transmission case, and after passing through the oil pump housing and oil pump cover, it enters the suction port. As the inner rotor rotates, the outer rotor also rotates. This motion causes the ATF to be sucked up through the suction port and discharged from the discharge port. The discharged ATF flows through a passage in the oil pump cover and then a passage in the oil pump housing. It then goes through a passage in the transmission case to the control valve assembly, from which the ATF is directed to various clutches, brakes, and torque converter lockup clutch for acting as hydraulic fluid and lubricating oil. Part of the ATF also flows to the manual valve, from where it is distributed to the circuit corresponding to the range selected by the selector lever. As engine speed increases, the delivery rate of the oil pump also increases."

Might have to try writing to Subaru to find out more . . . .