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LEGACY VENDOR
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Discussion Starter #1
I’ve posted this just on the off-chance that, out of the 6000+ members of the forum, someone might know something about the hard anodizing of aluminium?

I’m currently repairing a 5HP24 transmission from an ’03 petrol L322 - the owner was complaining about poor shift quality - and the first obvious thing I noticed was that someone had filled the transmission with Dexron (red) instead of Esso LT71141 (amber) fluid. The fluid was very new looking so I guess the fluid change had been a recent attempt to cure the shift problems – just a shame they’d used the wrong stuff.

However, now I have the transmission completely stripped, I can see that several of the spools in the valve block are suffering from some type of surface delamination. The three example spools in the photo are from the modulation pressure valve, the ‘B’ clutch valve and the main pressure valve.

Clearly the spools are made from aluminium (to avoid differential expansion with the valve block) and I expected them to be hard anodized for durability. They certainly have a dark-coloured surface coating of some kind and it is non-conducting (like anodizing) yet it can be relatively easily scratched with a file (I understood the anodizing produced an incredibly hard surface). Anyway, whether it is anodized or just some form of coating, the surface treatment is clearly becoming delaminated somehow and this will obviously affect the performance of the transmission’s hydraulic circuit.

I just wondered if anyone recognised this phenomenon and maybe knew what caused it?

My concern is that possibly this is a common failure of this transmission at higher mileages and that others amongst us with ’02-’05 petrol models might end up suffering the same symptoms at some point.

Phil
 

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hi phil,
it looks like it has worn a little of the dye off, hard anodizing can be as little as 25 microns thick, but still needed caustic to remove the plating coat. are these in oil all the time? if not then i recon that a little of the anodizing has worn and revealed the ali underneath thus corroded a little . oh by the way i used to do anodizing for 6 years, but without it in my hand the pics are still hard to judge.
sorry for the little info
ian
p.s we only used a 02 type file to check that it was hard enough which is quite small but that was the legal limit so to speek. the plating on these bits might have been in the process for that bit longer and been over plated making them that bit tighter in the holes so less tollerances.
 

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Phil my knowlege on this is a little limited as I'm locks rather than transmissions, but from my experience anodising chemically converts the surface of the ally and as such wont flake off and de-laminate as those spools have. It looks more like a plating process to me that is indeed coming off of the ally.
 

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Locky said:
Phil my knowlege on this is a little limited as I'm locks rather than transmissions, but from my experience anodising chemically converts the surface of the ally and as such wont flake off and de-laminate as those spools have. It looks more like a plating process to me that is indeed coming off of the ally.
Yes, anodising effectively corrodes the aluminium, converting the surface to aluminium oxide, which as you say Phil is extremely hard and also has a much higher melting point than aluminium. If you wanted to anodise that I think you'd need to get all that flakey stuff off or it wouldn't be smooth.

By the way, I only know this because I got really interested in boat construction, but my experience is purely theory!
 

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LEGACY VENDOR
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Discussion Starter #5
Thanks all for your comments. I must admit that I’m coming round to Locky's point of view that the stuff peeling off is some kind of coating/plating process as opposed to an anodized surface which surely wouldn’t behave like this :? .

These spools sit in the valve block assembly of which the majority is permanently below the fluid level line, so they are continuously immersed in ATF. It’s obviously extremely unlikely that there’s some chemical compatibility issue with Dexron which doesn’t exist with LT71141.

This is the first time that I’ve seen this issue with the 5HP24 transmission (& I’ve stripped some high mileage Jaguar & BMW ones too, as well as the Range Rover ones). Clearly I have no choice other than to replace these valves – plus one of the valve blocks too as its bores have suffered as a result – so I’m just hoping that this is a one-off caused by a temporary glitch in one of ZF’s material processes.

Otherwise it’s potentially a fault from which many of us (with petrol ’02-’05 models) might suffer transmission shift problems in the future. I sincerely hope not :pray:

Phil
 
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