I have a Mercedes Benz 300SD and it's been great. I can drive for a few hundred miles without any trouble; until the troubles come in. I've had all sorts of problems like engine overheating and hard starting. It was all resolved now. The recent problem is the door locks—either one of them does not work or none work at all. Any tips would be appreciated.

It really is annoying when that happens—power door locks that do not work—not to mention worrisome for thefts. Your car's power door locks have individual solenoid that activates the lock mechanism. There is also a switch that powers these solenoids. When you lock the doors, the switch will send voltage to the terminals of the solenoid. When you unlock them, the switch reverses the polarity of the voltage. Power door lock failure can be caused by a faulty switch or solenoid. It can also be due to wiring problems or mechanical failure in the linkage. What you need to do is to first diagnose the fuse box and see if it is not blown. You can check your owner's manual for the location of the fuse box. Try each door lock switch to determine if any door is responding to what switch. If any door responds to any switch, then you will need to replace the switch. If all door switches work but one door fails, then you have a problem with a defective solenoid.

I've been driving this 300SD for a few years now and it's got quite a mileage already. The good news is that it's still driving fine and I have no problem with its performance. The current problem is with its ignition lock and tumbler. Sometimes the ignition lock doesn't work. What should I do?

One of the main security features of your car is the ignition lock. This is where you insert the key to start the vehicle as well as lock it so no one can drive it without the key. The ignition lock is connected to the electrical assembly/ignition switch of the car. Signs of wear and failure of ignition lock includes difficulty in inserting and removing the key from the switch and when the key turn intermittently or not turning at all. What you should do is to inspect the key for possible wear and have it replaced if it's damaged. If you have a spare key, you can test the ignition lock using it. If the other key also does not work, it means the ignition lock cylinder is the one you need to replace.

I noticed that my Mercedes Benz 300SD is taking a longer than usual cranking cycle. Sometimes, it even fails to crank and I have to do several tries. What could be the problem and what should I do to solve it?

The one thing that can cause a longer cranking cycle or crank failures is when the high pressure fuel pump has leaks. This is especially true with vehicles that use direct injector. This injector works under a lot of pressure so leaks can happen—either when the engine is running or resting. The leaks will then cause severe carbon buildup and a rich fuel reading. You will need to submit your can to a leakdown test which only professionals could do because of a sophisticated scan tool needed.

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