You have to forgive me on this one. My car has some kind of factory alarm and I didn't know it. I was under the flooring on the passenger side and mounted on the lower firewall is a box with two plugs; one with maybe ten wires and the other six I think. The box said Becker on it, so I'm thinking something for the old radio. Both plugs were out; so I said what the hell and plugged them back in. Now when I get back to the car and unlock it, open the door and the lights flash and the horn beeps.
Ok, so I have an alarm I never knew was there.. How do you shut it off? It goes for maybe 30 seconds then off by itself. How do I arm and disarm this thing, and is it even supposed to go off randomly like that in the first place? I never even knew my car had this feature. Thanks

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide

Yes, that was the alarm module.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I once found one on my 85 300D. It had been disconnected. It was connected to the car radio antenna. It apparently could be activated/deactivated using a remote switch like modern cars. I didn't have the remote so pulled it out along with all the wiring that went to door switches etc.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

From what I've read in service bulletins for this, the driver's lock has a micro-switch that is supposed to disable the alarm. The bulletin's solution is replacement of the driver's handle/lock assembly. The new handle is supposed to have a special guard to protect the micro-switch from moisture.

I've run across this same problem on my W126, during a rainstorm. What makes me wonder is if a homemade guard could be made for this. I haven't run across any solution, here on the forum, other than the nominal "disconnect it".

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Sam

84 300SD 350K+ miles ( Blue Belle )

I'm going to take some educated guesses here. Most of the factory alarms of that era had a common use scenario.

1) Locking: It assumes you pull the key from the ignition, then close all doors, then lock the car with the master key. When done in this sequence, the system arms itself, typically by checking that all doors are closed, and that the key has been used to lock the car. There is typically some kind of electrical switch that is associated with the driver's door lock. (That was mentioned above).

2) Unlocking: Assumes that the key will be used to open one of the three entry points: Driver or passenger door, or trunk. There are typically small switches that are integrated with the locks on each of these entry points. If the key is used, then the action of unlocking will disable the alarm.

3) Alarm: If any of the doors or trunk, hood, etc. are opened without using the key (i.e. the alarm module didn't "see" the use of key as noted in #2 above, then it will honk the horn, flash the lights for 30 seconds, or until the key is placed in the ignition and turned to the 'on' position.

Therefore, I'm going to assume that there is probably a problem with the switches that are associated with the locks on your car. You need to pull the door apart and check if the switch is working.

Other than that, you'll likely want to find the schematic for your alarm mechanism so you know how to trace stuff out.

I'm merely guessing based on other systems I've worked on of that vintage.

Good luck!

Packman

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!

Yes, it is supposed to go off randomly like that. It is designed to let you know it's working, and it helps the local police departments to raise money by ticketing you for having the alarm go off, which helps with government financing. Also, neighbors love being disturbed by the honking horn, and will come out of their houses and greet you when that happens. It is a social responsibility and socialization feature.

Packman.

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!

I once had an '87 300D Turbo like the OP's. The alarm had been disconnected and there was some mickey mouse aftermarket alarm kludged in. I removed the kludge and left the factory alarm disconnected. It's probably not worth bothering about unless you need to have a working alarm in your socio-geographic location.

The early alarms had a "tilt" sensor under the rear seat, left side. This was a constant source of problems. Most owners disconnected it and the factory stopped including it after a couple of years into the 124 model. Later cars have the "convenience module" in this location.

As others have said, the alarm is activated when you close and lock the doors with the key. The alarm is supposed to deactivate when the door is properly unlocked.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

People at the food store were looking at me like I was an idiot because I couldn't figure out how to shut it off. Going to disconnect it for now. I have other things to worry about. Thanks everyone.

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide

What, all that and you're just going to disconnect it?

Well, that's probably what I would do. People think it is hilarious that I put a "Club" on my 240D. My friends say "Who would steal it?"

Sincerely,

Packman

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!

I had a factory or dealership-installed alarm on my old 1982 240D. Or... at least I assume that's who installed it.

It was controlled by a Mercedes rocker switch in the glove compartment. When I parked the car, I opened the glovebox, hit the switch, and locked the car. When I got back in the car later, I had to turn it off manually.

I only assume this was a Mercedes-installed part because of the rocker switch. I THINK it was identical to the one for the rear dome light. I have clear memories of this, but never tried to track down where the brain and/or alarm horns were. I'd take a closer look, but I sold that car 10 years ago.