W210 when car alarms get dementia

[GregMN]

1996 W120, 350k

The other day the door locks kept trying to lock when I was driving it.
When I got home and went out to the car later, it had locked itself.
When I opened it with the key, the alarm when off until I turned the key in the ignition. (In 1996 these cars still had a real key.) It would not respond to the remote lock/unlock.
Later that evening the car's alarm started to go off. I turned it off by turning the key in the ignition. This happened twice.
Later the courtesy lights came on and stayed on until the battery got low and the car turned them off.

I currently have a charger on the battery so I can start it and get it in the garage to try to figure out what's going on and fix it or disconnect it.

Any thoughts ?

[Jeremy5848]

My '96 E300D has only 300K miles—something to look forward to! Strange problems of this sort are often caused by a weak battery, or a battery with a weak cell. Bad conduction through a fuse can also result in strange activities but perhaps less so in the W210 and newer cars because of their spade fuses, which typically maintain better contact than the older fuses/fuse holders.

[GregMN]

I just took a look at it. I pulled the alarm fuse under the back seat next to the battery.
40amp fuse for the alarm and rear window defroster. That seemed to stop all the nonsense.

If someone wants to steal a rusty 1996 diesel they can just knock on the door and I will give them the key.
My security lies in the fact that even with the key they will not be able to start it from cold.

[Dan Stokes]

We have a dog alarm that goes off unpredictably. There doesn't seem to be a fuse. His main job is to keep elephants off the lawn and so far he's very good at it. However, sometimes he alerts on non-existent elephants - or he chases them off so quickly that we never even see them. He does this from inside the house. He's either very good or demented.

Dan

[KarTek]

My friend's aunt had this problem with her 2002 E320. If you take the vacuum pump/door lock control out of the car, you'll find a circuit board in the bottom of it. This can get really cruddy, especially if a drink or water was spilled in the back seat.

I'm thinking that battery vapor may cause it damage as well if not vented properly.

On her car, the alarm would go off while she was driving and the trunk would magically pop itself as well.

[97 SL320]

On the 97 E320 the multi function convenience control unit is in the under hood box, passenger side and I'm guessing the 96 is the same.

I'd open the box, pull control and open it up. There is a cryptic sequence to remove this control that I don't recall at the moment. I think the connector also holds the unit into the mount. Capacitors can leak causing all sorts of odd behavior. Your car likely does not have DAS X ( chip in a key used in 97 ) and relies on the remote locking function and a starter interrupt relay. I think that relay is next to the shifter but don't quote me on that.

The alarm on mt 97 SL320 would would not disarm / trigger and the remote stopped working. I ended up unplugging the alarm box as it is separate on these cars.

[GregMN]

Originally Posted by KarTek

My friend's aunt had this problem with her 2002 E320. If you take the vacuum pump/door lock control out of the car, you'll find a circuit board in the bottom of it. This can get really cruddy, especially if a drink or water was spilled in the back seat.

I'm thinking that battery vapor may cause it damage as well if not vented properly.

On her car, the alarm would go off while she was driving and the trunk would magically pop itself as well.

The rag wool insulation in that area was damp, so I will check into that. Thanks.

Originally Posted by Dan Stokes

We have a dog alarm that goes off unpredictably. There doesn't seem to be a fuse. His main job is to keep elephants off the lawn and so far he's very good at it. However, sometimes he alerts on non-existent elephants - or he chases them off so quickly that we never even see them. He does this from inside the house. He's either very good or demented.

Dan

Up north here it is mastodons and woolly mammoths. My Alaskan Malamute keeps them away.

[Dan Stokes]

Lucky thing you have that Malamute! Those suckers can do some real damage.

Dan