In common with most modern vehicles a starter motor lock out relay is fitted, so ensure this is checked for correct operation before undertaking work to replace a defective starter motor.
Unfortunately there is only one relay of this value fitted on the A class so if you suspect the relay it does mean purchasing a replacement unit.
However in the case of some relays the cover can be easily removed you will then be able to see if the relay closes when the power is applied. In the case of this relay it can be seen the the contact points 'A' are worn which is what would have caused the intermittent problem when starting by this owner
With cover in position, if the relay is working, when the ignition is turned to the start position the relay will be heard to, or felt to click, I.e. closing of the contacts within the unit which allows power to pass to the starter motor solenoid which powers the motor. Revised part number. A0002 542 76 19.The photo on the right is of the old unit removed, and bears the old part number.
Note the lugs on the sides of the relay , these engage with the white retention clips ensuring the relays remain fully housed, use other manufacturer's relays and you may not have this facility.
The history to this defect was that even after replacing the ignition switch the owner still had spasmodic problems starting the car. On some occasions only the SRS light would show when the ignition key was turned, when this happened the owner knew the car would not start, turn the key on other occasions and all instrument cluster lights would show, when this happened he knew the car would start.
Remember a Mercedes-Benz 'Star' diagnostics short test will cost you £50.00 approx, that's the price of two relays so the relay replacement is the cheapest option initially, the worst that can happen is that you finish up with it as a spare part. The other option is to clean the points with fine' wet & dry' and then re-test, although I would replace it as soon as convenient.
However Ch of France who is an aircraft engineer sent the following information:- There is a big mistake on the existing sheet in that the total value of the resistor has been calculated in serial despite of they are not. You now if you use normal law R=R1+R2 but in serial only. Here it's paralel so 1/R=1/R1+1/R2
You will find average 85 that is the true value.