I've removed the center differential part(s) from a 2001 subaru legacy to find that one of the internal parts lost its alignment/position (began to move axially) and began wearing away small areas of the aluminum housing. This has resulted in fine aluminum particulates in the gear oil. To clarify, the center differential is in a aluminum housing bolted to the rear of the transmission, sharing the gear oil lubricant. I have not removed or disassembled the transmission itself. Disassembly of the transmission is not likely to happen.

I'm hoping to flush out as much as I can with a couple drain/fill cycles of gear oil. However, I realize that this will never completely remove all of the particulates.

My question: Will any remaining particulates cause further problems internally in the transmission? Would they wear out bearings or other internal transmission parts? I assume the hardened steel gears can deal with the contamination. I checked a parts diagram and don't see much other than roller/ball/thrust bearings, splined shafts and gears in the transmission.

Edit, follow up question: Would using an lighter or different oil in the transmission for a short time (1 of 3 flush/fill cycle) help wash down particulate/contamination and or would that cause other problems with the residual oil that would remain in the transmission.

Parts diagrams/reference from Subaru

  • Welcome to Motor Vehicle Maintenance & Repair! I take it the tranny and transfer share fluid? You replaced the transfer? – Pᴀᴜʟsᴛᴇʀ2 Sep 9 '19 at 15:32
  • Thanks, yes both the transfer gears/diff and transmission share the same gear oil. The plan is to replace the failed center diff part, and a couple bearings and oil seals in the transfer section if the particulate contamination wont cause more problems soon. If the contamination will cause transmission bearings to fail then this project may be abandoned and vehicle scrapped. – alex Sep 9 '19 at 15:52
  • I've not dealt directly with what you're talking about, but I have a feeling the transmission would need completely cleaned, if not a rebuild as well. The aluminum as it floats through the oil, would get squished between the gears as they mesh. This, in turn, would most likely weld the aluminum to the gears. The gears may still go around, but it's going to possibly do a couple of things. First, it may throw clearances out of spec, which will cause excess wear. Second, it's most likely going to be noisy. This, in and of itself, is not an issue, but it's going to be terrible to drive. – Pᴀᴜʟsᴛᴇʀ2 Sep 9 '19 at 16:40
  • If it's only a little aluminum, you have a good chance of being okay.. While it's obviously not good, assuming you flush the fluid, and the transmission seems to work, just go with it. Even if some aluminum is attached to the gears, the gear wear pattern should wear through the aluminum, allowing the gears to continue to mesh like they're supposed to. Also, given that the particulates were very fine, I doubt there would be clumps causing the aforementioned welding, rough operation or incorrect clearances. So my opinion is to fix the issue you see, and it'll probably be fine. – the_storyteller Sep 9 '19 at 16:58
  • I appreciate the input. Can you expand on what you mean about the welding of aluminum to the gear faces? I didn't think that would be possible or a concern. The worst has already been done in terms of particulate density in oil, driven @ temp for probably 75-100mi with this condition. I should see evidence of that on the transfer gears, but hadn't noticed anything (yet). Assuming I refill/drain 3 or more times, the particulate density should be much lower then. My concern is about accumulation in the recessed areas like bearings and splines. – alex Sep 9 '19 at 17:03

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