In need of someone that has a bit more electrical knowledge than I do. I'm trying to attach an extra brake light to my bike rack that hooks up to the trailer hitch using the trailer hitch wiring. Thought I had it figured out until I tried it. Went to try the turn signals and both sides on the coach blinked. disconnected hitch wires and turn signals went back to normal. My thought are since both sides are connected in the light on the bike rack, it's running power back through the side that isn't being used. What I probably need is a couple of diodes to prevent reverse flow on both turn signal lights. Here's where I need the help. How do I determine what kind/size/rating diode to use? I know the theory, just never had to use any in a project that wasn't already pre set up for me.

Maybe you can use one of those adapters that are used on vehicles with separate brake and signal bulbs.Someone else may explain it better since it's been a long time since I dealt with itThis

Got one controlling the 3rd brake light on the coach. it's too big to fit on the bike rack, and don't want to mess up the hitch wiring if I need to tow something.

The EASIEST solution is to pick either the right brake light/turn signal or the left brake light/turn signal and hook to that. It will function perfectly fine as a brake light, but will flash with that turn signal. By tapping into both the right and the left signals, you don't get increased functionality as a brake light, but you get flashing on both turn signals AND backfeeding, as you have found.The harder solution is to find the wire feeding your third brake light and tap in to that.photobug wrote:Got one controlling the 3rd brake light on the coach. it's too big to fit on the bike rack, and don't want to mess up the hitch wiring if I need to tow something.After posting, I saw you posted this just before me. This gives you the perfect answer. Tap into the "STOP" wire and "GROUND" wire and run them to the light bar with a 2 pin connector rather than the trailer hitch connector.

shouldn't need an adapter. simply tie into only 1 turn signal light.

Your basic problem is that there isn't a brake light line on the trailer connector; there's just a left brake/turn and a right brake/turn. It sounds as though you just shorted the two together and fed that to your new brake light, and apparently the same circuits power the vehicle's turn signal lights. Even with steering diodes to prevent back-feeding, you'll still have the problem of the brake light on the bike rack blinking whenever a turn signal is on and the brakes are not on, say when changing lanes or merging onto the highway, which I suspect could be rather confusing to drivers behind you.If you have a third brake light on your vehicle (or no third brake light but separate signal and brake circuits at each side), probably the simplest approach would be to tap into that line and run a separate connector for the light on your bike rack, not plugging into the trailer connector at all. If you don't have a separate independent brake light circuit anywhere at the back of the vehicle, it will be pretty hard to do what you would like to do; you'd probably have to run a wire to the front and tie into the wiring harness somewhere in the dash area, perhaps at the brake pedal switch, and the details on how that would work probably vary somewhat from vehicle to vehicle.

electrical expert | Good Sam Community

Bobbo
Explorer II
Jun 25, 2017
Much easier to just install another one of these

at the rear of the coach/car (or on the bike carrier) and only tap into the "STOP" and "GROUND" wires. Doing this would let you use the 4 pin or 7 pin hitch connector. At 4"x9" it is fairly large, but at $22 it is cheap and easy. (Clicking the photo above loads the Amazon.com page.)
wnjj
Explorer II
Jun 25, 2017
Bobbo wrote:
DrewE wrote:
The bike rack light will flash whenever a turn signal is on and the brakes are applied, though.

True. We are back to tapping the 3rd brake light wire or the output wire of the adapter.

It can be done right with 2 relays and 3 diodes but that's getting pretty complicated.
Bobbo
Explorer II
Jun 24, 2017
DrewE wrote:
The bike rack light will flash whenever a turn signal is on and the brakes are applied, though.

True. We are back to tapping the 3rd brake light wire or the output wire of the adapter.
DrewE
Explorer II
Jun 24, 2017
Bobbo wrote:
wnjj wrote:
How about a relay which is controlled by the left turn signal? Then feed the right turn signal through to relay to your 3rd brake light. Then it will only light when both lights are on. A solid state relay would be best since it will cycle quite a bit.

