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Thread: Tail light LED upgrade - now cruise control doesn't work

  1. #1
    Senior Member gulwng3's Avatar
    Join Date:  Mar 2013

    Location:  Champaign, IL

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    Tail light LED upgrade - now cruise control doesn't work

    Problem: I upgraded to LED tail lights (not replacement bulbs, but all new boards and bulbs from DMC). My car has a cruise control from the mid 80's that worked fine until the replacement of the tail lights. Now it doesn't work. Also, the tail light stop bulbs (2 in each tail light) glow dimly now when the cruise is hooked to the stop light wires (green/purple wires). I can unplug these connections and this goes away. Cruise does not work with new LEDs.

    What I know: I switched back to my old incandescent board on one side and the cruise now works and no more dimly lit LED on the other tail light. I need a resistor that behaves like the old incandescent stop bulbs and only need this on one side.

    What I need to know:
    1. What is the total resistance of the 2 stop light incandescent bulbs? Despite watching several YouTube videos, I cannot figure out how to measure ohms. What size resistor do I need?

    2. How do I properly install a resistor? I think I have to splice it in across the ground and positive... What color wires do I splice in to?

    Thanks for any advice,

  2. #2
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

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    Sounds like your cruise controller is pulling up the brake light circuit. So you would need one resistor on the brake light circuit and the other end going to ground. Our four OEM brake lights draw 100 watts but I would suspect you can use a much smaller load resistor and therefor not have the resistor dissipate so much heat.

    If it was me I would try some different resistors to find out the most resistance I could use and therefor save power (heating). But if you just want to buy one resistor, I would install one 10 ohm 25 watt resistor.

    I would guess a 100 ohm 5 watt would work but not sure if you want to buy a bunch of resistors.
    Dave M vin 03572

  3. #3
    Senior Member
    Join Date:  Feb 2013

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    You could also switch that cruise control sense wire to a relay. This will work unless your cruise is a funky one that requires resistance.
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    Early 81 5spd conversion- DMCH Ground Effects, Double Din, Custom Instrument Cluster, QA1 Suspension, 3.0 PRV with MS3

  4. #4
    '82 T3 FABombjoy's Avatar
    Join Date:  May 2011

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    I have a Rostra Microcruise 4 and tried some fairly large value resistors. The relay mod worked much more consistently.

    The full version of the above diagram w/ explanation is available here:
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

  5. #5
    Formally hmm252000
    Join Date:  May 2011

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    I had this issue too. For me, I simply added a spare relay to the brake circuit (I did this at the cruise controller wiring harness). When I press the brakes, it powers the relay coil. The coil alone was enough for the cruise control to be happy. No need to wire anything else into the relay or brake circuit!

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