Doing it manually with the key bypasses the central locking system, it is just a function of the lock cylinder direct to the latch. If it won't open manually there is a problem.

the Euro model does open the trunk by pressing in and turning the key counter clockwise.
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300 euro

My euro 500SEC works fine with the key alone, identically to my US spec 300SD, so I don't know what difference might be seen.

Honestly, looking at the lock assembly from inside the trunk, they don't appear to have any real lock wiring to them, just the linkage to the vacuum pod to let the central locking system unlock or lock them when it's in the right key position.


Back to the original question though, there shouldn't be any issue jump starting the car at the battery, as long as you hook it up correctly.
I've had no issues jumping my car, or jumping from my car to another's car, just take care to not cross the wires.
I also often make the last ground connection to the engine itself, rather than to the battery. That prevents any chance of a spark next to the possibly flammable hydrogen gas output of a charging battery.

Best of luck!

My '85 500SEL has TWO trunk locks. One works by the key and the other appears to be a "fail-safe" mechanism. It looks as though it is a spring loaded bolt which is retained by a solenoid. It is definitely an original part.

I'm guessing it is part of the 1st gen alarm system. No battery and the solenoid is not charged. The bolt slides shut when the trunk is closed. Stops anyone from disconnecting the battery to disable the alarm. Yeah, I know, doesn't make any sense but there it is.

Picture of that?

It'll have to be a nasty cell phone pic. My digital camera took one too many falls from the fender. I'll try to get it posted today.

I think your locks are just gooked up with crud. Mine works every time.