1992 525i Theft-protection? going off randomly

[JesterMeister]

I have a 1992 525i I recently bought and randomly when I turn the car off the theft protection or something starts going off. The car starts honking and if i try to start the car it turns over but it won't actually start. Normally if it happens I need to unplug the battery but the most recent time this happened it shut off after about 45 seconds. I don't know what would set it this off and where to even start looking to fix this and I can't seem to find anything on the internet about this. Thanks in advance for any answers.

[JesterMeister]

Ok I've done some a lot more searching on the internet and would unplugging the alarm control unit under the rear seat make it stop forever, because if so I'll just unplug it and never deal with an alarm again.

[shogun]

if it would be so easy to stop the alarm by just unplugging the alarm control, the alarm would be not worth the money. There are wires going in the system and out of the control system.
Is it original BMW or aftermarket? Here some info,
DWA system https://etk.club/eba_docs/582en.pdf
EWS I/EWS II/EWS III/EWS III D E31/E34/E36/E38/E39/E46/E52/E53 http://www.e38.org/EWS.pdf
https://www.meeknet.co.uk/e38/E38_Immobiliser_System%20EWS3.pdf
https://www.meeknet.co.uk/e38/E38_Immobiliser_System.pdf
https://www.bimmerforums.com/forum/showthread.php?2151974-Exasperating-Alarm

[danespann]

92 doesn't have factory EWS or anything so the car must have an aftermarket alarm. They're usually behind the driver side kick panel or under the back seat.

[JesterMeister]

It was under the back seat, or I hope it was cause whatever box was under there has been unplugged for about a week and a half, so far no issues but the alarm would just go off randomly after a while of doing nothing so who knows.

[shogun]

more details please what you unplugged, what is written on the box? Or make a picture and post it here? If you still have the alarm going off, then you unplugged maybe another module. Also 1992 is not enough. Month? Therefore here you have all ETM https://shark.armchair.mb.ca/~dave/BMW/e34/
1992 could be 2 different ETM, you see diagrams what is where and component locations views, built dates 7/1991 - 9/1992 or 9/1992 - 8/1993
add the production month to your profile

[JesterMeister]

Sorry for taking a while to respond, but what I unplugged was the part 65811378027, the onboard computer was already bugging out before I unplugged it so I didn't even notice it wasn't working and haven't cared to plug it back in. The car hasn't done set off the alarm so far but I might just be getting lucky since its random. although in my small amount of research I do remember reading something about the obc having a anti-theft code function which maybe since it was bugging out maybe it was setting that off? idk

[shogun]

That part is the BMW E31/E32/E34 Onboard Computer Relay Hella 65811378027 with a 15A fuse, located under rear seat. Not in the fuse box but directly under the seat support hidden. I posted pics in 2006 here http://www.bimmerboard.com/forums/posts/236235/
the alarm horn is not the normal horn, alarm horn is hidden, difficult to reach, located near the ECU box in the wiper mechanism area

OBC has a anti-theft code function, see owners manual. Also see here code- alarm feature OBC http://www.e38.org/e32/BMW_pdfs/OBC_info.pdf

ignition lock position R. Code button: 1000, 100, 10 and 1 buttons.
Code is set with the S/R button.
Now turn ignition key to 0 = LED on 36 hours.
The engine hood is now monitored regardless of the ignition lock position (alarm horn).
Driving away protection is activated when ignition is switched on.

Deactivating code: The reminder gong and flashing display are activated via terminal R or 15. Input code: and not via S/R. 3 wrong inputs or starting attempt = alarm.

Emergency deactivation
emergency starting procedure if the code has been forgotten
disconnect for 1 minute and reconnect the battery.
turn ignition key to position 1, the alarm will sound
a time display will appear and run down to zero for 15 minutes
after 15 minutes, the engine can be started
emergency deactivation can be stopped by inputting crrect code.

[JesterMeister]

I plugged the obc relay back in and I turned the car on, turned if off and the alarm went off, thus my assumption was that my glitchy obc was messing with the anti-theft code function. So I ordered myself a used obc off ebay that isn't glitchy and swapped them and so far I've had no issues so I think my assumption was right and I think the issue is solved at this point.
Thanks for all the info shogun