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

could not find information, so now posting this question/request for help.
Am stuck on a hill side and the transmission is not getting into differential lock.
If I turn the switch form II to III position I hear no click (not sure if I should - since I hear a click when shifting High/Low).
there is a air-puff when moving switch from III to II position.

Also, none of the instrument-lights are on, but that seems to be a main-fuse issue, since also the green LOW gear light is not turning on (nor the buzzer or emergency blinkers work either - but the heater does). Oh yes I checked all 24 fuses and all are OK.

Any thoughts appreciated.

Even if that hill side is close to the house, I'd try to use the loader and/or backhoe to get unstuck first. Then, once back home, I'd start the trouble shooting.

Stuck diff locks apparently aren't uncommon, but did they work before? If not, try spraying some penetrant up into the "drain" holes. After removing the plugs, of course.
I don't recall hearing any clicks when engaging 4WD or the diffs, and I wouldn't expect to. I'm pretty sure it's all air operated and nothing but.
Does the air pressure go down when engaging 4WD or the diffs? If not, the system probably works fine, except for the stuck parts in the axle(s).

I'm no expert on the indicator lights (either) since I almost never have them turned on. But I do know that they often show things other that what they're supposed to.
If memory serves me, the 4WD light comes on when engaging the PTO in one SEE...or something along those lines.
Lastly, that the fuses are good doesn't mean that they make good contact in the holders. Or, if you bought your SEE from Texas recently, chances are that the rats modified the wiring, and not for the better.

Update to the instrument lights - silly me the NATO light switch was in the wrong position.
The left light is for the PTO, which turns on/off as soon as I move the PTO shifter a bit.
The second is for the differential lock and turns on now when I move the switch to the differential lock position and turns off when switching in AWD position.

The instrument seems to work now. Even now with 'confirmed' differential lock only one wheel each axle is spinning.
Is the instrument light simply based on the switch position or is there a sensor on the differential?
I do not see any drop in air pressure when switching.

Thanks for the tip with the penetrant, will try it next time I am going to my property.
I have hardly used the lock and if so I may not have noticed it's not working since AWD may have done the trick, too.

One more thing, when I switch to rear-wheel drive there is a bit of an air-leak, I did not used to (as far as I can tell)

You may want to do some research on exactly where the differential lock drain plugs are, most are painted over and never have been drained. The manuals are completely silent on this procedure. I did post a pdf of the owners manuals page from my 1300 which has a similar setup at some point on the huge Unimog thread. I believe that the differential lock indicator is strictly an indication that air pressure has been applied to the differential locks versus they have engaged.

I haven't spent anytime trying to figure it out, but believe that the drivetrain (axles and possibly the driveshaft tubes) are pressurized to prevent water from getting into the components when operated under water. I expect that the system is the source of air leaks when 4wd is engaged, Unfortunately I cant tell you what is normal.

Basic Unimog maintenance calls for manually injecting anti-corrosive oil into the air lines going to the differential locks. That oil is needed to lube and clean the Pistons and seals of the lockers. Inject oil by removing an air line and squirting in oil. Operate the lockers, then remove drain plugs to expell excess oil. Imho there's not much point in putting oil into the drain plugs.

Edit:. There's another possible issue with supply air. Do you know your air pressure is good? So called 'priority' actions, like brakes, are in the air circuit ahead of the over-pressure valve and could be working fine, hiding low pressure beyond the over-pressure valve. Lockers are secondary actions and might not be getting air, hint, no clunk heard. I hear a clunk on my 406s under some circumstances.

Question I can't answer:. On a SEE is 4wd a priority action? What about transmission shifting? If 4wd and trans shifting are secondary, and are working, then the lockers problem is not an air supply issue.

tennmogger said:

Basic Unimog maintenance calls for manually injecting anti-corrosive oil into the air lines going to the differential locks. That oil is needed to lube and clean the Pistons and seals of the lockers. Inject oil by removing an air line and squirting in oil. Operate the lockers, then remove drain plugs to expell excess oil. Imho there's not much point in putting oil into the drain plugs.

Click to expand...

Not that I've tried it myself yet, but apparently the air lines usually won't come off as intended, due to paint and/or other reasons.
Unless one wants to replace or repair the air line - which may not be a bad idea since it would facilitate future maintenance - that leaves getting lubricant in through the drain plug holes.

Again, I haven't had to do it yet. All I've done so far is to remove the plugs for a peek. A few had oil dripping out, some were dry and had some debris on them. The latter I "lubed from below".

