The body of the locomotive truck (15) is cast metal; a plastic side frame (23) holds the wheels in place. Under normal circumstances, you probably do not need to disassemble the trucks, but they can be removed from the locomotive frame. Note the two oval slots in the locomotive frame. A steel shaft (18) holds the truck to the frame; it fits through a large gear at the top of the truck. The oval slot allows the truck to pivot, letting the locomotive go around curves. On top of the truck assembly, you will see two phosphor bronze “fingers”. Take great care to not bend these pieces. The fingers conduct electricity from the wheels up into the motor. If you encounter erratic operation, in many cases it is because the wheels of the locomotive are dirty, or these conducting fingers have become bent or are dirty. As with the rest of the locomotive, a light oiling of the gears and shafts will be helpful. Try to avoid giving a light oiling of the electricity conducting parts such as the pickups. Also, avoid getting oil on the coupler spring (21).
Another important matter involves keeping the wheels clean. Because the locomotive emits oil and carbon from the motor brushes, and the locomotive operates in the real world, you will find deposits of dirt and crud on the wheels. This may include pet hair, carpet nap and other foreign debris. This stuff is nonconductive and eventually the locomotive will stop running because of the accumulation.
There are a couple ways to deal with it. You can use a toothpick to manually clear off the deposits. Ken Brzenk, of Märklin, Inc., has suggested making a simple device to correct this. Take two straight pins, solder a brown wire to one, and a red wire to the other, then connect the wires to the transformer. Turn the locomotive upside down, propping it so that it cannot shift around. Turn the voltage on and put one straight pin on one wheel, and the other pin on a wheel on the opposite side of the locomotive. The locomotive starts running, and the points of the straight pins clean off the deposits.
If you do not feel confident in maintaining your locomotives, your dealer should be able to offer you assistance. There also is a group of authorized Märklin service centers around the country which are qualified to perform repairs on Märklin trains. These dealers are required to maintain an adequate inventory of parts and tools which make speedy repair possible. Also, the repair centers have handled many of the problems that you may encounter, and Märklin’s technical manuals are quite explicit and detailed. Don't feel too self-conscious if you are bringing your locomotive to the repair center in pieces. I guarantee that you are not the first person to have done so.