How can I tell if my 300 E has an orginal alarm system. (broken of course)
 

The 91 300 E I just bought has an extra horn in the engine compartment, the trunk light won't pull off/out and I hear a clicking noise while messing around in the fuse box. I'm thinking this thing has an alarm that has been disconnected. How can I tell for sure if it has an alarm and is it worth trying to fix if it does? How would a dealer disconnect it if it was going off all the time?

To test to see if your factory alarm is connected, lock your doors but leave a back window down -- reach inside and unlock a back door, then open it. If the alarm goes off then it's working; if not... well it's either been replaced or it's disconnected.

To turn alarm off, put key in front door and unlock all doors.

G'luck!

~Paul

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'91 300E, 207k miles

Is the box behind the fuse box the main alarm unit? It doesn't have any quick access to it?

Also, the factory alarm just uses the factory horns when activated. If you have an accessory horn, then perhaps an aftermarket alarm had been installed at one time...

Most alarm installers locate the alarm "brain" somewhere inside the dash, usually underneath the driver's side cockpit. The "brain" may still be active (as the relay "click" can still be heard).
Some installers do mount the alarm computer in the engine compartment, but I think that's rare, since most manufacturers don't design them to withstand the heat and moisture that they would be subject to in that location.

Typically, aftermarket installs use black convoluted tubing to protect the wiring harness, so that would be a clue as to where stuff is located...

The engine horn is welded to the side of the compartment. It doesn't look as if it was added. Who knows. Anyone know what the long rect. box behind the battery is? Has a slector switch on top.

The later model (1989 and on) have a seperate hon for the alarm.