Faulty Trailer Lights

[Shanes Hornet]

Hi All,

I need some help in fixing trailer lights.

Nothing works & when plugged in it blows the hazzard light fuse.

I've just picked up a multimeter, but have no idea how to use it.

Cheers Guys,

Shane

[Ashore]

Shane , Trouble shooting on trailor electrics can be difficult at the best of times , if you are unsure of how to use a meter then I would say that the problem may be beyond your skill level in this field
Try an auto electrician , or a mate that help you on site as you really need to be there to diaginose the problem

It sounds like a massive short or a major problem with the wireing:mad:

How old is the trailor
Has this ever happened before
Has the trailor undergone any modifications or had any damage recently
Have the lights worked previously on your present car



Ashore

[Daddles]

Bought a Tarago once, with tow bar, and plug. Trailer lights didn't work, so pulled off some body pannelling - the bloody plug hadn't been wired up. The wires were there, the plug was there, the two weren't connected though :eek:

Richard

[Gumby]

Nothing works & when plugged in it blows the hazzard light fuse.

I think that means the problem is in the trailer end, not the car. There must be a short in the plug or inside the lights. That's where I'd be looking. If you get a new trailer plug, it will have a diagram of the wiring set up.

[arose62]

Google is your friend - try a search for
how to use a multimeter

It even returns a discussion on that subject, right here on this bulletin board!

But to add some value;

I'd start with the trailer unplugged from the car.
You'd use the resistance measurements on the meter (R, or ohms), probably one of the lower ranges.
Touch the probes together to see what measurement you get when there is a dead short.

Then, touch one probe to bare metal on the trailer, and the other to each connector inside the trailer plug.

Any that give you the same reading as touching the probes together, are suspect. (Now, maybe it is OK for one of them to be grounded, but at least you can eliminate some of the wires).

Cheers,
Andrew

[Wood Borer]

Remove all the globes in your trailer so they don't upset your resistance measurements.

Don't plug it into the car but like Arose said, measure the resistance between each wire and the chassis of the trailer. Ensure the probe connected to the trailer chassis is making a good electrical connection by scraping off the paint (or rust) .

Also check the resistance between each of the wires. they should all be a very high resistance 10K ohms or higher. Ideally they will be infinite resistance but we are talking practicalities here.

The resistance of a cold globe is quite low which is why I suggested you remove all the globes.

There will possibly be one wire connected to the chassis of the trailer but some trailers pick up the earth from the car via the tow ball. This is not ideal but is the way on some trailers. This arrangement can sometimes cause your brake or parkers to flash when you are using your indicators.

If you find a wire that is low resistance to the chassis and it is not the earth wire, then you will have to find the short. It might be a wire that has been crushed or damaged against the chassis. It might be a loose wire in one of the lights or an incorrectly mounted light.

To identify which wire does what:

Switch your multimeter to the volts scale (greater than 12V DC) and measure the voltage between the chassis of your car and each pin on the trailer plug whilst you have someone else use the brakes, the indicators, parkers etc. Only measure one at a time to avaoid confusion.

Get back and let us know what you found.

[meerkat]

hi Shane,

when you've worked it out let us know coz I have the same problem with mine. Either blows the globe or it just doesn't go on.

Didn't know about the earthing through the ball thing, thanks for that Wood Borer.

Andrew

[Wood Borer]

Andrew,

If it is blowing globes, this cannot be caused by a wiring fault. Vibration, the globe sitting in water, wrong voltage rating etc can cause globes to "blow".

The globe filament will burn out if there is too much current flowing through the filament. The current through a filament is determined by voltage rating of the globe. If your charging circuit in your car is not regulated correctly then the excessive voltage will blow cheaper globes. This can be overcome by either fixing the charging circuit (may be expensive) or reducing the voltage across the globes that are blowing.

One way of reducing the voltage across your globes is to use a couple of silcon power diodes in series with your globes. The diodes cost less than 20c each. Each diode will have a voltage drop of about 0.6V across it.

For example if the voltage supplied to your globes is as high as 14.5 volts then your globes will have a short lifetime. Use 3 diodes in series with each globe giving you a voltage drop of approx 2 volts. The voltage across the globes will then be 12.5 V which will greatly extend the lifetime of the globe considerably.

Earthing through the tow ball or having a bad earth will cause the lights to do strange things. The earth is intended to be the return path for all current going through your globes.

If there is significant resistance in that return path, then there will be a voltage drop across that earth connection which will vary depending on the current flowing. So if you are using your indicators, the current will vary as the indicators turn on and off. This in turn will vary the voltage drop across the return path and also the voltage across any other globes operating at the time. In other words, your brake lights will flash too.

Good connections, good light fittings, good globes and good wiring will ensure you get your precious cargo of timber home to be transformed in your next creation. Bad connections could be a contributing factor to a collision which could cause your precious cargo to be transformed into match sticks.

Be Responsible

[dazzler]

Hi Shane

One of the problems with trailer wiring is that in most cases the wires run along the inside of the drawbar and along the chassis and eventually wear through. It can be a real pain to find the short. Also not all cars are wired correctly so going off wiring colour charts sometimes doesnt work.

My suggestion is to go and by a new length of 7 core and do a full rewire. It is far less stressful, you can do it properly and though it may take a little longer it will be as good as new.

Ditch the multi meter and get a test light, the ones with a globe in the end and a length of wire and a sharp bit on the end.

