Voltage loss in brake wire when light is hooked up.

[Dgill82]

Hi All,

I have a 88 Vmax, i have a very strange problem with the brake light wire. It had got layed down previously and upon putting the bike back together from mostly minor damage I got new LED dual running and Brake lights. But when i hook them up which i have done on a few other bike with no issues. the brake wire is showing 0 voltage when a light is hooked to it. However if the wires are open just hooking them to a Multimeter it reads 12 volt with every squeeze of the brake lever. I tried to hook up the old lights also even with no bulbs and still with the same result. Any input would be very appreciated for this has me stumped.

thanks,

 

[ninjaneer]

so you have the yellow brake wire going to the LED brake wire? where in the wire harness are you measuring the 12 V? is the yellow brake wire shorting to ground between where you measured the 12 V and LED light? is the LED light grounded?

 

[Dgill82]

the new lights are self grounding, and the tail light turns on no problem with the new lights so i know the lights are good.Also i have tired this with the original harness also. for the yellow wire when its hooked to the harness for the light the yellow has no voltage when hitting the brake lever, but with the wiring harness unplugged i just connect the open yellow to the positive and the ground to ground and when i hit the brake lever i get 12v every time. and i have gone over all the fuses. So when ever the harness or any light is connected to the yellow wire the voltage is 0 when applying brake but has power when not connected to anything.

 

[ninjaneer]

which LED taillight are you trying to use? have you tried to directly tap the LED taillight's brake wire on your battery's positive post? have you tried directly tapping the LED taillight's turn signal wires on the battery post? have you tried to tap the LED taillight's running light wire on the battery post? have you tried simultaneously tapping the LED brake and turn wires on the battery post? etc .....

 

[ninjaneer]

humor me and attach a length of wire from one of the bike's ground wires to one of the taillight's mounting screws

 

[jfeagins]

If your situation is anything like the front turn/running LED's I installed on my 09, you may need a diode between the running light wire & the turn signal wire near the LED unit. I had all kinds of flakiness going on during my install, and isolating that running light input wire going into the LED unit with a simple little diode made all the difference in the world.

 

[VMAX1260]

humor me and attach a length of wire from one of the bike's ground wires to one of the taillight's mounting screws

+1
Dude ground your tail lights with a wire to the chassis. i cannot understand when you say the lights are self grounded. i dont know how a unit like lights or tail lights are self grounded if there is no actual wire from any device to the ground

 

[Dgill82]

I got some dual LED chrome Bullet brake and tail lights, they only have to wires coming from the light one hot tail light and one hot brake light, for grounding the housing is all metal and when connected to the frame the light is grounded. but for easier testing purposes I have been just using the factory tail lights. with the light hooked up as normal the running tail lights work great no issues. but when applying the brake the dual filament doesn't light up. so I put the voltmeter on where light plugs in, the factory male to female connector. Upon doing so the voltage reads 0 when applying the brake light. so I then disconnected the yellow and tested the voltage with the blue and ground connected to the light as normal I get 12V everytime I hit the brake.

 

[Dgill82]

So to test the wire I went all the way up to the first connector by ignition switch and spliced into the brake wire there and ran a that new spliced in wire down to the lights. still got the same issue no brake light. But then I had the idea of maybe just the switch itself is bad, I have a spare switch and when plugging that in and with the new wire hooked to the yellow on the lights and the lights ground wire going directly to the battery ground. I finally got some action with the lights lighting up when the brake is applied. however this only worked once while it was hooked up, So I try to hook it all back up with the new switch and I still get the same response

 

[ninjaneer]

it is a bad assumption that grounding an electronic to the frame will "ground" the light. if the metal grounding plane of the light is suppose to make contact to the negative battery post that contact to the frame needs to be clean. and by clean i mean no paint.


a simple test is to just a continuity check from the negative post to the metal surface of the light once it is installed
Regards from my Taptalking Hercules Android

 

[Dgill82]

So I finally found the issue it was the connector to the switch was the problem. I noticed it was loose just a bit, so I replaced both and bingo brake light works as normal. Thank you all for all your help.