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Old 03-09-2010, 08:38 AM   #1
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LEDs: tricking TV's diagnostic system

I want to upgrade to LEDs but heard that my MB GL450 will not recognize them due to the low voltage draw and will show a "trailer light out" message. Anyone know of a way to trick the computer? Thanks.

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Old 03-09-2010, 10:08 AM   #2

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Try checking on THIS Forum, lots of info on towing and elec stuff.

Read the first few posts, your not alone!!

AF #1

"Sticks & stones can break your bones...and hail will dent your Airstream"

So when is this..."old enough to know better" supposed to kick in?
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Old 03-09-2010, 10:54 AM   #3
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You can use an adapter cable available from VW dealers, part #ZVW 808 004 (7 pin to 7-pin). Only some dealers stock it, so if your local dealer doesn’t have it, ask them to search other dealer inventory. You can also buy a similar cable made by Valley at 7-Way to 7-Way Trailer Adapter for VW, Audi and Porsche Towing Trailers with LED Lights Valley Wiring V39010

The cable simply adds a resistor into the line to simulate the resistance of incandescent lights. We later eliminated the need for the cable by permanently wiring a couple of incandescent lights into the circuit. The lights reside in the wiring closet inside the trailer.

If you don't use one of these solutions, you won't get a "Trailer Light Out" indicator. Actually, the computer will not be able to tell that there is a trailer connected, and as a result it will not supply power to the brake controller (assuming that you followed the factory recommended method for wiring in the brake controller). So you need to resolve the issue by one method or another before you can tow. Fortunately the fix is simple.

This problem is common to most other European SUVs as well, including the VW Touareg, BMW x-series, Audi, etc. I'm told the cable will not work with Land Rovers, but I have no personal experience with that.
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Old 04-23-2010, 11:52 PM   #4
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I am working on the same problem. A solution similar to what Rich mentioned is using a snap-together resistor circuit at the stop/turn signal LED light conductors to imitate the resistance of filament bulbs. I'm giving it a try - the resistor kit is available at
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