I have been trying to chase down an ABS problem for the last 4 months.
MY 93 K1100RS has been laid up for most of the last 2 years whilst I recovered from multiple surgeries on my left arm and shoulder arising from a superman impersonation after hitting a very large kangaroo on my trail bike.
The original error code was 4 - rear brake sensor.
I swapped the front and rear sensor under tank wiring over. The fault code did not change to the front sensor (3).
I exchanged several emails with Tosi Tabu in Japan and he convinced me that the ABS brain was faulty.
As Christmas was fast approaching and I had set myself a mental milestone to get back on the horse (bike) by then, I ordered a "refurbished" ABS 1 brain rather than having mine repaired.

It turned up a week later.
But no matter what I tried, I couldn't get the new ABS unit to reset (it kept on flashing code 7).
I followed all the instructions on Tosi's Web page to the letter http://bmwk10075abs1fix.web.fc2.com/detaila.html#resetpro .
The battery is fine - Odyssey PC680.
I had a friend, who is a retired electrical engineer, test numerous circuits on the bike.
We found that the ABS dash switch wasn't the best, so a new one was sourced from Motoworks.
Tosi was of not much help in the matter.
His emails seemed to infer that I was incompetent and to have someone else look at the bike wiring.
Eventually, the ABS unit was sent back to him.

Tosi confirmed that it displayed a code 7 error, but that he was able to reset it.
I had to pay the return postage to Japan as well as a postage and handling fee to get the ABS unit returned to me.
Tosi stated that I should just reinstall it and ride the bike.
Tosi also stated
Instead of many mail exchange I introduce you a K bike forum where many OZ riders belong to.
https://www.k100-forum.com/f8-electrical .You can post as many question as you like and people there will answer you.
I know a person who has many K bikes and know very well about them.
He is around 200 km away from your residence so he or you may ride up to the place and study how to error reset as hands on training.

Well, the returned unit still shows a code 7 error when reinstalled in my bike.
I cannot get it to reset (clear the code) either!

I'm at my wit's end as to where to proceed now.
It's hard enough trying to recover from my injuries.
My kids want me to give up bikes.
But I hang on.

I'm posting this both in the frame and the electrical section of the forum as they both seem to have ABS related questions.

cheers,

Andrew
Ipswich, Queensland

That's me Tosi is referring to. The ABS 1 is very sensitive to under voltage. If you haven't already done it clean all the connections, the multi pin plug with deOxit and scrub them with a tooth brush then spray them again. Clean all the ABS sensor and modulator plugs the same way. Clean all the earth points, use a pot scourer pad and deOxit then replace the earth wires and use some Loctite antisieze on the threads.  

I have found that 0.1v voltage drop due to bad connections can have an effect on those things and most of all keep trying to reset. I heard of one guy in US getting it to reset on the 304th go and as to date it has been OK. I made a wire that gets permanently attached to earth at the 3 pin plug (while you are working on it but not riding) and a special switch to work around the non-locking switch then all that needs to be done is key on and off and the switch on and off which makes it heaps easier.

Tosi is a very clever guy and get a bit impatient when others have problems that should not occur.
If it still keeps it up go for a ride while its flashing and get the battery up to top charge (20 minutes) then while riding in 5th gear turn the ignition off for 1 second and back on see if the lights stop flashing. Sometimes you need to have a few goes at it but if it's a genuine code 7 then it will work.

The units are strange things and the correct code doesn't always show. I had mine come up with a 7 and it was one of the wheel speed sensors had become unplugged because I probably didn't plug it in properly in the first place.

I have found that even the voltage drop from starting on a good battery will cause a fault code 7 but turning the key off and on seems to fix it OK if it is ridden regularly which is the secret to it. If it doesn't get ridden at least twice a week for a good few ks then they do play games with your head.

I had one that was sent to him and came back with an OK but persisted and I just persevered and eventually got it going but it has just started to give me the shits again.

Some riders have actually installed switches so they don't turn on till the engine is hot or a time delay.
The damn things will do your head in if ya let them.

It would not have gone from a 4 to a 3 code because the 4 code had to be cleared first, they only deal with 1 fault code at a time.

+1 on Rick's advice on cleaning the connectors, especially the one on the brain.

Just yesterday I took my K75RT out for the first time in several months.  It had an ABS fault, but when I tried to check the codes and do a reset, it showed no faults, and wouldn't reset. 

I cleaned the brain connector with DeOxit, and reassembled, getting the fault code #6 for the ABS relay.  Did a reset and now everything works properly again.

The more I live with my three old bricks the more I realize that electrical connector corrosion is probably the main problem you have to deal with, and you can't do too much cleaning and maintenance of the connectors. 

One caution, Rick is absolutely correct in advising to use a toothbrush and a Scotchbrite pad for cleaning connectors.  The connectors are plated to resist corrosion and the loss of continuity.  The fact we are experiencing corrosion problems indicates that the plating is beginning to break down.  To attack the remaining plating with abrasives will only make future corrosion problems that much worse. 

Best to clean with a cleaner like DeOxit followed by an application of silicone dielectric grease to prevent future corrosion.

I had already done the multi pin connector with the last of my deoxit. I've ordered a large aerosol can online so I can go through all the rest.
I hadn't considered that the fault would be stuck until it was cleared. However, the old brain now gives out a code 7 whereas it was giving a code 4 before.
Buggered if I know

What is the condition of the ABS system wiring?  People have issues with other wiring on the bike, why not the ABS wiring?  A wire by wire inspection for broken wires or chafed insulation might be in order if nothing else seems to work.

Is there anyone with a bike you can put your brain onto to see what it does?  That might help isolate the problem to the system wiring if that's where the problem is.

In the end it was a combination of corrosion in the connections and the pins in the connections spreading when I pried them apart.