What is this?
 

I have attached two photo's of the same unit that I discovered under the passenger front carpet against the firewall.

It is unplugged, which tempts me to plug it in again but as I have no idea what it is... I thought it might be better to ask!!

From a 1986 420SEL

Thanks for your help.

Alarm.

Hey man, thanks!

I will try connecting it and see what happens!

Don't do that unless you want your neighbors pissed at you when it goes off at 3am for no good reason.

X2

The PO unplugged it for a reason.

Yes

I found out why.

Nothing happened when I plugged it in. I tried to activate it but nothing happened.

Later when I got in the car to go to the store, as soon as I turned on the ignition I had the whole horn blaring and lights flashing thing going on!

So, any ideas about what might be the fix? Or am I looking for a new black-box controller?

It's activated/deactivated when you lock and unlock the doors. As you found out, it goes off for no apparent reason. Even with a "new" black box, you will still have that problem. Do a search here and you'll see that most just unplug the box and leave it that way. If you want an alarm, I suggest you pick an aftermarket one up.

Bad connections...
 

The alarm problems are usually caused by bad lock switches and/or the wiring ...these maybe or maybe not worth fixing...

If you open the front driver and passenger doors, you will see a plastic plug on the door side of the jamb. In the plugs are the connectors for the alarm system. There are three wires. These plugs generally fail when the plastic turns to dust.

On some of the older w124's, there was also a "movement" (I think MB called it a "towing unit") unit under the left side of the rear seat. It triggered the alarm if someone or something bumped or tried to tow the car when the alarm was set.

I repaired mine and got the alarm system to work, but it took days. There is also a switch that the hood contacts when shut.

I disconnected the horn when troubleshooting the system...it's much quieter that way...

Hope this helps...

The Tenor Man

The alarm on our 300E hasn't had a single issue in the whole time I've owned the car....did they improve the system on the later vehicles? I set it off myself on accident once, so I know it works! :D :eek:

Alarm
 

Mine works great too! 1989 300E!