LED 3rd Brake Light Logic Unit Issue

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hazard147@hotma

Member - Posts: 4

Joined: September 17, 2006

Posted: September 20, 2006 at 10:16 PM / IP Logged

I have a 93 full size Chevy Blazer and recently installed a LED 3rd(upper) brake light. To make the setup work properly, I also installed a logic module for this application from JC Whitney which will only light the 3rd brake light when both taillights are on. (This is because I have brake lights that double as turn signals)

I have it all wired properly, checked it multiple times, but the LED's barely light up whenever either turn signal is on, or when the brakes are on.

The module is a 3 wire unit, one wire to each of the factory tail lights and the final to the new LED unit.

What do you think is wrong? I think that I either

A: got a bad logic unit

B: since the logic unit has only three wires, it is counting on a ground source through the 3rd tail light. A standard incandescent bulb is only a couple of ohms, basically a connection to ground. Since my new tail light consists of only LED's which are not allowing any "reverse current" to supply a ground for the module.

If B is correct, do I need to put a power resistor in parallel with the new tail light(from the module output to ground) so that the logic module can "see" a ground? What value and power rating do you think would work?

Any other possible causes to my problems?

Thanks in advance

-Mike

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ill-logix

Member - Posts: 39

Joined: April 07, 2006

Location: United States

Posted: September 22, 2006 at 10:49 AM / IP Logged

I dont know about the logic unit but uou can do this with 2 relays. Wire the coil of a relay to to each turn signal. Then wire +12 to one relay's normally open contact, wire from the other side of the contact to the normally open contact on the second relay. Now wire from the other side of the normally open contact on the second relay to your third brake light and ground the light.

ie:

- denotes wire

] [ denotes normally open contact.

******* denotes brake light

+12--------] [-------------------------] [------------------------*******--------------------GND

left relay right relay 3rd brake light

ill-logix

Member - Posts: 39

Joined: April 07, 2006

Location: United States

Posted: September 22, 2006 at 10:52 AM / IP Logged

Forgot to add this

O denotes relay coil

+12v Left turn signal --------------O--------------------GND

left relay coil

+12v Right turn signal ------------O--------------------GND

right relay coil

Mad Scientists

Silver - Posts: 380

Joined: February 07, 2004

Location: United States

Posted: September 22, 2006 at 1:55 PM / IP Logged

Measure the voltage on the wire that goes from the logic unit to the third brakelight (referenced to ground). If the voltage is 12v then you have a ground problem or a problem with the LED unit. If you don't see 12v then you might have a problem with the logic unit (obviously measure voltage when the brake lights are on)

Let us know what you find..

Jim

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hazard147@hotma

Member - Posts: 4

Joined: September 17, 2006

Posted: September 22, 2006 at 5:47 PM / IP Logged

Thanks ill-logix, hadn't even thought of that, I would really like to get this logic unit working so I have the fewest points of failure possible.

I tested the voltage at the unit's output, with no load, with either turn signal on or the brake lights on I see 11.5v. At my trailer wiring I see 12.55 volts.

When I have the load applied, the LED brakelight, I see 7.45v with either turn signal on, or the brakes on.

JC Whitney sent me a second module for free because they didn't understand my problem :) I hooked it up, exact same prob with the brand new module.

I am on my way to Radio Shack now to get a resistor to try my original idea, I am about 95% certian that there is just not enough resistance in the "load" side of the circuit.

Will keep everyone posted and thanks for your help,

-Mike

ill-logix

Member - Posts: 39

Joined: April 07, 2006

Location: United States

Posted: September 22, 2006 at 6:12 PM / IP Logged

I doubt that the problem is not enough resistance. If it was anything like that it would be that you dont have enough load or to much resistance. I imagine that the logic box you have bought it solid state so double check that you have a quality ground connection. The fact that you are getting an output when just one turn signal also indicates that there is floating voltage due to a lack of ground. So the output is actually current leaking through the solid state gates. Relays rule :)

hazard147@hotma

Member - Posts: 4

Joined: September 17, 2006

Posted: September 22, 2006 at 7:54 PM / IP Logged

Success, I added a 50ohm 10watt resistor in parallel with the LED tail light and it now works perfectly!

Thanks for your advice and I hope if anyone ever has a problem with this, that they will find this thread.

-Mike

ill-logix

Member - Posts: 39

Joined: April 07, 2006

Location: United States

Posted: September 22, 2006 at 8:06 PM / IP Logged

awesome, I didnt realize that the ground was provided through the brake light. clear as mud. Congrats :)

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LED 3rd Brake Light Logic Unit Issue

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