Tail light /Brake light problem??? Help.

[monkeywrench]

When I apply either brake my tail light dimms instead of brightning up and if I hold it long enough the fuse blows. I am not sure what is going on here. Any ideas. This is the only thing that is holding me up from getting it in the road.

[horror1972]

bad ground or bad contact.
Clean the contacts and check the ground wire

good luck

[monkeywrench]

bad ground or bad contact.
Clean the contacts and check the ground wire

good luck

The contacts are good as far as I can tell and the ground seems to be fine.

[knotzilla]

It sounds like the brake circuit is shorting out. Trace out all the wires for the brake circuit and look for frayed wires that might be contacting each other or grounding out. Also, check the bulb socket in the tailight itself. It could be rusted out. Ted

[kurifu]

Bad contacts and rusty sockets are not likely to cause a blown fuse, since they would typically add resistance causing a lower current flow.

Remember: V = IR ir I = V/R, so as R increases (resistance) without bound, I get closer to 0 (in amperes).

I recall that the light circuits for these vehicles are really simple. A few points you might actually want to check:

1) Measure the current flow by removing the bulb and using an ammeter to touch the contacts, have someone pull the brake level.

2) Make sure you are using the right fuse, the reading above shouldn't exceed the rating on the fuse.

3) Make sure you are using the right type of bulb, too many watts could cause the same problem.

If all of that doesn't answer the questions than you need to start looking for a short to ground between the brake switch and the light bulb, you might be able to illiminate mutual wiring if the problem is only happening with one of the switches.

Good luck, electrical problems blow (pun intended)

[monkeywrench]

I disconnected both brake wires from both brakes and the tail light is still bright. There are no bad grounds that I can find along the wires. My tail light is brand new, so no rust or loose connections. I had it working correctly about a month ago and now it's wrong. I didn't change anything except pull the battery and put it on a tender.

[SqDancerLynn1]

Check the ign switch Could feed back thru park position. My lights used to dim when brakes were applied/ didn't blow fuse. Hi resistance thru ign switch.

[phaedrus]

I disconnected both brake wires from both brakes and the tail light is still bright. There are no bad grounds that I can find along the wires. My tail light is brand new, so no rust or loose connections. I had it working correctly about a month ago and now it's wrong. I didn't change anything except pull the battery and put it on a tender.

I noticed you indicated the tail light assembly is new. Has it ever worked? Also, is it an OEM tail light and are you using the correct bulbs? I'm thinking more about miswiring here as opposed to incorrect loading.

Your tail lights have 2 filaments. A P21/5W bulb, for example, has a 5W filament for the tail light function, and a larger (I'm not sure if it's 21W or 16W) filament for the brake light. It sounds like the brake circuit is shorting to chassis. This prevents the brake light filament from energizing, and pulls the battery voltage down, which dims the tail light. I suspect the rest of the lights on the bike dim, too? Eventually (I hope not too long), the (10 amp?) fuse pops.

I like what you did by disconnecting the brake light switches. Try this again, but disconnect the tail light assembly. I realize you won't be able to see your tail light dimming, but you should be able to watch the rest of the lights on the bike to see if they dim. Also, I guess you could wait long enough for the fuse to pop (or not) as part of the test. This test will point to the light assembly or the wiring (between the brake light switches and the tail light). If the test 'passes', meaning no dimming / blown fuse, you could hook the tail light back up but remove the bulbs and repeat.

Even though it sounds like corrosion is not part of your problem, corrosion can most definitely cause shorted or partially shorted circuits and blown fuses. It's true that corrosion causes high resistance connections. It's also true that corrosion has a much lower resistance than air. Short circuits occur when corrosion bridges the space between 2 or more circuits. I use silicon dialectric grease on just about every electrical connection on my bike, including the lamps. It inhibits corrosion, lubricates (no tug of war getting them apart), and protects.

let us know,
phaedrus