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Alarm switch. If someone open the hood while the alarm is armed it's going to activate the siren.
That sounds redundant (not that I doubt your answer :)).

To open the hood, don't you have to open the door which is locked if the alarm is activated.

Is there an underhood light? With German engineering, you would have to have the light switch on for it to operate since you don't need the light in the day.
 

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It's a momentary alarm switch, not a latching switch for a courtesy light. :)

If hood is opened while alarm is on, it triggers the alarm.

It is feasible to break into the car when it is locked by opening the hood and applying pressure to one of the vacuum lines, which pops the door locks. So MB fitted an alarm switch to protect this possible entry point.

I was told by an AA guy that this was the favoured way for the car thief on early models. Open hood; dispatch the under hood siren (if fitted); pop the locks; bypass the immobiliser (if fitted).

It looks like the early cars have this switch fitted regardless of whether the MB alarm option is fitted or not, so in some cases it could be regarded as redundant.

My '92 500SL was fitted with the MB alarm option, but the immobiliser was unreliable (intermittently couldn't start car!) so I fitted a Clifford alarm, which also utilises the switch.

While I removed the old MB alarm I found that the blue wire from the drivers door lock to the central locking pump in the trunk had been purposefully cut down by the seat, presumably as part of the original MB installation (?). In practical terms this meant that the drivers door latch would not operate the central locking until I reconnected this wire. Worth bearing this in mind if anyone has similar problem. Check continuity of blue wire between door latch switch and connector to pump in trunk. But I digress. :)
 
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lowesguy
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