OK, so I went to have the bike inspected and the brake light is not working when you use the rear brake. Works fine with front brake.

Took it home and reluctantly got out the multimeter. I started at the switch and found that I have 12.5ish volts coming into the switch and basocally nothing coming out when the brake is not depressed. Mash the brake and you get 12.5ish coming out of the switch, so that ruled out the switch in my book.

Traced the wire from the actual light back to the junction block under the side panel. From the light side, I have about 11 volts when the front brake is depressed and the light goes on. When the rear brake is depressed I only get about 4 volts and it jumps around a good bit.

So, my diagnosis would be that something has gone bad in the wire from the switch to the junction block. Sound right? It is a wrapped wire loom from the switch along the frame and back to the box, but it does run close to the engine so maybe it just wore out or melted or something. I hate to have to tear it all out of there, but that is all I can think is wrong.

Being that the multimeter is not my friend, I figured someone here might double check my thinking before I went to tearing it all out.

One other thing - I have a number of wires going into that junction block that do not have a corresponding wire coming out of the junction block. Plus a few wires just floating around inside the seat area. Makes me wonder....

 

All I can say is I share your pain. I have a 1990 International model, for which the wiring diagrams in the manual are next to useless, so I cannot trace stuff. I have got close to the point of thinking of going through all my electrics, to disturb stuff clean and grease with dialectric stuff, in the hope it will last another 20 years!

Give that sig pic of yours a cuff round the ear for sticking his tongue out!

 

Dan - the multimeter IS your friend, and it won't lie to you! It's very odd that the rear brake switch wire would have developed enough internal resistance to drop the voltage down to 4V at the junction, unless it's shorting out on something within the bundle - but a short would probably drop the voltage all the way down to zero, not to 4V. Sounds like corrosion on the terminals to me. Before you tear everything apart, I'd disconnect both ends of the wire, clean the wire ends and the terminals well, connect it back using dielectric grease, and then repeat your voltage drop test.

If it's still dropping voltage, instead of tearing everything apart you could just run a new shielded wire along the existing bundle, and use that instead of the existing wire, and secure it with tie wraps. It would be better to unwrap the existing bundle and rewrap it with the new wire, though - because it's down there where it will get wet and dirty.

I've unwrapped and rewrapped many of the existing cable runs on Bertha with heavy-duty electrical tape, after I had to get into them to access this or that wire for various purposes. For example, when I installed her Twin-Tec single-fire ignition, I had to run a new blue wire from it under the tank up to the new coil, along the existing bundle along the top of the frame. I unwrapped the entire bundle and rewrapped it to include the new coil wire. It wasn't difficult, and I used tie wraps about every 6 inches to secure the wrapping. You can also use friction tape around the bundles in areas where chafing is likely.

I understand about those mysterious wires! Being a Police model, Bertha also has LOTS of orphaned cables and junction blocks with nothing coming out of them after they removed all the police equipment from her. Drives me nuts, because I can't find a Police electrical diagram to tell me what all of these wires were once used for. I'd like to remove them all eventually, to clean things up. Maybe this winter...

 

Back brake light switches are notorious for going bad. Before you start chasing down a short I would replace the switch. You can get one for under $20. I use the Accel, the HD one uses rivits to hold the contact and they loosen up and you lose the connections. The Accel is one solid piece. The only hassle is bleeding the back brake. If you use a piece of clear hose it isn't that bad.

 

Jump the switch,see what happens...............Run a ground jumper off the tail light........see what happens.........check wires inside rear fender,make sure none are rubbed through from the tire

 

Ken is spot on with his description. Being a car guy, and having to teach technicians, I go through this all the time.. The meter is your freind, however, thats up to you to use it correctly. Sounds as if you are if you can see the 4v drop. My question would be, where do you see it? Are you checking at the blade coming off the switch or back probing the terminal end? Basicly, visualize a water hose.. You have 12psi filling your hose, all the way to the nozzle (the switch in this case). When you open that nozzle (close the switch) you are providing a path to ground. It is now up to the amount of resistance in the hose (circuit) to consume that pressure (voltage) before it actually goes to ground.. Now, if you have a sprinkler (brake light) that requires that 12psi to opperate and kink the hose (add resistance) the 12psi will be reduced at the sprinkler because its being used to push through the kink, creating a low pressure point at the sprinkler. With me?? So, by using your pressure gague(meter) on a live circuit, curent MUST be flowing to ground, you can now look for your pressure loss.. I'm gonna switch back to electrons, but its the same idea.. You can do a voltage drop. If you start with the power side(battery to light bulb) at the B+ cable and check to the hot side of the switch. You should have 0v there and it sounds as if you do.. Then check the outlet with the switch closed (peddal depressed), you should have 0v (actually <.5v is typically acceptable). Again, it sounds as if your good there. At this spot, you could be checking both the sw terminal and the harness connector, you should see 0v at each... Continue following through the ckt until you see your missing voltage... HD wire diagrams SUCK!!! But, thats all we really have, try to break it down as simply as possible. If the front sw works, you know its not in the ckt past where the front and reas splice together.. You can also rule out a loose ground or bad bulb..

Hope that might help a little. Many guys are intimidated by electricity and testing it. This is where the test light comes in. If you've got a light that has the same resistance of the component you are testing, it ill light if you have enough voltage/curent flow and not if you dont.. So, it really takes allot of though out of the equation. The problem comes when you are testing electronics. The current flow through the test light might be too much for the little drivers in the component to handle. This causes the magic smoke to come out and gets expensive..

With all that, I just had my buddy tell me my brake lights werent working.. I use the rear more than anything, so we checked and the fronts were good, but the rear wasnt. I stopped by the shop and picked up a rear switch w/o even testing it and put it on, fixed.. Its a reckless way of working, but I knew in my gut it was the switch.. Like these other guys mentioned, you may just fix it with a switch.. But the testing could be a valuable learning process for you. either way, good luck..

 

I like HGMs(greg) comparison with the "garden hose" to electrical problem solving.
I am hopeless with electrics and a guy once tried to describe "electricity" like this............"think of a large river and its tributaries" and went on to explain.
It must have been the Mississippi Delta he was talkin about,because 25 years have gone by and Im none the "friggen" wiser.!!! At least we can still have a laugh.
Good luck with repair,Bungo.

 

+1

Keep it simple, besides you don't have license to operate high-tech machinary anyway

Sorry Dan, couldn't resist. A bad ground will make your voltage go away too, when it feeds the hot to a bad one, it throws positive to everything around it. Wouldn't suprise me to see a test light burn with one end grounded and the other touching the fender... then you'll scream. LOL But I'd bet it's your switch and be sure to use DOT5 in that one when you bleed it. If it's yellow when you take the cap off, suck that out before you do any bleeding. DOT5 is blue and turns yellow after it get way old.

As suggested, jump the wires and see if it'll light. Those switches WILL go bad and just checking voltage there will be no load and when the bulb is connected the load draws the voltage down. Mine was bad for years, just got enough to make it look like a taillight and never kick the anti-dive.

Standard (brand) part number is SLS34 about 13.00 and most parts stores stock them. I used an Accel that lasted all of 2 months and the last one was a "Standard" and it was identical even down to the fine print on the metal...

PS: there are two wires connected at one side of that switch and one of those comes FROM the front brake. So if the front works, that already tells you the wire going to the brake light bulb is OK.. There's only one wire going to the bulb beyond the rear brake switch...

Call me if you need a cheerleader