Floating Ground

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, (Lititz, PA)

This sounds like one my old favorites, 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 issue 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.