Date registered: May 2005
Vehicle: 1991 300SEL W140, Brabus, W220 facelift
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Unfortunately, I don't have schematics of the W126 (which I assume it is??), and the following is then from a general point of view.
When you disconnect the battery, the generator still generates electrical power. It appears as if the ignition power is not cut, when you switch the ignition off. The ignition switch has several circuits, so even if all other electronics stops, you may still have power to the ignition circuit, if that part of the switch i broken (this is where a schematic would come in handy).
Another possibility is if the engine selfignites due to fouled spark plugs, which may again be due to incorrect mixture. This is mostly seen on engines with high compression (which is not your case).
1991 300SEL W140, Brabus, W220 facelift
1987 190E 2,3
1987 Honda Accord 2,0