RV Doctor – My Dinghy Tow Wiring Is Causing Me Confusion

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September 24, 2009

Dear RV Doctor,
My motorhome is towing a Ford Escort and I am having a problem with my tow wiring. When I have my lights on and then turn on a turn signal, I notice a really faint blinking from the turn signal that is not supposed to be blinking. I’m really confused by this. Any ideas? – Craig Burkholder, (Lititz, PA)


Gary BunzerThis sounds like my old favorite, Craig, the lifted or floating ground. Occasionally a ground fault can be very difficult to find, but fortunately you’ve provided some clues.

First and most importantly you have not mentioned that you are experiencing other problems like faulty braking or running lights, so for now we will assume that the faulty ground is restricted only to the turn signals. I am further assuming you are not having this problem when the running lamps are off.

The first thing to check is whether the ground wires feeding the signal lights on the coach have become broken, damaged or disconnected. If this is the case the current has to find a different path to ground. Since your tail/brake/signal light combination utilizes a single bulb with two filaments, the signal light filament can ground itself through the tail light filament, as long as the tail lights are not turned on. With the lights off, there is no current flowing through that filament so the signal light can find a ground path through it. Once you turn your lights on, positive voltage is flowing through the tail light filament so the turn signal cannot use that same path as a ground path.

Does this problem only occur when the towed vehicle is connected? And, does the problem occur on both vehicles? If the problem occurs regardless of whether or not you are towing the car, remove the lenses from the motorhome tail and/or signal light assemblies and carefully inspect the full ground path from the rear of the assembly. Inspect this ground plane carefully to make sure there is good continuity all the way from the bulb to the coach ground. Make sure that all the bulbs and sockets are free of moisture, rust, and corrosion, and that the bulb is making a good positive contact inside the socket. Hopefully the problem is limited to a dirty, wet, or corroded bulb or bad socket connection.

If the problem occurs only when you are towing, you should perform the same inspection on the towed vehicle. I include this for completeness, as you did not mention that the problem occurs when you are driving the towed vehicle by itself. Although it would likely cause other problems which you have not mentioned, it is a good idea while you’re investigating a ground problem to perform an inspection of the wiring harness. In addition to the taillight assemblies pay particular attention to where the wire connector is grounded to the coach frame. Be sure all ground connections are clean, dry and tight.

(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)

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  2. Bill

    I have a 2011 Ford Edge that I tow behind my Bounder. I had the towing kit installed on the Edge by my RV Dealer. It is a Blue OX Towing setup which works with the Aventa Tow Bar and Patriot Brake.

    When I apply the brakes in the RV, the brake lights on the Edge come on except for the center brake light. When I (or the Patriot) put on the brakes on the Edge, all three brake lights on the car work.

    I assume it is a wiring problem but I don’t know if it is a Blue Ox wiring harness problem or an installation problem. All the other lights and turn signals work correctly.

    Any ideas?

  3. Ken Murphy

    I need help. I have a ’92 Fleetwood Coronado with Chev Chassis. Recently on a trip, I noticed my oil pressure was out of sight and amp meter was going crazy, but everything was still running normal with no problems. When I got home, I bypassed my instrument panel gauges, replacing them with after market old style gauges for temp., amp meter, and oil pressure. Instrument fuse good. Guages now work fine.

    However, when I hooked up my dinghy and checked my lights, I noticed my clearance lights on the MH and brake lights and running lights on the tow vehicle were not working. I found a blown fuse, replaced it, and everything worked fine–for a period, and the fuse blew. It’s not an instant thing like a dead short, but it’s an unpredictable period of time before the fuse blows again. Any suggestions would be greatly appreciated!

  4. Darthvagrant

    Speaking of toad wiring- just wait until it’s necessary to hook up a late GM or other CAN bus system. I have two “toads”, one being a 2006 Saturn (Penske?) ION quad coupe. ALL of the external lights are on their own separate circuits. It is no longer possible to fire both tail lights or stop / turn lights with one feed from the tow vehicle. ….meaning…there is no continuity between the front park lights and tail lights, front and rear turn signals, etc. I was obliged to find a 12 pole double throw switch to isolate the car from the tow vehicle and use the toad vehicle’s lights. Believe-it-or-not I finally found a 14 pole double throw (push button) switch in a discarded VCR. It resides in the trunk with the two positions reading “tow” and |”no tow”. (with a cotter-pin lock-out of the “toad” position) I refuse to use the magnetic add-on tow lights as IMHO that indicated someone with no electrical knowledge taking the easy way out. I want the “toad” lights fully operational from the tow vehicle (lights). That said, however, one misstep with the wiring and it can be bye-bye for the body ECM module in your CAN bus toad..
    My Suzuki Sidekick, my other “toad” is used more frequently as it’s four wheel drive and is really well suited for National forest trails and roads, and the like. It’s “old school”, just fire the tail lights from the front park lights and likewise with the turn signals. Boy! Did that older **stuff**used to be a piece of cake compared to the new CAN bus vehicles!

  5. Jim


    Just go to your local RV service or parts store and tell them you need the round window crank knob, they will be able to fix you up

  6. Anthony Staffa

    I need help in the repair of 2 louvre windows in my rv and I cant find anything in your blog about these type of windows!!!!!!. The round circle to open and close has broken on both windows. I have a Winnebago/ Adventurer. PLEASE PLEASE HELP ME GET THESE REPAIRED
    Please email me soon!!!!!!!!!!!!!!!!!