A number of customers have contacted us about problems they have when installing LED third brake lights in trailers or custom applications using a logic module.  Symptoms usually are all or some or all of the LEDs glow dimly when a turn signal is applied.  Other strange behavior can be seen, but it always involves a logic module and LED third brake lights.

Why you may need a logic module-

A logic module is required when installing a third brake light in a trailer or towed RV.  This is because neither the 4 flat connectors nor the 7 way RV plugs have a separate wire for a “brake” signal.  Rather, they use the same signal wire to indicate a turn or stop.  The logic module connects to the left turn/brake and to the right turn/brake and has an output wire that goes to the newly installed third brake light.

Why do they do this?

Most of the third brake logic modules were designed for use with incandescent bulbs.  These bulbs take a significant amount of current to light up.  Small amounts of current bleeding through the logic module caused no issues with incandescent bulbs.  But LEDs require such a small amount of current to operate, that even the small amount of bleed through is enough to get some of the LEDs to glow.

How to Fix:

Connect a 1 watt, 330 Ohm resistor between the ground wire and the hot wire (from the logic module) close to the third brake light may fix your problem.  When small amounts of current trickle through the logic module, they will move through the resistor, bypassing the LEDs entirely.  When the full rush of current arrives when you are really stopping, the majority of the current will be forced through the third brake light as expected. 

Depending on what your local electronics store has in stock, you should look for a 330 ohm resistor (I’ve had some customers use a 400 Ohm resistor successfully) but it needs to be rated for at least 1 watt.