Disconnect all the old wiring from the lights etc and then get it all free. Go to the front and disconnect the trailer plug, get a new one if it is cruddy, tie the new 7 core onto the old one where the trailer plug was. You can use electrical tape to bind the two together nice and tight.

Now pull the old stuff out the back so that the new stuff runs through the channels. You can also put the new stuff in a clear plastic hose to give it further protection from rubbing and then pull it through. Run it to the further set of lights making sure that it is not tight. Tie it off there. Run the rest across to the lights on the other side. Tie it off. I usually slice into the black core and expose about 4" of the coloured wires so that I can splice into them later. Cut an extra length that will run to the lights
as a T.
Now strip the black sheath away at all ends about 4". Bear the wires on each at the front end and attach your plug back on. Dont worry about what colours go where (if you get a new plug it will have a diagram so follow that if you wish but dont worry too much). Make sure that the trailer is on the car so the earth is good and that none of the wires are touching each other or the trailer. Leave the top bit of the plug open if you can so you can see the wires/brass bits.

Go and put your parking lights on. Get the test light, attach the clip to the trailer on a bit of bare metal (chain is good). Now work across the plug touching the brass bits till you get light. write down the colour and "Tail lights". Go to the back, find that wire at the lights and attach it. Turn off tail lights, put in bulb and check. Should be on. Now go back to the T spot, pare back that wire with a knife and splice the same coloured wire that goes to the other tail light. Solder this spliced bit and wrap well with electrical tape so it cannot touch any other wires. Connect it to the other tail light and check.

Next do the right indicator, left indicator and then get passerby to step on the brake while you find that wire. Make sure that you solder whenever you join wires and make sure that they are taped well. Write down the colour and what it refers to.

Use Cable ties/clamps to secure all the wires so that any future rubbing is prevented.

To save your back if you have some car stands jack it up nice and high to save your back/knees.

Just remember if anyone suggests you use connectors or "scotch locks" just punch em in the head......thatll learn em.

Good luck.

Dazzler
PS. Send me a PM if you get stuck or sms on 0418 663 897

[Ivan in Oz]

If the problem is ONGOING and you cannot find the Fault.

Replace the FUSE with a 12VOLT light Bulb

If [or when] you get an EARTH fault this Light Bulb [Acting as a Fuse]
will Glow Fully,
BUT!!!!!!!!!!!!!!!!:eek: :eek: :eek:
you will still have power AND No fuse to replace
Then go through and locate where the Short to Earth is.
You will determine when the EARTH FAULT is gone/found/hiding
The "Light Bulb [Acting as a Fuse]" will go Dull, or out.
You can leave ALL the correct sized lighrt bulbs where they belong.

[Ivan in Oz]

Replace the FUSE with a 12VOLT light Bulb

This will take a bit of SkullDuggery :eek:
and maybe some soldering

[Daddles]

People are making far too much of this. Trailers are dead simple.

First, hook your neighbour's trailer up to your car and see if the lights work - if they don't, ask him if they work on his car. If they do work on his car, your plug needs attending to and the easiest way to do that is to rewire the rotten thing - if you've got one dud fitting, others'll be crook too.

Assuming your car is right, rewire the plug - same reasoning as the first step and it doesn't take long.

Once the plug is right, check the lights. If they look rusty or bodgy, buy new ones - they cost stuff all and you'll probably need them anyway.

If things are still acting up, replace the wiring. It's just a matter of buying a length of cable, running it, putting a junction in where it crosses the trailer, and hooking it all up.

This is easier and quicker than stuffing about with all the fancy meters and such like and has the benefit that you wind up with a trailer that has good lights as opposed to a trailer who's lights are dodgy but behaving at the moment.

And yes, when it comes to electrics, I still subscribe to the smoke theory

Richard

[Wood Borer]

It all boils done to what test equipment you have, your skills using it and your electrical/electronic test equipment.

My job revolves around electronics so using a multimeter and interpreting the results when it comes trailer wiring is not difficult however if electronics is a mystery to you, then the rewire and test light is the best way to go or even better, get a mate who is experienced.

Just be careful with the rewiring, many faults can be caused by the light fittings and the car wiring.

In other areas of electronics where globes are required to flash, a "dark current" is used. This involves a small current flowing through the globes even when they are off. The current only keeps the filament warm but not enough to produce light. The life of the globe is substantially increased and you can flash the globe on and off at faster rates.

I only mention this in case this technique is used in modern cars. I have heard it is used but I am not sure if it is used in all cars let alone trailers. If it is used then testing of the circuits may give confusing results.

This technique has been around since the early 1900's when globes were one of the few choices for alarms or signalling.

[bennylaird]

If all household wiring had this feature or a "soft start" we would put the bulb manufacturers into recession.

Heat causes more resistance hence lower current, when cold low resistance, lots of current, blown globe.

If you want to impress, turn an appliance on and off on and off a few times, easy to blow a power supply for the opposite reasons, a thermistor had high resistance when cold and low when hot so it protects you from a turn on current surge, but turn it off and on and it's still hot so again a surge and usually enough to blow a circuit.

Isn't electronics fun, so many ways to do damage?

[bennylaird]

Trailer wiring has it's share of problems with bad connections, plugs, globes and damaged wires, earth loops, cars being of different configuration etc. Sometimes it can be better to rewire and use the correct colour coded wire to get back to standard.