On the 1991 560SEL, a strange smell similar to burnt plastic (not fuel) comes from the engine compartment as soon as I start the car. I have checked the fuses for any that might be burnt, they all look fine. any ideas?

If you mean like 20 seconds after you start the car, and the engine/coolant is not even warm yet, then that should be some electrical gizmo. Some electrical things like for cold engine start would only be activated if the engine is cold.

See if the smell is also there when you first start the engine after it has been warmed up.

Also try just turning on the ignition, but not starting the engine. See if you get the smell.

And another factor is if the outside temperature is near freezing. Things like the side mirrors will automatically turn on to defrost. But they will not turn on when warmer - like 40 degrees F. Is it cold outside when this happens? Does it happen when warmer? Maybe the smell is a mirror?

check you electrical wires from starter to alternator,for warm wires.My w126 has a place on passenger side fender down by alternator where the wires junction.I would asap carry a fire ex. and think about a battery cutoff switch.Older cars are bad for going up in flames.Also check if you had heater on when you start the car.The heater and air cond. control are bad to catch fire,if the aux. water pump has become stuck.They reccomind attaching a 1 amp fuse to aux. pump. to save car.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Where is the aux water pump located?

I found it, near the monovalve. OK, I'm assuming I need to install a fuse (amperage?) into the power line feeding this little monster......