Hi, I would always recommend a full diagnostic check on Mercedes STAR diagnostic tool to help pinpoint the problem but on the SL model the most common issue is the PSE pump which controls the central locking which is located in the left of the boot. This can fail on its own or also commonly due to moisture or water ingress through poor roof seals - especially the lower rear screen seal.
In the first instance you should check fuse no 71 in the rear fuse box (located under the rear storage compartment behind the drivers seat) - if blown you can try replacing this - sometimes the system will work again for a short while or may even just be a one off blow - if it blows again then the PSE pump will certainly need replacing.
The pump can also fail without blowing the fuse so it is not a sure fire test but simply the first step - if this is ok then the car will need checking on STAR to see if communication is possible with the PSE control unit
There are of course other possibilities but this is the most common on the SL
Best regards, Paul
can I buy a replacement pump without going to mercedes garage.
Any idea what the PSE pump costs and how much labour involved in replacing it?
Hi - from memory a new pump from Mercedes is around £800, it will also need coding on STAR after installation. Actually replacing it is not too bad - you need to remove the boot floor covering, unscrew the roof pump on the left of the trunk and lift it out of the way & the PSE pump is located under here
Another cheaper option if your unit is faulty is a specialist company have yours repaired
Here is a company I have used before: BBA Reman
It also has the added benefit of not needing coding after installation if they repair yours