So I just got home with my new to me 1991 K100LT and after looking the bike over I noticed that the rear modulator has been bypassed and does not have any brake hoses going to it. The system intitalizes with the two red lights on the dash and after you activate the front and rear brakes they start flashing together then go off after you ride. I would like to get the system working. The previous owner and mechanic that did the work have passed away so I can not ask them what the issue was. My friend said he thought the rear brake was locked so they bypassed the modulator but that was 5 years ago so he was not sure if it was the modulator or brain that was bad.

My question is there a way to make sure it is the modulator that was bad and not the brain? It would seem to me that if the brain was bad it would not function correctly and send error lights and he said the front ABS works although I have not tried it. He did give me a used modulator but he never installed it and did not know the hoses were missing. If the modulator is bad is there any tests to rule it or the brain out? I plan on blowing compressed air into the modulator to see if it is open or blocked. It would seem that if it failed closed it should not flow air and if it is open it could be ok? Is this a fair assumption? I just wnat to rule out the brain before I spend the money on hoses to reconnect the modulator. If it is the brain I will not fix it due to cost but hoses are cheap to install the new/used modulator I was given.

First check if it`s any fault codes stored in the ECU (beside the obvious, rear modulator).

http://www.largiader.com/abs/absfault.html

I will check for codes but if there were any wouldn't I have a flashing light?

You have a point there...but it is also strange that the warning system behave like everything is normal,with the rear modulator disconnected.

I will have to look at it but it is still electrically connected just not the brake hoses so all I can thing is the system assumes everything is ok. I am going to start by seeing if the fluid circuits are open and verify with my spare modulater. I am hoping the one on the bike is stuck closed but one would think the brain would detect this. Not sure how smart the system is.

Don`t know the ABS I that well, but on ABS II you hear and feels that the hydraulic being tested during the self test.
But what feedback does the system rely on, no idea...
Haven`t got any problem with this (so far), so no need to study the different functions in detail.......

You might also find some info on this page:
http://bmwk10075abs1fix.web.fc2.com/

If it turns out the ABS brain is what has the fault the guy at the link posted by Inge is "The Man" when it comes to repairing ABS brains, when mine failed he fixed it very quickly and posted it back to me for less than the cost of what people were claiming as a working 2nd hand unit on ebay at the time. There is never a day when I ride my K when I don't offer up silent thanks to Tosi for his work in fixing mine. ABS has really worked for me saving my bacon on more than one occasion, I would never want to be without it and it was part of why I bought a K in the first place.
Regards,
K-BIKE

K-BIKE wrote:If it turns out the ABS brain is what has the fault the guy at the link posted by Inge is "The Man" when it comes to repairing ABS brains, when mine failed he fixed it very quickly and posted it back to me for less than the cost of what people were claiming as a working 2nd hand unit on ebay at the time. There is never a day when I ride my K when I don't offer up silent thanks to Tosi for his work in fixing mine. ABS has really worked for me saving my bacon on more than one occasion, I would never want to be without it and it was part of why I bought a K in the first place.
Regards,
K-BIKE

+1 Tosi has fixed 2 brains for me and both are still going well and as K Bike said for a heap less than an unknown used part of fleabay.