Originally Posted by sfrantz
All of the lights on our tow vehicle (2007 Honda Odyssey Touring) work fine, and the turn signals and brake lights work when we apply them with our motorhome (2008 National Pacifica 40D). BUT, when we turn on the headlights on our motorhome the tow's tail lights do not turn on. It seems they need to be on for safe night time towing.
Any ideas where to look?
Well Sir, a couple of items here. First, you say when you turn on the "head lights" your toads tail lights don't work. Well, do you mean HEAD LIGHTS or, do you mean tail lights/marker lights on the coach? You see, the headlights on your coach are separate from the tail/marker lights. I don't know if you know that or not. But, it's your tail/marker lights that are used to power up the toads tail lights through the pig tail.
Now, speaking of that. One thing you need to do, right off the bat is, get either a volt-ohm meter and set it to the 12VDC side and, start a pin check on the 7 pin plug on the back of your motor home. Or, use a test light. Both will do the same thing. A test light is much easier. But, what you're looking for is signals from each pin. You should have:
1. Right turn/brake
2. Left turn/brake
7. 12+ power for charging your toad while driving down the road
At least that's the normal arrangement. Some will be a bit different. So, you need to ground that VOM or, test light, which ever one you are using and, have the DEAR WIFE go to the drivers seat and start operating each of those. Do make sure you have a REALLY GOOD GROUND on your test light or meter. By the way, you'll need the ignition on for testing the right and left turn signals. And, to make things easier, and lessen the chance of starting WWIII, the use of small, two-way radios is real good for this partner testing thing.
Now, if you find that your pin for the tail/marker is working, then, it's highly possible that you have a connection issue on the toad side or, in the wiring of the pig tail or ???? Grounds are a very common problem with many electrical issues in lighting. Sometimes the signal(s) is or are going through but, due to a bad ground or, weak ground, they're just not presenting at the filament(s) in the bulb(s).
And, if you find that the designated pin for the tail/marker lights IS NOT WORKING at the trailer plug on your coach, well then, you've found the problem but, not the cause. So, at that point, you'll need to do some investigation into either fuses or, other problems. Some coaches/manufacturers, such as our '04 Itasca Horizon 36GD with the C-7 330HP CAT, utilizes a completely independent set of fuses for the trailer plug out put. Don't know if yours does or not.
It's been suggested that you may need a "three to four wire" or, "four to three wire" system. Well, that's only if one or both of the lighting systems have AMBER tail lights for turn signals. Much of this depends on just how your toad is wired for the toad lights.
Do you have separate bulbs, installed in the housings of the toads tail lights or, do you utilize the stock tail light bulbs in the toad, for toad lights too? There's a few variables here. Hope some of this info helps some. Good luck.