On my 99 ML320 , a few times now when I use the remote to lock the doors, the passenger door lock clicks up and down, trying to lock, but will sometimes stay up. I unlock all the doors, try again, and maybe it will lock. Lately it has been ok, until again today, the wife tried a couple times, and ended up setting the alarm off (at least that works fine,and was her mistake). Once home, I pulled the small rubber plug on the door, above the lock mechanism, sprayed a little contact cleaner towards the moving assembly inside, and "Presto!!" it works fine again.
Is this common, and is it just a lack of cleaning or lubrication needed? Anyone else deal with this situation?

"Machine gun" doorlocks were a common problem on the old mls. Surprised they waitied so long before acting up on your vehicle.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)

Sooooooooo...what's the fix/problem?

Maybe others know; mine was repaired under warranty several years ago. I'll try to find the dealer invoice for the number of the replaced part.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)

hi, That door actuator needs to be replaced. Good news, if you replace it yourself, no rivits to drill like a front door replacement. Spraying didn't do anything, Its a sealed unit.

Well, since my last post and a 1200KM trip, not a problem since. Could have been a little grit or something got cleaned out of any moving parts? Anyway, works fine now!

Our problem was the rear tailgate lock actuator. Replaced under warranty.

This is not an uncommon problem with ML's. The problem usually will set fault codes in the control unit for the central locking system (AAM- All Activity Module). The only real fix is replacing the faulty door latch assembly on the problematic door. Each door latch, including the rear tailgate, has a microswitch inside that recognizes the latch open or closed. When this goes bad, the vehicle does not know the position of the latch & tries to repeatedly lock the door, to no avail.
If your ML is still under warranty, it should be covered. This same scenario has been found in some other MB models also.

Anyone have the part number for the tailgate replacement parts? Pics on how to replace it?
Any pics on the location and part number of the microswitches?