Headlight Fuses

[Stewart]

Just came to do a final lighting check as soon as headlights switched on fuse blows,tried all other cobinations on that fuse and is definately headlights,disconnected each headlight in turn and fuse still blows,stuck for idea`s can anybody help??

[sj-bradley]

If you have disconnected all lights from the circuit you should have a large / infinite resistance.

 

Stick an Ohm meter on the feed to the lights and ground the other end. What reading do you get?

 

In other words, you have a short somewhere. What fuse rating are you using?

 

Simon.

[Stewart]

original fuse was 15amp tried 20amp and even 30amp all blew as soon as the switch was activated,seem to recall someone having this problem a long time ago and the result was switch failure does anybody recall or had similar problem

[sj-bradley]

Yes, one tip though. Don't just keep uprating the fuse like that as they are there to prevent the cable getting too hot and melting.

 

The cable is only rated for a certain current and if the fuse is too big, you will cause a fire in the loom. Something you DON'T want.

 

There is definately a short on some part of that circuit which you must find before trying again.

 

A switch failure could be the problem. The only action I can suggest is that each section of the circuit must be isolated until the short disappears. The last bit disconnected will contain the fault.

 

Did you put an Ohm meter across the circuit? What resistance value did you get. Sounds like you are getting close to zero ohms (a short) when you should be getting almost infinte (with bulbs out of circuit).

 

There is no short cut to this. You have to take the bulbs out, measure, disconnect one leg, measure again etc until you find the one where the reading goes high. The last one disconnected contains the short.

 

Work your way from the farthest point back to the fuse.

 

You can put the ohm meter across the fuse terminal (One end to light feed other to ground DISCONNECT BATTERY THOUGH)

 

If you get a swtich failure you will see (bulbs OUT) the reading go from high to very low. It should NOT change if the bulbs are not connected.

 

I would expect Meg Ohms for the cable only with the bulbs out. With you blowing 30 amp fuses the reading should be more like 0.3 ohms or less which is a short circuit.

 

Simon.

[sj-bradley]

If the problem occurs when swtich activated, disconnect the wire from switch to lights and test with ohm meter again.

 

The reading should be Meg ohms whatever the switch does. If it changes to a very low reading, then the switch sounds like the fault.

 

DON'T think there is only the one fault though. You still have to test the rest of the circuit before energising it.

 

Simon.

[Longboarder]

A few questions.

Did they used to work OK or have you just wired them up?

How have you wired them up? Can you draw a quick pic?

Are you using the sierra column switches to control the headlights or other aftermarket switch?

Have you used any relays?

If sierra column switch is it from a pre 1987 sierra or post 1987?

Two relays with the column switch earthing the relays?

Just one fuse in line for the pair of headlights? One fuse for dip one for main? Is any fuse before or after the relays (if any)?

Is the link wire 56 - 56 between the two halves of the column switch in place?

Anything else sharing the fuse.

As I said a drawing of the wiring diagram you have aimed for would be a very helpfull thing. If you do one sitting on the car in the garage you may well have a eureka moment as you work through it.

 

Nigel

 

My lottery style guess. You have taken the earth wire that joins relays to column from pin 87 on the relay or have fitted relays which dont have the standard pin layout to holders that do have the standard layout.

[Stewart]

Yes they worked OK when first wired,using sierra column switches from 1992 sierra,using relays,think seperate fuse for main and dipped,the fuse is also for the horn and digital dash they all work its only when dipped headlights switched on does fuse blow.

[Longboarder]

For starters switch the headlights to main beam before switching them on. Then switch on. If the same fuse blows then it must be part of the lighting circuit common to both dip and main which pretty well narrows it down to the switches. If the fuse doesn't blow the it must be from switches to somewhere in the dip wiring.

You can also try the main beam flash without the lights on. This checks out some different bits. Until I'm looking at a circuit diagram I'm lost. Some have a much more practical approach of bit by bit testing.

[Stewart]

Tried switching headlights to main beam before switching on and they worked OK,removed dipped beam relay and switched lights on fuse blew.?What size fuse should the dipped headlights require,the digital dashboard and horn are also on the same fuse.

[Stewart]

Thanks for all the help problem solved faulty rear fog light switch.

[Longboarder]

That's unexpected! I take it some part of the dipped circuit feeds the switch. Well spotted.

 

Nigel

[sj-bradley]

Well fog lights are only supposed to work with dip and main (IVA state this) so not "totally" unexpected.

 

Well spotted though

 

Simon.

[Longboarder]

'fog lights are only supposed to work with dip and main so not "totally" unexpected.'

 

What was unexpected was that it only blew the fuse on dip. Not on main as well.

 

Nigel

[sj-bradley]

 

What was unexpected was that it only blew the fuse on dip. Not on main as well.

 

 

Ahh, now THAT was unexpected (Tales of the unexpected - Hm ! Remember them )

 

Simon.