How do I test my BMU?

[Kontoboy]

I pulled my '87 K75Cs bike out of the shed after sitting for three months.  It fired right up but the console brake check light remained on after hitting the front and rear brakes.  The rear brake lever/light work fine, but no brake light with the front lever.  I pulled the tank and checked the switch at the connector with an ohm meter--the switch works fine.  I then checked the front switch continuity back to the BMU harness and it is fine.  I also have 12 volts at the BMU harness and back up to the switch connection under the tank.  I'm using a stock 12v bulb (no LED) and even swapped it out to be sure it wasn't the problem.

The harness connections all look fine--clean and no frayed wires and fits snug on the BMU. 

I am thinking the BMU is bad,  so I pulled the top off and visually it looks fine--no fried resistors and loose solder joints.

I read the post on by passing the BMU (http://www.motobrick.com/index.php?topic=906.msg3606#msg3606) but didn't see a post on checking the BMU itself.  Any ideas how to do this?  Or do I just bypass the BMU, and if the brakes work assume the BMU is bad?

Thanks mates!

[Inge K.]

Also check the Circuit for Your tail light, it's also controlled by the BMU.

[Kontoboy]

Since the brake light worked when actuating the rear brake I assumed the wiring was good from the BMU to the brake light. Is the a separate circuit from the BMU to the brake light for the front brake?

[Scott_]

Since the brake light worked when actuating the rear brake I assumed the wiring was good from the BMU to the brake light. Is the a separate circuit from the BMU to the brake light for the front brake?

Check for power on fuse #2 with the ign switch and lights on. (hint if you have the small park light above the headlight lit, then fuse #2 should be good)

Terminal E1 of the bmu is the input from the front brake switch. grey/red
Terminal E2 is input from the rear brake lever switch. grey/green
Terminal 15 is +12 power from fuse #1  green/black
Terminal 58E is +12 power from fuse #2 grey/white
Terminal 54A is output to the brake lamp  grey/yellow
Terminal 58A is output to the tail lamp grey/black
Terminal K is the output to the inst cluster warning lamp white/blue
Terminal 31 is frame ground. brown

[Kontoboy]

Thanks everyone for all your insights.  I found the problem.

Got the bike home on Saturday and dug into today--checked fuses, tested switches and continuity and voltage on all the BMU circuits, then wa-la, I noticed that one of the running lights was out.  (I didn't notice it before working outside in the bright Florida sun).  The PO wired the turn signals for 1157 dual filament bulbs to serve as running lights and I simply didn't notice that one was out.  I swapped the bulb and abracadabra, the BMU is working and the warning light goes out as the front and rear brakes are applied.

Still some more work to do--the bike fell over on the trailer bringing it home. (I know--trailering is a sin but the wife wasn't up for 1000 mile ride!)  The bike came to rest with the wheel well on the fuel rail.  The tip over bent the fuel rail enough to tear a hose, snap a rail mountling bolt and pull an injector out.  Luckily I had a spare rail and was able to drill out the broken bolt and retap the threads.  Now I'm just waiting on fresh o-rings for the injectors.

I also picked up a flea bay fuel guage and wire harness.  It works but the calibration is way off, and I need to figure out where and how to mount it. Time for more research!

Thanks again.

[Kontoboy]

Well I thought I found my problem.  Then I took the bike out yesterday and the brake indicator light stayed lit and once again the brake light wasn't working when I squeezed the front brake lever. 

I dug into the electronics once more determined the find the electrical gremlin tormenting me.  I rechecked continuity and voltage at the connectors and all was fine.  Then I noticed the brake light was flickering on as I moved the BMU relay around.  Thinking a bad connection at the relay plug I checked all the pins at the connector and they were good.  Then I pulled the lid off the BMU and checked continuity from the connector plug to the circuit board.  Aaha!

The E1 connection to the board was intermittent.  While looking straight at the board the solder joint looked fine.  Looking at the connection from the side of the board I detected a hairline crack in the solder joint. 

Pulled out my bench soldering iron and re-soldered the joint, and so far so good!

I hope that gremlin is finally gone.