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First Day with G500, First Problem (Center Diff Lock)

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smoltz
Posts: 3

First Day with G500, First Problem (Center Diff Lock)

Post#1 » Sat Feb 28, 2015 1:39 pm

Picked up a low mileage, dealer maintained 03 G500 today. Meant to check that the diffs functioned properly on the test drive, then forgot to, went to pick it up today, left the dealer, tried it and having some issues. The center diff lock engages in a reasonable amount of time, but when I try to disengage it, the orange light turns on, but the red light never goes out. After driving for a while with it engaged, it eventually disengaged. So I thought it might just have been sticky, but I have engaged it several times and it just doesn't want to disengage when the button is pressed again. I just downloaded this very useful PDF (http://www.mercedestechstore.com/pdfs/G ... (ACB-IC-TF)%202-05-03.pdf) so I have an elementary understanding of the system at the high level. But where do I go from here? When the diff is engaged, and I push the button to disengage it, I hear what sounds like the vacuum pump running. That seems odd to me since disengaging should just involve releasing the vacuum pressure. What else am I missing here?

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AlanMcR
Posts: 4581

Re: First Day with G500, First Problem (Center Diff Lock)

Post#2 » Sat Feb 28, 2015 4:58 pm

The dog clutches won't disengage until the stress is taken off of the teeth. Off-road, on uneven and slippery terrain, that generally happens quickly, but not instantly.

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G Nome
Posts: 4211

Re: First Day with G500, First Problem (Center Diff Lock)

Post#3 » Sat Feb 28, 2015 5:01 pm

Try doing engaging and dis-engaging while making some turns both ways on not dry pavement.

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syncro_G
Posts: 821

Re: First Day with G500, First Problem (Center Diff Lock)

Post#4 » Sat Feb 28, 2015 7:52 pm

...or just save your testing for when you're in a dirt lot.

smoltz
Posts: 3

Re: First Day with G500, First Problem (Center Diff Lock)

Post#5 » Sun Mar 01, 2015 6:44 am

I had a GX470 with a locking center diff and a Tacoma with a rear locking diff, so I understand that if there is any load it won't disengage and that sometimes it requires a little coaxing.

I took it to an empty snow covered parking lot today and it exhibited the same behavior it did yesterday. It engages quickly, but it won't disengage. I tried driving staight, slight turning. Stopping, backing up, going forward again. None of those attempts worked. It exhibits the same symptom in that when the diff is locked (red light on), and I press the button to disengage (causing the orange light to come on as well), I hear a vacuum pump running. If I press the button again (to cancel the unlock), I hear the pump go off when the orange light goes off (red light remains on).

I was able to get it to disengage by shutting it off,then starting it back up. At that point the orange light was on but the red light was off,and then I pushed the button again, and the orange light went off.

I'm going to drop it off at a dealer (either my local one or the one where I bought it from which is a several hour drive away unfortunately) and I'll report back.

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Crank
Posts: 816

Re: First Day with G500, First Problem (Center Diff Lock)

Post#6 » Sun Mar 01, 2015 7:54 am

I think you need to get on some uneven terrain to get all four wheels turning at different speeds.

Your diff locks may just need to be exercised. They can seize up from lack of use, but is highly unlikely on a G that is as young as yours to have fully seized lockers. Mine is an '03 with 29k miles on it, previous owner was about 60-65 and did zero off road driving.

Find yourself some bumpy dirt and drive around on it at lower speeds, snaking back and forth, over undulations, etc. Getting the truck into a turn, with the inside wheels on flat ground and the outside wheels climbing up and over rocks.... Those large wheel speed differences will help the lockers engage and disengage.

I would bet yours are just fine. These things are meant to be used.

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Gelande
Posts: 3949

Re: First Day with G500, First Problem (Center Diff Lock)

Post#7 » Sun Mar 01, 2015 11:52 am

I am confused by why the orange light would be off while they are engaged.

It should go like this....

You are driving slowly, you press the center diff lock button on, orange light immediately comes on, and vacuum pump starts running, then once diff is actually locked, red light comes on.

Ok now you are driving with the diff locked, on loose ground (no pavement), and BOTH orange and red lights are on.

You want to shut off the diff lock.

You press the button again, orange light should instantly turn off, vacuum should stop making noise, and then red light will remain until diff actually opens.

Then at last, red light should go out and diff is open.

smoltz
Posts: 3

Re: First Day with G500, First Problem (Center Diff Lock)

Post#8 » Mon Mar 02, 2015 5:44 pm

mserpe wrote:I am confused by why the orange light would be off while they are engaged.

It should go like this....

You are driving slowly, you press the center diff lock button on, orange light immediately comes on, and vacuum pump starts running, then once diff is actually locked, red light comes on.

Ok now you are driving with the diff locked, on loose ground (no pavement), and BOTH orange and red lights are on.

You want to shut off the diff lock.

You press the button again, orange light should instantly turn off, vacuum should stop making noise, and then red light will remain until diff actually opens.

Then at last, red light should go out and diff is open.


Thanks for explaining this. I read the manual, but it was in a state of panic and I misunderstood. In that case, it seems like the main issue is that the diff won't stay locked. What I am seeing when I press the button is that orange light turns on, red light turns on and then orange light turns off. I think it's also a little sticky on the disengaging possibly because lack of use, but seems to be getting better.

Been talking to the dealer on this, they claim that over 25mph the center lock disengages (they got this info from MB USA Tech Line). But I haven't read anything like that online nor does it make any sense from a mechanical standpoint, nor have I seen it documented. Can anyone with an MBUSA W463 confirm that the center diff lock stays engaged above 25MPH?

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Chris
Posts: 3123

Re: First Day with G500, First Problem (Center Diff Lock)

Post#9 » Mon Mar 02, 2015 10:29 pm

The center difflock is activated by vacuum pressure, there is a rubber diaphragm on the front of the transfercase that pushes the difflock actuator, this rubber part can/will develope a vacuum leak when it gets old.

Sounds like it's time for a new center difflock diaphragm, which requires a special puller tool to remove the front output flange.

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Gelande
Posts: 3949

Re: First Day with G500, First Problem (Center Diff Lock)

Post#10 » Fri Mar 06, 2015 6:51 pm

isn't vac leak or diaphragm crack going to halt the center diff from locking (which is not the problem here)

vs. halting it from unlocking?

Seems like when vacuum is lost, it automatically unlocks no? and if not enough vacuum or no vacuum, seems like it would never lock in the first place

thoughts?


either way, Chris505 is correct it is most easy to remove the flange because it is rather in the way of getting the cover off the diaphragm

also... before you do that... check to see if the vac pressure is as it should be in both modes, center diff switch orange....should have vac pressure at the banjo bolt input.... there is a psi amount for that in the tech books... I forget it sorry...but check service manual and use mity vac tool or similar to check vac pressure at the banjo

when orange light is turned back off, vac pressure should drop

well anyways, its a cheap part unlike the rest of the parts on that gearbox.

if you need to borrow a flange puller let me know i have one

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