Right rear door will not lock when pressing key. They is a sound like trys to lock then unlocks. I can maunelly lock that door first then lock all doors with key. Also when going 9mph doors use to lock.now they recycle at right rear door every time I get below 9 mph. I disarmed the system from auto lock, but can not find the locking problem. HELP RVT

 

you need a new lock actuator for that door most likely. there are numerous fuses and relays for the central locking system.

 

It may be too late, but I became a member of this forum today.

I had exact same door lock problem of my 2000 ML320 two weeks ago. MB Dealer charged me $850. It made me crazy, and I am trying to fix my 10 year old ML320 by myself.

Last week, I had changed my ML320 front left window regulator by myself with few tools. Please visit following web page. This will help you changing door lock actuator.

http://autorepair.about.com/library/faqs/bl556c.htm