Does anyone know what this lock is for /goes to?  My ignition key doesn't work to it either.  My bike is a 85 K100RT

As far as I can ascertain it's not an OEM fitment but I will hazard a guess that it is a security alarm of some sort.

Thanks Rick

That's a factory alarm.  My 91 K100LT has the same thing.  If the alarm is present you will have the alarm box inside of the tool tray.  There will be a hole in the tool tray cover for a pushbutton switch to stick through for when the seat is opened the alarm will sound.  I think there is also a motion sensor in it.  The backup battery is probably dead by now.  Luckily mine was just a  9 volt battery but I believe earlier ones had a special battery.

Ah so what I was shown and told to be a factory alarm probably was not. It had a key lock under the seat and the key needed to be inserted and turned of within a few seconds or the alarm went off.  Something more to add to the memory banks, gees it sure is getting crowed in there.

It is a factory alarm.  Look at Max BMW here http://www.maxbmwmotorcycles.com/fiche/DiagramsMain.aspx?vid=51750&rnd=12082016 then go down to diagram 65_0324.  I turn my alarm by just turning the key after parking it.  Does your bike have the alarm box mounted in the tool box?

It was not my bike it was on. I think from memory the alarm box was in the tail section. It was  on a bike I did some work on and when I asked the owner said it was factory but apparently he didn't know either.

I'm sorry, when I read your post I was thinking the original poster came to the conclusion that it wasn't a factory alarm from my post.

I've had my 91 K100LT since new and this alarm system was on it the day I bought it.  I seldom use it but I do keep the backup battery live in it since I found out it has one.

RicK G wrote: I think from memory the alarm box was in the tail section.

It was in the tail section on the 4V and low seat models, then the pushbutton for the seat was disabled.

Here's a picture from the under seat mounting.

I'm struggling with the same thing.  I bought my 85 a few years ago and after it sat a while I had to replace the battery and now when I turn the ignition on, the horn blows. I'm trouble shooting and found the device under the seat and finally found the keyed switch on the relay box.  I had to order a set of tri-wing screw drivers to get the cover off of the under seat module and I see a battery that is swelled on each end, so its surely bad.  Its a varta of some type.  Still working to get the wiring connector off of the module.

On my 2 bikes I can slip the module, with the wiring still connected, out of the plastic case and let it hang whilst I deal with the battery. The less I disturb connections that are behaving the better.

ainsjac wrote:On my 2 bikes I can slip the module, with the wiring still connected, out of the plastic case and let it hang whilst I deal with the battery. The less I disturb connections that are behaving the better.

+1

If you're looking to replace the battery for the alarm system, here's a link:

http://www.ebay.com/itm/Varta-4-V80H-2-Pin-4-8V-80mAh-NiMH-Battery-55608304015-FAST-USA-SHIP-/271436814984?hash=item3f32e51a88