FLU419 stuck in 4-wheel mode, not engaging in differential lock

Are you applying air from an external source on the front axle or are you just turning the selector switch to lock and listening for air?. If you are not hearing air on the front with the selector in lock I expect you have a plugged air line upstream of it.

I have not done a locker seal but my understanding from several posts on the Benz World forum is that the parts are cheap but the axle needs to be disassembled to replace them.

Good Luck !

I used the SEE air, and I had first the air-line removed from the top of the axle and air is coming out and has the 106psi.
For sure the inside is clogged up.

a week has passed, but still no luck getting the differential engaging.
Next will be to open the axle and replace the parts.
Will do the rear first. Do I need to remove the entire axle, or can I just remove the side with the differential lock?
Anyone has experience in removing and opening the axle? Any tips/hints appreciated.

I would seriously consider getting it off the hillside before taking the axle off or apart.
Also, keep in mind that depending on whatever penetrant you're using, it can can take days, weeks, or even months, before things get freed up. If it's regular WD-40, it may never happen.
Another way to get the SEE out of there would be to put better tires on it. As I recall, for whatever reason you couldn't walk it out with the bucket and backhoe, but with good tires and using the brakes a bit (to mimic working diff locks) it may work.

Yes, I was able to drive it out. it was downhill to the exit anyways (uphill what was not working....)
Right now it's parked on asphalt with the rear-right wheel in the air. Ready for the work - once I got all the parts, which may take another week to find places to buy and ship.
Would I need a spring-clamp? or can I just drop the axle (after removing the wheel and the shock).

anyone knows of a mechanic in the north San Diego County area - who knows Unimogs?

I'd leave the tires on, making it easier to move the axle around. Also, for the removal, the backhoe and outriggers are your built in "spring compressors", making leaving the tires on more appealing yet.

Of course, digging into the axle would be my very last resort. Unless I absolutely needed the lockers to function within a few days, and had unlimited time (to perform the job), I'd simply let the penetrant do its thing. Aided by mechanical encouragement through the drain (if that is possible - I still don't know).

Easy for me to say, come to think of it. I won't need functional lockers until several months from now, when pushing three foot deep wet snow piles with chains on the tires.
Not that I'd really need the lockers then, either. I could just stop biting into more than I can chew. But it's unlikely that I'll learn to do that, so the lockers are a fast and convenient way to get going again.

here is a picture I took yesterday from the rear drain-hole:


I have till September to complete the septic system and this location is where two trenches need to go totaling about 130feet.
The size of the bucket is just perfect for the job and easily goes the 5' deep.
I also 'fabricated' an air-hammer module (3-way valve with electro-switch) to pulsate the air multiple times per second - may need to fine-tune the ON-OFF timing for best impact.

So out of curiosity, and maybe you already mentioned it, but what penetrant are you using? And are you filling the void through the hose end and letting it sit and do its job?

using Marvell Mystery Oil - which was mentioned in other posts for this job.
And fill it from the top, but it seems not much goes into it - if there was a way to 'suck' the air out at the same time.
Or maybe I find a way to push it from the drain hole till it pours out from the top - but how to close it?
Or should I remove the nipple to have a bigger hole - I can try this and use the bigger opening to get a tool in...

Everybody has their favorite, but I thought that MMO was more of a maintenance than a penetrating liquid.
Be that as it may, the penetrant should displace the air all by itself, even if you cap the system back up. Unless that air tubing is a really small ID. Not having had one off myself yet (I know, I should) I don't know how large or small it is inside.

Making an adapter for filling from the bottom sounds like a good idea, if nothing comes out when filled from the top.
Removing the fitting on top and try mechanical means would possibly be my first choice - especially if I knew what it looked like inside, and was in a hurry. And you may be able to get a peek through the hole.

is there a better penetrating oil? I don't mind using other one.

For almost anything as far as every day use goes, I use JB-80 (aka known as "Twice as good", which I think is an understatement).
When it comes to very stubborn stuff, I use a 50/50 mix of acetone and ATF.
Not sure how this mix would act on the seals, so some caution may be in order. Without reading up on it first, I'd definitely be hesitant to leave it in there for days.

Thinking more about your problem, specifically assuming that the piston is stuck in place and will not slide to engage, getting higher pressure might be the key. Where are you putting in penetrating oil? At the differential? What if you disconnected the tubing and screwed in a Zerk fitting, as for a grease gun. Then fill a grease gun with heavy oil and start pumping.

You are at a point where disassembly might be required anyway so if you blow an o-ring, no big deal. With the size of the piston, a grease gun can exert huge pressure, so use good sense. Obviously the idea here is to displace the air with oil so you can build pressure. You should feel the pressure in the pumping force.

While you are applying pressure keep in mind that the piston and dog clutch (the square teeth that engage) will not engage unless the alignment is correct. You would have to rotate that lifted wheel while applying pressure. When the alignment is right, the dog clutch will engage.

Did some search on JB-80, but it's probably 'so good' it's not available in California. Found it on eBay - but it would be another week delay.
Any of those at Grainger I could probably get the next day:
https://www.grainger.com/category/penetrants/lubricants-and-penetrants/lubrication/ecatalog/N-118b

I also thought about adding a grease-nipple and use the grease-gun, but it's (1) a metric threat and (2) if I can get it going how to remove the grease?

Update:
OK I think the drain-screw is a M10 (but have not measured), so I could get:
https://www.grainger.com/product/GRAINGER-APPROVED-Grease-Fitting-5PU18

That's odd, considering that Justice Brothers is based by the 10/605 intersection. Too good for CA is probably a correct guess on your part, though.
Anyway, there are many other penetrants, and I once found a study of their effectiveness - which is how I found out about the acetone/ATF mix. If you find it, take your pick from that list. I don't remember the results.

Bel-Metric (ask for Matt) should have the hardware to fit the axle. Not sure if taps are readily available for Zerk threads, but then it wouldn't have to be very precise.
Use a basic, crappy grease and it'll "wash out" with oil.

Note that oil was suggested for the pressure test. A grease gun will pump oil too, you just might have to keep the tube/cylinder held high to keep down the mess.

Oops. Forgot about that part. Sorry.

Found this page:
http://www.oocities.org/nharrison314@rogers.com/dansunimoglockerpage.html
it also shows the piston and dismantling of the U1300, but should be very similar to the U900.

Dang! I really want to see that illustration, but Malwarebytes won't let me.
Does it look like you could poke things physically, or are the moving parts inaccessible?