Why Is My Mercedes Having A Starting Problem?

Why Is My Mercedes Having A Starting Problem?

About 2 months ago, my Mercedes Benz W203 C180K Kompressor's starter motor started slipping when attempting to turn-over the engine. On average the bendix drive slipped twice for every three tries. Then it progressively got worst, requiring several more attempts before it would finally engage. My wife was at her tethered end and refused to drive the car unless I fix it. So I planed to do so the upcoming weekend, encouraging her to use the car until then. However, as fate would have it, I was forced to fix it much sooner than I anticipated. My wife drove into a Fish Hoek garage for fuel and when the attendant was done, she started the car but nothing happened, the starter didn't turn over, it was silent, not even a click. After a few more attempts she gave up and phoned me to relate the above problem, so I grabbed some tools and jetted to her rescue. On arrival, I checked that the starter fuse was OK and I knocked the starter relay with a screwdriver handle a few times, but without success.

Then I asked her to turn the key, while I knocked the starter a few times with a ball pane hammer, but to no avail. It started to get dark so I phoned for a roll-back and had to car taken home. The following morning I started my diagnoses. I suspected that the starter was fault but wasn't willing to remove it, not until I made sure it wasn't anything else that caused the same symptoms. So I removed the SAM cover and once again systematically checked all the fuses with a multi-meter. I even switched on the ignition and checked for voltage on either side of every fuse just to make certain I didn't get an ohms reading through some other circuit previously. It all seemed fine, all the fuses were OK. Next I pulled out relay the S - starter relay, removed its cover to inspect its contact to see if it wasn't burnt and checked its solenoid for continuity.

It seemed fine, but I wasn't convinced that the relay per se was OK. Now since a relay is a electro-mechanical active device I needed to test it, to see if it performs as it should. A relay is just a remote switch, implying in order for it to switch, it must be remotely controlled by a voltage originating from another switch contact. In our case, when the ignition switch is turned, it sends power to the the relay solenoid which then activates the relay. The diagram on the relay cover, shows where each of the terminals connects to, but it doesn't follow international relay convention. On the diagram, the terminals are marked as 1 and 2, 3 and 5, with 3 and 5 being the physical switch, 1 and 2 being the solenoid winding. In the above image, the pin marked 3 would normally be marked as 87 and the pin marked 5 would normally be marked as 30. Have a look at this relay.

These pins carry the load which is normally Open-Circuit and only closes when its solenoid winding is energized. With my multi-meter switched to ohms, I checked the solenoid winding and as you can see, it measures .3 of an ohm. The solenoid winding is connected to the multi-meter with 2 dog clips that is connected to the multi-meter's leads. In the image below, I'm applying 12 volts to the relay solenoid with the dog clips connected to the switch and the relay activated as it should. Satisfied that the fuse is OK and that the relay was working as it should, I needed to bypass the relay, thinking that the Ignition switch may not be sending the required voltage. So looking at the image below, the relay in the far left corner, sitting behind the red fuse is the starter relay, which I reinserted. Mercedes Benz agents are fully aware of the starting problems Mercedes Benz owners are experiencing with the cars.