This is a wonderful idea. One of the turn signal/brake lights activates the relay, but sends no power to the new light. The other turn signal/brake light sends power to the new light, but if the relay is not activated the power does not get there. The third light will ONLY light up if BOTH turn signal/brake lights are lit.

The bike rack light will flash whenever a turn signal is on and the brakes are applied, though.

Frankly, if it is not too bright of a light, I'd be tempted to hook it up to the running/tail light circuit and have it on all the time the headlights or parking lights are on. If it's more bright like a stoplight than a tail light, that's probably not a good solution. I don't know about the precise legality of such a setup, but I suspect it's close enough to being legal to not get into too much trouble.
Bobbo
Explorer II
Jun 24, 2017
wnjj wrote:
How about a relay which is controlled by the left turn signal? Then feed the right turn signal through to relay to your 3rd brake light. Then it will only light when both lights are on. A solid state relay would be best since it will cycle quite a bit.

This is a wonderful idea. One of the turn signal/brake lights activates the relay, but sends no power to the new light. The other turn signal/brake light sends power to the new light, but if the relay is not activated the power does not get there. The third light will ONLY light up if BOTH turn signal/brake lights are lit.

On the relay, attach pins 85 and 86 to the hot wire of one of the turn signal/brake lights and a vehicle ground. It does not matter which pin goes to which point.

Attach pins 30 and 87 to the other turn signal/brake light hot wire and the hot wire of the new light. It does not matter which pin goes to which point.

Do not use pin 87a.
wnjj
Explorer II
Jun 24, 2017
Using diodes means the new light will flash if either turn signal is on. Not a good idea and may not be legal. Turn signals are supposed to indicate direction which a center-mounted light will not do.
MEXICOWANDERER
Explorer
Jun 24, 2017
Diodes. Go overkill city.

For something like this I will choose a 10 amp axial diode. I use heat shrink on each side but leave the plastic diode case exposed to lose heat. Twin 6-amp axial diodes can be used in parallel each side but that's a little lumpy.

Yet another area where theory gets the ---- knocked out of it by reality. When I used 3-6 amp 1,000 PIV rectifiers on a 2 amp incandescent bulb they failed. Theorists fail to acknowledge tungesten filament inrush.
wnjj
Explorer II
Jun 24, 2017
photobug wrote:
I'll have to look into how I would wire a relay, especially since I have a relay that the support tab snapped off, also still have the old turn signal relay I replaced to fix the hyperflash issue when I switched to LED lights.

12V automotive relays have pins with numbers in the 80's, plus 30.

Pin 85 - ground on 7-way
Pin 86 - left turn on 7-way
Pin 30 - right turn 7-way
Pin 87 - first pin of new light
Ground on 7-way - other pin of new light

This performs a logical AND which only lights the new light if both left turn AND right turn are lit. Your turn signal unit is probably a flasher and not the same thing as a relay.
wa8yxm
Explorer III
Jun 24, 2017
Your thinking is good. Two options, well 3
One, you can get an adapter, kind of like the one pictured above. Been a long time since I've seen 'em though they will turn on the added light ONLY if both turn signals light.

Option 2 is to go to a dual lamp system on the bike rack, one for each side. This way half the fixture lights if it is turn signal, all for brakes,, I kind of like this solution, Or two side by side single lights.

Finally diodes.. Put diodes (3 amp at 24 volt should do nicely) from your local electronics store in the line.

Diodes are electrical "Check valves' put it in but do not formalize (Solder) the leads and test, if it works the way you want great.. Oh the banded end goes toward the bike rack. (if the diode is backwards turn it around).
photobug
Explorer
Jun 24, 2017
I'll have to look into how I would wire a relay, especially since I have a relay that the support tab snapped off, also still have the old turn signal relay I replaced to fix the hyperflash issue when I switched to LED lights.

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