I'd think that the axle pressurization system, and probably the 4WD/diff lock part, too, is low pressure. Still, with one wounded hose at the steering knuckle, that relatively small leak on one FLU is enough to only allow 4WD (or diff lock) use for about 45 seconds at the low rpms I tend to run.
I think the FLU could benefit from higher tank capacity no matter what, but obviously that leaking hose should be fixed. One of these years it may make the list.

Thanks for all you inputs so far.
looking in 'TM 5-2420-224-24P-1_Parts Tools' page [FONT=Arial,Bold]0074 00-2 (306 in pdf) it shows figure 73 with 4 sensor switches, but does not identify which is which, however it shows they are all on the transmission and not on the axle.
My FLU has 194miles and shows 71 on the hour meter would this indicate a high likelihood that the lock is stuck on both axles?
To make it un-stuck I like the idea to remove the air-line and inject lubricant.
Some questions:
can I use silicon based lubrication spray? Or what is recommended, available from the ordinary shop?
Also when I remove the line on the axle, I suppose there should be airflow when the locks are on.
To un-stuck can I apply my own air pressure and what is the psi I should not exceed.

On another note I think the same air-supply is used for the RPM switch on the backhoe and this is working.
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Just about any liquid lubricant can be injected. I have injected motor oil and tractor fluid (hydraulic oil) and all is well. A pump oil can is handy for this job. The lockers operate on full system air pressure so no problem using shop air to engage them.

While you have the tubing disconnected is a good time to verify the pressure being applied to the lockers.

It's a valid point that damage to the hoses or tubing should be avoided. One of the first tools needed for this kind of work on any Unimog is a set of metric tubing wrenches. Plain open end wrenches can easily round the corners of a tight tubing connector. The only mog I have had trouble on with these fittings was one that was completely rusted out underneath. In the case of your SEE, you should have no problem opening the line to inject oil.

Bob

On my SEE, the air lines connecting to the axles can not unscrew without cutting the air lines as the nut doesn't spin independent of the air line. That means that as the air line fitting rotates, the air line has to twist up. I realize that's not the design but rather is probably related to overzealous application of paint. I realize that I could cut the air lines and then get splice kit but would prefer not to go to the trouble.

I was lucky, today I disconnected the air-tubing from the axle. It's a 14mm wrench and actually no problem, any paint on the tube came right off.
The volume inside the axle must be very small, the lubricant filled it up quick.
I then cut the tube about 1 foot along the drive shaft where it was easily accessible and connected the air supply side first to a 1/4" quick disconnect coupler to hook up an 1/4" poly-tube long enough to reach into the cabin with a pressure gage on the other end.

The pressure was right along the working pressure and turned on/off as I toggled the differential lock switch from AWD to diff-lock. So this would eliminate any air-supply issues and really leaves a stuck differential.
With the quick connect coupler in place I reconnected the original line and screwed the nut back in place.
Not sure how long I should wait for the Lub to penetrate, but still after 30min I cannot hear anything moving when applying the air (rest air-pressure with engine off).

Next I lifted both rear wheels using the backhoe support legs and was able to turn the driver side in one and can see the other wheel turning the opposite direction.
In the hopes that by 'wiggling' the wheels the differential lock would kick in. I suppose the wheels would just turn the same way with or without differential lock.

Could any of you who have it working check if you could hear anything with engine off and just turning the knob? Perhaps getting closer to the axle and have an assistant turning the knob.

Also can I use a hand-pump (or foot pump) to apply pressure above 106psi? how much could I apply?

The wheels will turn in opposite directions with an open diff since there's less friction and resistance in the diff than upstream from the pinion.
After you prior post I wondered if there's a way to "poke" the locking mechanism manually, freeing things up a bit quicker, but never did look up an exploded view of what's in there. You may just have to wait while the penetrating oil does its thing. It can take days and weeks for corroded things to free up, even when using good stuff.
It may be helpful to leave the lines pressurized, as it should help force the penetrant in.
Oh, and another thought; How about dumping whatever you put in there and filling up with Evapo Rust or something similar? Most of those liquids are perfectly safe to use on seals and such. After all it should better to get rid of the rust than to just free things up.

If the loader stand gets built today, I'll fire a SEE up - and will try to remember to listen for any sound effects from the lockers.

Update: Tried the lockers while stationary and engine off. Couldn't hear any sounds from the lockers, and I tried several times. If they do make a sound (which rightfully they should), it can't be heard from the cab.
By the way, is the SEE still on the hillside or did you drive it home?

Thanks for checking on the sound, but your differential lock IS working, right?
With both wheels lifted off the ground, do the wheels turn the same direction with locked differential?

I am looking at figure 139 in TM 5-2420-224-24P-1: is this the spring which would push back the lock? and the parts to the left which would move?

my SEE is till on the hillside, the next rain is probably a couple of months away, so no need to move it for that reason. But I would need to get going with digging trenches on that hillside though.

