Third Brake Light Wiring Problems

[cjfgraham111]

Hi,
I'm looking for some help with wiring a new third brake light that I bought from Moss Motors (they look great, by the way). It is an LED unit that has about 10 LED's in it. I thought that I'd be able to simply splice a wire into the existing brake circuit and all would be well. But, not surprisingly (for me anyway), I've been defeated by the black magic that is automotive electricity. I know that for most of you this is a fairly basic issue.

When I spliced it into the left hand brake wire the LED unit worked but the normal brake light (left one) stopped working. The right-hand brake light still worked. So I got fancy and thought that I'd take the left hand side of the brake circuit and feed it directly to the LED unit and just run another wire from the right-hand light. This worked, sort of. All three lights worked but at a significantly reduced level.

Do I need to run a completly seperate wiring circuit from the brake switch to get power to this light? Any suggestions?

Thanks, Chris g.

[alan71]

Did you connect it between the green/purple and black wires?

Alan

[John Larkin]

Chris,

LEDs have a much lower resistance than conventional car bulbs. For instance, when you replace original indicator bulbs with LEDs you need to add a resistor in line at each bulb or the indicator relay will not work properly because it needs a specified resistance to work against, and this is provided by the filament in the original bulb. When you wired up the new brake light across the the power and earth wires for the original left brake light I suspect that the current took the line of least resistance and most went through the LEDs rather than through the relatively high resistance original type of bulb, so the left brake light did not glow. I would have expected the right unit to have been affected as well though. I imagine that replacing the original brake light bulbs with LEDs also would overcome the problem, or putting a suitable resistance in line with the power feed to the new high level brake light.

John Larkin

[RotoFlexible]

Hi,
When I spliced it into the left hand brake wire the LED unit worked but the normal brake light (left one) stopped working. The right-hand brake light still worked. So I got fancy and thought that I'd take the left hand side of the brake circuit and feed it directly to the LED unit and just run another wire from the right-hand light. This worked, sort of. All three lights worked but at a significantly reduced level.

This depends on what you mean by "spliced". The LED unit you bought should have the necessary circuits inside so that it doesn't need additional resistors etc. You should be able to hook it up by "tapping into" any of the green/purple brake light wires - that is, leaving the wire electrically intact, but stripping away some insulation and connecting from the exposed wire to the LED unit. (You also have to run a ground wire from the LED unit.)

In electrical terms, this is wiring the LED unit in parallel with the other brake lights.

What I suspect you have done is cut the brake wire and run wires from both cut ends to the LED unit. This would be wiring the LED unit in series with the brake light, and would produce the symptoms you have observed.

As far as the actual wiring - I would (in fact I just did) use connectors rather than exposing the wire as described above (I was just trying to make a point). I cut the brake wire, then used a four-way bullet connector (it connects up to four wires) to re-connect the cut ends to each other and to the LED.

[neilsjuke]

Have you got the polarity right neg / pos earth on the black . depending on your car battary to earth.
Neil

[alan71]

When I spliced it into the left hand brake wire the LED unit worked but the normal brake light (left one) stopped working.

It sounds like you have wired it in series with the left hand light.

The resistance of the light bulb would be so low, compared to the resistor in the LED unit, it would have no effect and the LED would light up. The current flowing through the brake light would be to low to light it.

Alan

[alan71]

Hi,

As far as the actual wiring - I would (in fact I just did) use connectors rather than exposing the wire as described above (I was just trying to make a point). I cut the brake wire, then used a four-way bullet connector (it connects up to four wires) to re-connect the cut ends to each other and to the LED.

That would be the easiest way. If it?s like mine (S4) the connectors are already there, you just need to change the 2way for a 4way.

Alan.

[cjfgraham111]

Thanks for all of the suggestions.

I'm pretty sure that I have the polarity correct (but I'll check anyway). I'm using the correct circuit. When I said splice I meant that I wired it in parallel with the left-hand brake circuit. When I did that, the LED unit worked but the regular left-hand brake light did not work (but the right hand brake light did).

The second way I tried was when I took the left-hand side of the rear brakc circuit, disconnected it from the left-hand brake light and used the wires to power the LED unit. Then I wired the left hand light in parallel using the wires that power the right-hand brake light. This resulted in all three lights working, but at a significantly reduced light level.

From all the suggestions I received (Thanks, by the way!!) it sounds like is may be a resistance issue (or lack of resistance). Is there anyway to determine what level of resistance I need or is it just a trial and error process. It also sounds like I need to put the resistance into the line that leads to the LED unit?

Thanks again for all the thoughts.
Chris g.

[alan71]

A LED typically requires about 2V, so to run it on 12V you need a resistor in series. If you bought a LED brake light designed for 12V the resistor would be built in and you just wire the unit in parallel with the existing brake lights.
If you have all the lights working but at reduced brightness it sounds like a bad connection, always a risk when you disturb old wiring. Check that you have full 12V, if not measure the voltage between battery ? and the black wire and battery + and the green/purple wire (assuming negative earth).

Alan.

[neilsjuke]

I would remove all the bullet conectors and give them a good clean and or replace with solder bullets and clean the conectors do the side /stop lights while you are at it and make sure you have a good earth. tip with a solder bullet strip the wire long and heat the brass bullet and feed the solder into the wire sticking out.

[billwill]

A LED typically requires about 2V, so to run it on 12V you need a resistor in series. If you bought a LED brake light designed for 12V the resistor would be built in and you just wire the unit in parallel with the existing brake lights.
If you have all the lights working but at reduced brightness it sounds like a bad connection, always a risk when you disturb old wiring. Check that you have full 12V, if not measure the voltage between battery ? and the black wire and battery + and the green/purple wire (assuming negative earth).

Alan.

The LEDs in the light are probably wired in combinations of series & parallel, so that you don't waste the power represented by 10 of the 12 volts.