My mercedes 300E does not start, the crankshaft won't try to turn, the waterpump has just broke down few days before, but now it just does not start at all... wonder if when over heated the relay wouldnt work..
When you say that crankshaft won't try to turn, do you then mean that starter solenoid does not activate (a noticable click) and that starter motor does not try to drive the crankshaft?
And is it with automatic or manual transmission?
Anyway, the voltage to the starter solenoid has a very simple path: Through "start" position of the ignition key and through the starter lockout switch at the gearbox (if automatic transmission.. it will only start in P or N).
If the starter motor tries to drive the crankshaft without succes, you have a serious problem in the engine.
If the solenoid does not activate, you should troubleshoot the described path. The starter lockout switch is a probable cause (are you sure it is in P or N?). Also if you have antitheft starter lockout, it would be in this path. It could also be the "30 circuit" fuse (I don't know the fuse number for your model), but then there would be no lights in instrument panel lamps when ignition key is in "Run" position.
If the solenoid activates, but the starter motor does not try to drive the crankshaft, the problem is the starter motor (or the battery line to it).
Is the battery good? If no, charge (or change) it first.
If battery is good, is there clicking sound from the starter solenoid when trying to start? If no, solenoid (or cables to it) is bad.
If yes, does the starter make noise trying to turn? If no, starter (or the battery cable to it) is bad.
If starter does make short period of noise trying to turn but the crankshaft does not. It might be engine internal.
Was the car driven with a broken water pump? How hot and how long was the overheat? Was the overheat the last time this engine ran? If yes, there is serious problem.
There is a very remote chance the engine is water locked, meaning the head gasket broke and coolant leaked into the cylinder. Liquid filled cylinder cannot be compressed hence crankshaft won't turn. To check water lock, remove all spark plugs to check if liquid pouring from cylinder. But since the overheat just happened, I highly suspect internal damage.
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