Mercedes: 2001 c220 cdi diesel estate blowing starter fuse, compression stroke, spontaneous combustion

Question
hi my starter motor fuse on a 2001 model c220 cdi 2.2l diesel keeps blowing the starter motor fuse. This is a 15amp fuse which i was advized to change to 20amps it also blows that fuse aswell i have been told the starter motor could be bad or the starter motor cable is grounding on the heat shield or the starter motor control circuit is at fault do you agrre or have a possible solution? this problem has caused my wife to be late so many times her job is at risk please help  many thanks ben.

Answer
Ben,
Has the starter made any unusual noises, or seem to drag during the start cycle. How many miles on this vehicle.
Because of the higher compression to over come, the starter must produce more torque to turn over the crank under the compression stroke. It is having to squeeze the diesel fuel to cause spontaneous combustion.
Consequently, the amperage the starter draws is much higher than on the average gas engine.
Or, if the starter engage time is longer than normal this will cause a heating of the wiring.

I tend to look for the least expensive and easiest and obvious thing to repair first.
For those electrons to flow smoothly, all the attaching circuits need to be clean from corrosion.

The starter has actually more than one point of electrical contact. You have the switch circuit and then you have the power to motor circuit. The power to motor circuit will be the larger size wire, and it will go directly from the battery terminal or a direct connect bus because this wire provide the mass of electrons for the starter winding to torque up for start.

The second smaller wire is a lower amperage wire that attached to a magnetic switch/solenoid. When you turn the key, power is directed to this solenoid to engage the larger wire and motor start.
ALL the terminals must  be clean and free from corrosion.
Corrosion is resistance and resistance is heat and heat burns off the wires unless of course there is a fuse in the circuit. Thus your fuse is saving the vehicle wiring harness.
Raising the fuse amperage,just raises the odds that you will melt down the wiring harness.

So.... Time to check all the grounding wires. Start with the battery and clean the terminals and post. Pull the fuse and clean the holder and fuse tips. If they are aluminum you really need to polish them Copper is a much better conductor but economics has now cause aluminum to be the bad choice.
Look for ground wires between the frame and engine. Remove and polish these points on both ends and reinstall tightly. Look for any wires that have bad or broken insulation. If they are near a grounding point tie them up and or wrap with electrical tape.

If after you make sure all your grounding and power points are clean, both switch circuit and power lead to starter, you still have the fuse blowing problem, I would remove the starter and have it checked, at an auto parts house.

Many time the bearing in the motor will become worn enough to allow the armature to drag the field winding. Or the Bendix gears will bind during engagement, and cause dragging of the starter which raises the current draw.

The brushes inside the motor cold be wearing out and causing arcing on the commutator, again causing heat. Finally, if the field windings get a short that too can cause excess draw of amperage and heat. Any of these problems could lead to fuse blowing.

Hope this helps. Let me know what you find.
Good Luck
Robert