Airstream LED lights and European tow vehicles

Since we’re back at home base for a while, I’m going to be posting mostly about Airstream maintenance stuff.  Those of you who are looking for pretty pictures and stories about the family might want to avert your eyes for a while.

Several times a year I get inquiries from new Airstream owners who have European tow vehicles (mostly Mercedes, but also BMW, VW, Audi, Porsche, etc) and are having trouble getting straight information about hitching the two vehicles up properly.  I can’t cover the entire topic because it’s quite complicated but I’d like to cover at least one common problem.

The Europeans have been using clever computers in their cars, which measure the resistance of the trailer lights to determine if there is a trailer attached.  If there’s no trailer, the computer turns off the 7-way plug.  I don’t know why this matters, since American tow vehicles leave the plug constantly powered and it doesn’t seem to cause problems.  It may be a case of being just a little too clever, because this resistance-sensing scheme is baffled by trailers that have LED tail lights, as all new Airstreams do.

So imagine the happy new Airstream owner with a fancy BMW/Mercedes/whatever to pull it, and you’d think he’d be on Cloud Nine but when he goes to hitch up, the brake lights don’t come on and (on some vehicles, like Mercedes) the brake controller has no power.  The darned computer has turned off the power because it thinks there is no trailer.  All that money spent on a nice car and a nice trailer, and yet it’s stuck in the driveway with no lights.

LED lights on trailers are nothing new, so you’d think that the European vehicle manufacturers might have figured this one out by now.  Indeed Volkswagen has.  They sell a special patch cable that contains a resistor, which you can buy (if you search carefully on the Internet or have the part number at the dealer) for about $40.  This works, and it’s stupid.

It’s stupid because the resistance cable adds in a couple feet of length, so the cord is now too long and must be secured in some kludgy way.  Secure it incorrectly and one day you’ll find it dragging down the road.  And the patch cable is stupid because it adds another point of connection, and the connectors on 7-way cables are famous for corroding in the weather, so you’ve just lowered the reliability of your lights and brakes.

IMG_2078Andy Thomson at Can-Am RV helped me out with this one when we bought our Mercedes GL320 in 2009, and I’ve passed on the knowledge many times since then.  His solution is the best one, I think: just wire in some incandescent lights into the system.  (You could use resistors but light bulbs are easy to mount, and easy to find and replace on the road if needed.)  Andy uses the clearance lights that were found on older Airstreams, because they have two bulbs.  If one goes, there is some redundancy and you can swap a bulb from another light for a while.

The photo above is from our trailer.  We just mounted the clearance lights right on the floor in Eleanor’s closet, with all the other main 12-volt junctions.  This is normally covered with a box so you can’t see it.  Because the lights are kept out of the weather, they should last a long time.  We’ve been using this system for about four years.

LED Lighting FixThis solution is really easy for the DIY’er to install.  You just wire the lights into the relevant circuits.  The easiest place to do this is in the “rats nest” of wiring where the 7-way connector enters the trailer. This is usually in the front closet or under the front sofa, or behind an access panel in the front storage compartment, on the street side of the trailer.  (The diagram above is by Andy Thomson of Can-Am RV.)

Once you’ve made this simple modification, your Airstream lights and brakes should work with any tow vehicle.  If you ever have a problem on the road, check the 7-way connector for corrosion first, because the LED lights and this modification should be highly reliable.