Motorcycle brake light issue...how the hell is this possible?
 

Ok guys I'm at my wits end here with an inop brake light/turn signal on my bike.

When you step on the brake the light on the right side won't come on but the left does. The odd part starts here though. If I probe the socket with a test light the test light comes on. And if I measure it with a volt meter I have around 11V. I also checked the socket for ground continuity and it checks out good. And I tried 2 different bulbs that do in fact work in the other sides light socket.

The wiring for this light runs up to a trailer brake controller...this allows me to use the single filament bulb as both a brake and turn signal. If I seperate this connection and run 12V from the battery to the connection for the rear brake light the light comes on. You would think that means the power feed to that connector must be bad...but after probing that with a meter I have 11V there also. So how is it that with voltage at that connector and voltage all the way at the light socket, with good ground and a good bulb the light won't come on????

Paul,

Did this work previously?

Yes....for 2.5 years

Custom wiring? My buddies sportster did something very similar after we installed new handlebars. After I cut and re ran the wires through the bars, then re soldered them he had tail light voltage issues. I re cut them and did a more careful soldering job and it fixed it.

I want to say when he put the headlights on the brake light worked but the tail light didn't or the other way around

Hmm wouldn't be my idea then.

Tail light socket going bad maybe?

Mike, that was what I was thinking too.

I mean could be but if I'm getting voltage there and when I feed it right off the battery it works, wouldn't that rule it out?

Maybe that's where my buddies issue is similar, with direct isolated power it is still functioning. But with one little draw (split from the controller) it loses just enough juice to act up?

so both bulbs work in the one side and not the other.
you have power in the 'bad' socket. You verified ground there too?

maybe the socket is bad and the bulb is not making contact?

How are you attaching the jumper wire to test it? I was thinking that the connection between the brake controller and the socket hot might be weak. Hard to say.

Alligator clip into the female connector.

I would rule out the socket being bad since it illuminates with the upstream voltage from the battery. But when the brake controller is hooked up I'm getting 11v at my socket still. Shouldn't that just illuminate the light?? Maybe I'll try jumping the brake controller

I was thinking that even at 11v you should get SOME light, even if it's dim. This is defiently a head scratcher.

can you try switching the right and left wires right at the brake controller? that way you can tell if its just specific to one side of the circuit or specific to the designated output of the controller.

You're telling me!

Good idea Paul...I'll have to try using a jumper between them and see. Won't happen until tomorrow now though. Just came in from another 70 hour work week.

I was just going to suggest what Paul did. That should help clarify what is the issue. Or complicate it more :lol:

i also agree with Paul V and sbmxrdr. Try swapping the wires, see what happens. If it works after switching wires, its a problem with the controller, obviously. If it doesnt change anything, just burn the damn thing, lol.

Its possible to have voltage at the controller output, but due to internal issues in the controller itself, corrosion, wear, ect, adding a load to it will introduce enough resistance to the circuit (the lamp itself and then possible heat buildup from internal corrosion in the controller, which will create more resistance) to drop the voltage down low enough to keep the lamp off. Kinda like draining a swimming pool through a straw vs a firehose to put out a fire, sure there are 5000 gallons of water available, but the straw makes it worthless. Electricity can work the same way.

is it possible to check voltage to ground at the lamp when the brake is being pressed? Across the lamp and from the + side to the chassis?

i could swing by saturday afternoon or sunday and give you a hand if you need it. Give me a call.

Phil....if I had the voltage drop like in your analogy wouldn't my voltage at the lamp be lower than 11v?

Voltage to ground I should be able to test. Can I remove the stock ground and just run a meter between the light housing and frame to see the voltage?

yes, but only with the lamp installed, so you need to test with the lamp in. With no lamp, there is an open int he circut and no current will flow, so the restriction will not be evident

that will work see what it shows

maybe also see if you can check from the + of the bulb, before the bulb, to ground.

ultimately its a simple system, if the bulb works elsewhere, and there is a voltage at the socket, yet the bulb does not work, there is a restriction somewhere, and unless you have 100000 miles of wire before the bulb, its going to be an issue with the controller. Im confident that if you switch wires on the controller for the 2 rear brake lights, the now unlit bulb will work and the other wont.

Yeah I'm going to swap them tomorrow. I'm also wondering if the wire going to the light might be toast....it sits very close to the exhaust tip....but again I wouldn't have expected to see all the voltage at the socket. Is that possible if the wire is heat damaged?

yes, thats also possible. If its bad enough, it could cause this. I suppose that will depend on what happens when you switch wires.

is the wire all visible or is it run in the frame or something? if you can get it out without tearing the bike apart you can take a look. It will be pretty obvious if its damaged. However, if its that bad, it would have shorted out to the frame.

Everything from the controller to the light is basically visible.

Your issue sounds like lack of amperage and not a voltage issue, you can have voltage without amperage. Your car battery could have 12 volts but if the amperage isn't there you will not be able to crank the engine . A wire worn down to 1 or 2 strands, a wire broken inside of the insulation. I've seen wires start to corrode on the inside of the insulation and just rot. I've also had issues with ground wires in particular. My meter tests good, my test light shows the engine has good ground yet the starter can't get a good enough ground to complete the circuit due to a frayed ground strap.

I would start with swapping the wires from the opposite side on the controller as someone else mentioned. if it still doesn't work go after wires. If it does work it's something with the controller, or maybe a voltage/ground issue to the controller. If you have a voltage or ground issue going into the controller you will have issues coming out of the controller also.

Had 30 seconds to myself yesterday and clipped the signal wire going into the brake controller. Jumped it to the wire for the light and still no functionality....so the issue is before the controller. Maybe this weekend I'll have time to backtrack it.

Turns out it was the controller after all. Wired a new one in last night and everything is working properly again.