Hmm, good question. The lockers did work last winter, but that doesn't mean they do now.
I need to look at that figure 139 and see if it looks like something I'd understand (I'm a Detroit Locker kinda guy, not overly familiar with air lockers - other than having participated in numerous air locker fixes on the trails, all external, of course).

I'll be firing the SEE up again tomorrow, so I'll try to remember to engage the lockers and find out if they still work.

Addition: Yes, I'd guess that the spring is what normally keeps the dog gear disengaged, then air pressure is supposed to overcome the spring's tension and lock the diff.
I need to look at other illustrations to see how the air part works.

I guess it could make sense that there is no noice since the air (unless it would be VERY rapid) filling up the room and pushing the spring slowly as the pressure builds up, so it could be a soft touch.
But then I was able to hear when I use the low/high shifter.
Back to the wheel turning test: I am guessing that you would need to be in neutral so the drive-shaft can turn when you turn the wheel.
Or since all 4 wheels are locked there is no turning possible - unless you have all four in the air. In any case that would be a good indication that the locks work without driving.

RoJoGro said:

Back to the wheel turning test: I am guessing that you would need to be in neutral so the drive-shaft can turn when you turn the wheel.
Or since all 4 wheels are locked there is no turning possible - unless you have all four in the air. In any case that would be a good indication that the locks work without driving.

Click to expand...

No, I can try the 4WD and locker functions while driving and turning. Not that I'd want to do it on dry pavement, of course.
At least to me, it's very easy to tell if 4WD is engaged since the steering feels different and there are new (tire slipping) sounds and a different feel associated with making a turn. Also engaging the lockers exaggerates those sounds and the different feel of the vehicle.

That's one reason I'm not hung up on having working indicator lights. If I put the vehicle in 4WD and it feels like it's in 4WD, it probably is. Did I forget to engage or disengage 4WD? Reaching down to feel the switch answers that question.
Then again, there's been a few times where on a certain night run I forgot to engage 4WD in my Jeep. Always trying to do as much of the trail as possible in 2WD Low, I simply forgot. That could very well have been beer induced, but as long as the Jeep kept climbing the rocks it really didn't matter.
In those cases an indicator light would've helped me remember as it was too dark to see the lever (and a bit of a reach to feel it), but I would not want a needless indicator light ruining my night vision.
Sorry, this was a bit of a rant, but the thought of switches rather than levers, and indicator lights to indicate what I did or didn't do with my hand gets me wound up.
Actually, I'm surprised that new vehicles don't come with a light telling us that the wipers are on. They already let us know if we opened the door, put the key in the ignition, turned the lights on, and there's usually even a visual indication of the volume level on the stereo.

I'll let you know if the 4WD and lockers do work later today.

the pressure applied from the SEE is about 106psi, since I cut the line and added a coupler I can connect to another compressor and apply more psi.
I do not have one which produces higher PSI, but there is a 12V high pressure compressor up to 150psi I would invest the $50 in - is 150psi still acceptable for the mechanics in the differential - especially I am worried about the packing-ring (item 14 in figure 139).
What would be the highest psi I should be applying?

Sorry, I forgot all about checking diff locks today. Got caught up in a celebratory mode after removing a loader, removing a blade, and installing that blade, all without killing myself. Or even bleeding much.

Anyway, I would be very hesitant to throw more pressure into the axle(s). Leaving the penetrant to do its job seems far safer to me.
I'd rather try to poke whatever is in there, if it's accessible through the drain plug. Still haven't found anything showing those parts of the axle.

The main air tank has a over pressure relieve valve, That psi value would be the max I should be able to apply onto the locks, right?

The actual max air pressure seems to be all over the map on FLUs, so I guess that you could pretend that yours was set higher.
Still, since turning the switch on and off should basically accomplish the same thing as adding air from an external source, you could sit in the seat and hope that the locking mechanisms will eventually free up.
Let's hope that someone who has already done this, successfully, will take notice and offer up a cure. I think it has been described in the main (SEE, HMMH, HME) thread, but I don't have time for my third re-read at this point. Besides, every time I read that thread, more things come to mind that I should look at or fix.

from another post I saw the importance of the drain-plug. so I removed to drain any condense-water.
Well there was nothing coming out.
on the rear axle I could actually spray lubricant and it eventually came out the drainage hole, and so did air when pressure was applied - right now that side is soaking in Marvels mystery oil.
On the front however, not only no oil comes out, but also no air if pressure is applied - appears to be completely filled up with whatever!?

Any info on what minimum work to remove the pieces without dismantling the whole axle?