Thought I would document here a problem I had with my SL55 which probably applies to other cars as well, especially those where the key just initiates the starting sequence and the starter motor continues running until the engine has started, even if you let go the key.

The symptom is that when turning the key to start the engine, sometimes the engine will turn over fine, other times you hear just a click, sometimes nothing at all.

It’s tempting to think it’s a battery problem and I replaced my starter battery - the one in the engine compartment - because it was 17 years old and overdue for replacement anyway but the problem persisted.

The problem turned out to be the starter relay in the fuse box next to the starter battery. The relay is switched on by the engine ECU and its contacts supply power to the starter solenoid which in turn connects power to the starter motor and engages the drive.

It’s green, plugs in and a replacement cost around $12 from a Mercedes dealer in London. Replacing the relay solved the problem instantly.

The starter solenoid requires quite a high current to energise it so it was no surprise when I took the relay apart to find the contacts worn, with material blocking them making a good connection. What’s more, the solenoid is inductive which will tend to cause sparking when the contact is broken. There’s a diode in the relay which may be used to quench the sparking but that had failed and probably made the sparking worse.

So, a quick and easy fix and worth trying before suspecting the (very expensive) starter motor. The Mercedes dealer wanted $250 for an hour’s diagnosis time which prompted me to replace the relay first, which solved the problem.

Glad you were able to find an easy solution and thank you for sharing. If you can take some pictures that would be greatly beneficial as well!

Glad you mentioned this..I have this after the car sitting long periods of time aka 3 weeks. It feels like the car is possessed even after you let go it churns until you pull the key

I’ve had it happen a few times in my 2 years of ownership randomly and just today after sitting for 3 weeks.

It took a few tries as the starter just kept turning, I went away for a few minutes and then it started right up

Do you have the part number and fuse location?

The part number has changed since the car was new, different relay supplier, current part no is MA00 982 85 23. It lives in the fuse/relay box which sits alongside the starter battery. You can remove the lid and change the relay without removing the battery. It's easy to spot, has a green enclosure and 5 connecting pins/spades.

The relay is energised to start the car when you turn the key and everything else is OK, security and so on. Its contacts supply the current needed to energise the starter solenoid. That in turn connects the starter motor itself to the starter battery and off it goes... hopefully. When the engine ECU detects the engine is running, it de-energises the relay which then disconnects the power to the starter solenoid which in turn disconnects power to the starter motor.

For those who are not sure what a relay is, it's an electrical switch which is actived by passing a current through a coil to create a magnetic field. The magnetic field moves a moveable contact, called the armature which closes the connection. The reason to use a relay is to be able to switch on (in this case) the high current the starter solenoid needs using a much smaller current which is compatible with the car's electronics, engine management ECI and ignition switch.