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  Trailer Lighting Problems: All Lights Go Out When Brakes Lights Should Be On

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Author Topic:   Trailer Lighting Problems: All Lights Go Out When Brakes Lights Should Be On
fishinbob posted 06-04-2009 11:21 PM ET (US)   Profile for fishinbob   Send Email to fishinbob  
OK I'm stumped. had to rewire a boat trailer due to damaged wiring and lights. I'm using a standard 4 pole flat connection to my truck.The trailer is simple - two rear lights and nbo side markers. Both rear lights are grounded to the harness to the truck.

Almost everything works fine. Turn on the headlights and the trailer lights are both on. Both turn signals work. When I have my truck lights off, the brake lights work fine on the trailer.

Here's the problem - with the headlights on and I put on the brakes the trailer lights go off completely instaed of getting brighter.

Anyone have any suggestions?

Thanks for the help.

Plotman posted 06-05-2009 10:06 AM ET (US)     Profile for Plotman  Send Email to Plotman     
sounds like a tow vehicle wiring problem. You can get a little LED tester that plugs into your 4-flat plug for $4. Will confirm where the problem is.
Chuck Tribolet posted 06-05-2009 10:55 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
You have a bad ground. Might be the trailer, might be the
truck.


Chuck

White Bear posted 06-05-2009 12:11 PM ET (US)     Profile for White Bear  Send Email to White Bear     
Bad ground - check from the truck connection right through to the trailer.
jimh posted 06-05-2009 08:45 PM ET (US)     Profile for jimh  Send Email to jimh     
This is a very common and classic problem. There is too much resistance in the ground or return circuit. The total current of the running iamps is small enough to keep the voltage drop across the ground resistance low enough to sustain the lamp illumination. As soon as the cold filaments of the brake lights are supplied with current, it drives the voltage drop across the return portion of the circuit so high all the lamps extinguish.

Check the ground or return circuit. There is a poor connection somewhere.

jimh posted 06-05-2009 11:38 PM ET (US)     Profile for jimh  Send Email to jimh     
The trailer illumination circuit:

Schematic of typical trailer lamps

The current flows from the 12-volt battery positive, through the lamp, through a resistance in the ground return circuit, and back to the battery.

Schematic of typical trailer lamps

The running lamps are illuminated. The resistance in the ground circuit is higher than normal. Let us figure the current for the lamp is 1-Ampere. The ground resistance is 3-ohms. This mean there is a 3-volt drop across the ground resistance. The lamps are receiving only 9-volts, however they do illuminate at this voltage, although at reduced luminance. So the trailer lamps are burning.

Schematic of typical trailer lamps

Ww step on the brake. The brake lamps are designed to be brighter than the running lamps, so they draw more current. This means their filament resistance is lower, and when the brake lamps are off, their filament resistance is very low, practically a dead short. When the brake switch closes, the cold brake lamp filaments act more like wire than a lamp, and they drive the voltage drop across the ground resistance much higher, let's say to about 9-volts. The voltage drop across the cold lamps is only about 3-volts. This is not enough to illuminate the filament. This drops the other lamps to only 3-volts across them, and they go out. Once out their filaments cool, the voltage across them drops even more.

Schematic of typical trailer lamps

This explains why you often hear of a trailer whose lights "all work until you hit the brake." The high current of the two extra-bright brake lamps causes the voltage drop across the ground path to be too high to sustain the other lamps.

Chuck Tribolet posted 06-06-2009 09:10 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
There's another possibilty. Assume the trailer ground is
completely open.

With running lights only, there's a current path through
the trailer running lamp, trailer stop lamp, truck stop lamp.

With brakes only, there's a current path through trailer
brake lamp, trailer running lamp, truck stop lamp.

With both, there's no path back through the truck becuase
everything is at 12 V.


Chuck

jimh posted 06-07-2009 01:59 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is a sketch of the current flow when there is no connection or a high resistance connection between the vehicle and trailer. The resistance of the other lamps when off provides a sneak circuit for the running lamps.

Schematic diagram of trailer wiring and vehicle wiring.

Thirsty Whaler posted 06-09-2009 10:48 PM ET (US)     Profile for Thirsty Whaler  Send Email to Thirsty Whaler     
Thank you for the very informative and time consuming explanation to enable those of us who are "challenged" when circuit trouble shooting our wiring issues. I tip my cap to you, Mr. Hebert!
jimh posted 06-09-2009 10:57 PM ET (US)     Profile for jimh  Send Email to jimh     
Thirsty' --You are very welcome. And thanks for your kind remark.

I wonder if the originator of this discussion has returned to read the many replies.

fishinbob posted 06-10-2009 09:37 AM ET (US)     Profile for fishinbob  Send Email to fishinbob     
Hi and thanks for all the help. I'm most impressed with everyone's knowledge and thatk everyone for their well thought out and informative info.

Regards,

Bob

jimh posted 06-10-2009 09:45 AM ET (US)     Profile for jimh  Send Email to jimh     
Bob--Please let us know if you find the problem and what was its nature. It is always more fun to learn the outcome when working a problem to a solution.

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