Mercedes-Benz W201 battery drains overnight
W201: how do I find what is draining the battery while the car is parked?
Remove the fuses one at a time and measure the current with an ammeter. When the drain stops as a particular fuse is removed, the fault is in that circuit. This has turned up, for example, a rear window heater left on, the power window relay and the boot light.
W201: can the alternator drain the battery?
Yes. Worn brushes in the alternator voltage regulator or faulty rectifier diodes do not stop current from leaking back, and the battery can go flat while parked. A diode fault can drain the battery in just a few minutes. Check the fuse and the ignition switch base first, then the alternator.
W201: can the intake air preheating relay drain the battery?
Yes. The relay of the intake air heating element between the intake manifold and the cover can stick closed and keep the current on all the time. The preheater connector is under the intake manifold near the cover, and unplugging it helps determine whether the fault is there.