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HFD 4 LIFE
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Discussion Starter #1
I know alot of folks out there have had this problem where their cruise control doesnt work with the led 3rd brake light in place.The problem is the led 3rd brake light just doesnt give the system enough resistance and once the cruise control is turned on it usually activates the VSC,TRAC and ABS lights on the dash. I experimentated with various methods like;
having reistors in series with the bulb and it didnt work.Under advice from victor aka PCH i put the factory TBL bulb in parallel with the led brake light and it worked. here is what u need;
-Some wire
-atleast 2 wire taps
-Factory TBL bulb(tried smaller bulb and didnt work)
-electrical tape
-zip ties
-wire cutter and stripper

Start by removing the cover from the led 3rd brake light to expose the factory wire it is pluged into.

There is no need to disconnect the light so just cut the factory tubing that is covering the 2 wires(green,white with black) so that you can fit 2 wire taps

Now proceed to install the 2 wire taps with additional wire coming out of them.Install the taps on the factory harness with green and the white with black wires.

Now take the 2 wires you just tapped and attach them to the 2 leads on the factory TBL bulb. If u want to be extra clean there are universal bulb sockets for sale that will fit ure factory TBL

Now to clean everything up i descided to put put electrical tape at the base of the bulb where i attached the wires and then zip tied the bulb to the factory 3rd brake light harness up in the tailgate hidden away from view.

Take the car out for a test drive.you shouldnt get any codes after this and the cruise control should engage just fine

Finish everything off by reinstalling the TBL back cover

This picture elustrates what we are doing to fix the problem.we are basically going from circuit 1 to circuit 2


here is a picture of what my setup looks like when finished.If u can look closer you will see the factory bulb almost fully hidden away in the top part of the tailgate
 

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06.5 TCM
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Pishtaco
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3,956 Posts
Just curious if this only affects ATs, as I have no problems with my MT, LED TBL, & cruise control.
 

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HFD 4 LIFE
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2,563 Posts
Discussion Starter #9
Nice write-up, Will. I expect to see more DIY's from you, young man!
Rep point for me:D

---------- Post added 03-02-2012 at 05:37 PM ----------

Just curious if this only affects ATs, as I have no problems with my MT, LED TBL, & cruise control.
it affects all cars regardless of transmission
 

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Pishtaco
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3,956 Posts
Darrell, do you have regular bulbs in your tail lights?
Victor, yes, I'm running incandescent bulbs in all rear bulbs except the license plate & Oznium TBL.

I thought the issue with ATs is something like excess brake light voltage kicks the torque converter into slipping, which drops cruise control. When I installed my Rostra CC, the brake light microswitch on/off position controls CC disconnect, not circuit voltage.
 

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06.5 TCM
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You need to have at least one incandescent bulb in the whole brake light circuit. The way Rostra ECU knows when the brakes are applied is by monitoring the wire to the lights.
Common misconception is that it needs to see the positive voltage in that wire when the brakes are applied- that's not how it works. It actually wants to see the ground in that wire when the brakes are not on, and when the pedal is pushed the negative ground becomes a positive, so it knows to turn off the cruise.
With all led's in that circuit there is never a negative ground present because led's don't let it through like the regular bulbs can. So Rostra automatically assumes that the brakes are always on. Having just one incandescent bulb is enough to have the negative ground pass through to where ECU will register it.
 

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Pishtaco
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Doubled for Victor for the clear-as-crystal explanation of how it works. As I understand it, LEDs, being diodes, only allow current to flow in one direction. Apparently the Rostra CC wants to see it flowing in the other direction.
 
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