Thread: Trailer brake light problem - help please!

Hi Guys

Just picked up a new (second hand) trailer to replace my oldy. But the problem is that the brake lights don't seem to want to turn on. Tail and indicators work no problems.

I have checked with the multi meter and 12v comes out of the correct pin hole from the car whent the brake pedal is applied - so that is no problem. It is a 7 pin flat connector.

My old trailer uses separate brake bulb from the tail light bulb and this works fine when connected to the same car. But the new trailer has a dual filament bulb for the tail and brake light cluster. Is this the difference? Do I need to do something special to make this work? How do dual filament bulbs work anyway - does the tail light work on lower amps and the brake light is supposed to turn on when higher amps are applied?

I have pulled off the lense cover for the light on the right side of the trailer and can see that the wires going into the dual filament bulb are the 'neutral' and 'brake' wires when I trace back to the plug. So I gather that this is wired correctly. [But I can't see then how the tail light gets power]. Any idea what I need to do to get the brake lights working?

Could it simply be bad connection somewhere or do I need something else?

Thanks!

The dual filiment bulbs have two contacts on the bottom and need to be installed in the correct socket and will only go in one way.

The socket should have three wires, brake, tail and ground.

if you take the bulb out of the socket you should see the two contacts in the bottom, you can check with a multimeter that you have 12v between ground and each of the contacts when the tail lights are turned on and the brakes applied respectively.

Your problem may be caused by a bad ground connection, if the tail lights seem to work but go off when you apply the brakes this is definiately the case, the tail lights are only on because the current is returning through the brake wire, when the brakes are on the tail lights go out because there is now 12v on both wires.

Hope this helps

I shudder at the thought of trying to diagnose electrical dramas over the internet. I usually have a set routine to check from front to back with a tester to pinpoint where the problem is, but it would take ages to write it down.

However, we'll try some simple stuff first. Get yourself a test light, if you haven't got one already. And not the cheap $2.00 rubbish with a festoon (sausage) globe... get a good one with a proper bayonet globe.
Firstly, I don't quite agree with nihilism that a dual filament globe can only go in one way. They are meant to only go in one way, but I have seen them forced in round the wrong way. This will mean the brighter filament will be on with the taillights, and the dim filament will come on with the brake lights, but won't be noticable. You'll see it with the lens off.

Secondly, some of the contacts in a range of taillight assemblies are of a fairly poor quality, and don't actually make contact with the globe itself. This is easy to check with a test light. Sometimes the pins can be bent by forcing the globe in round the wrong way.

Thirdly, check the wiring from front to back to make sure there's no break anywhere. There'll be a join at the back of the trailer somewhere where the wiring connects the two taillights. I've seen some pretty shocking joins, even by proper trailer places, at these points. Wires just twisted together, wrapped in tape...

Fourthly, check the wire is in the plug properly. Should be a Red wire in No.6 pin.

Failing all that... I really hate chasing dodgy earths...

Good luck with it.

Originally Posted by Yonnee

Firstly, I don't quite agree with nihilism that a dual filament globe can only go in one way. They are meant to only go in one way, but I have seen them forced in round the wrong way.

Just have to comment on this. As you say they are meant to go one way but there are some out there that could probably be capable of doing them three ways. Reminds me of the time I had a look at a car of my brothers and would you believe, someone out there can actually install thermostats sideways.

Nev

Have you tried a new lamp (bulb)?

The problem will be a poor earth at the bulb socket. You mention two wires, which will be the Tail and Brake lines.

The tail filament will be 12V 5W while the brake filament will be 12V and approx 20w.
Filaments in the tow car will be similar power, but there may be multiple bulbs each side in the car.

When the tail lights are active, the current can flow from the car through the cable to the tail filament, then return with moderate resistance through the trailer brake filament, the brake wiring in the trailer, and the car brake filament(s) to ground. The trailer tails light but with diminished brightness.

When the brake lights are supposed to be on, it won't work in reverse because the brake light drays four times the current, and the tail bulbs have four times the resistance.

To check for this issue with a multimeter, measure the voltage from the brake terminal of the cluster to car ground with the tails on then from the tail terminal to car ground with the brakes lights powered. Ideally both should be zero, but a significant imbalance indicates a poor ground path.

hello everyone,just joinend 5 minutes ago and saw the problem about brake lights.always had trouble with lights on my road train and the easiest way i found to find faults was to run an earth wire straight from the battery to any earth that needs checking .hope this helps

Originally Posted by dozer3bandit

hello everyone,just joinend 5 minutes ago and saw the problem about brake lights.always had trouble with lights on my road train and the easiest way i found to find faults was to run an earth wire straight from the battery to any earth that needs checking .hope this helps

Good advice!

I always go to Sanitarium - the good earth people