Mercedes: central locking on a 500SL, key fobs, 500sl

Question
the central locking does not lock any of the doors(including the Boot) on my 1991 500SL even though the green or red lights on the door handles  do light up and flash. I did have a problem with the passenger side door not unlocking awhile back and now the red locking light on this side is very dull. I have checked the fuses in the Boot and moved them around to be sure that they are all working. The key fobs are both good and therefore I wonder if it must be a solenoid to one of the doors and possibly the favorite is the passenger side. What do you think.

Answer
Barry,
There is a good possibility that you have a corroded ground. The battery needs to by in tip top shape also. A high resistance connection can create all sorts of havoc with the computer. I would first try and reset the computer.
Here is the sequence:  Turn off all accessories and the radio. Pull the hood release. Exit the vehicle and lock each door with the master key.
Remove the (-) battery terminal and then the (+) terminal. Clean the cable ends to bright as well as the battery posts.
Locate the bullet fuses for the alarm and the door locks remove and clean or better yet replace with new (I would make sure that the fuses are copper not aluminum.)  but clean the holding clips with scotch bright or similar non metallic abrasive.
Wait 20 minutes and then attach the cables (+) the (-).  
Securly close the hood and then use the master key to unlock the driver door. Make sure that you remembered the correct direction to turn the key from locking.

Start the vehicle and then try the locking system.

Good Luck
Rob