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Headlight Fuse Blowing

Started by Peter UC , Aug 03 2008 04:16 PM

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#1 Peter UC

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Posted 03 August 2008 - 04:16 PM

G'day all

I've got a problem with my LX I am building as the headlight fuse blows every time I put the park lights on. I've unplugged the headlights and taillights as well as the park light plug underneath the bonnet. I've checked the parker wire for cuts on the insulation to see if it's shorting out on the body and the wire seems fine. I've also tried two switches to see if the switch is the problem but to know avail. Does anybody have any suggestions to what the problem is. The car is not currently running so it is a little difficult to get to an auto elecs who could probably fix the problem in about 5 minutes. :<_<:

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#2 _UNVSM8_

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Posted 03 August 2008 - 04:35 PM

have you checked behind the fusebox? could be earthing out behind there.. also are you using the correct fuse could be so sensitive maybe?

Ben

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#3 rodomo

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Posted 03 August 2008 - 04:38 PM

Does it have a trailer plug or old trailer plug wiring?

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#4 _luv the uc_

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Posted 03 August 2008 - 05:53 PM

Have you checked the number plate light? Or the dash backlights? They both run off the same circuit.

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#5 Peter UC

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Posted 04 August 2008 - 12:39 PM

The taillights was my first thought so I unplugged the connection that is under the dash. Also the fusebox and headlight switch are hanging loose so they can't earth with anything else. I have used upto a 30A fuse so there is definalty a bad short somewhere I just can't find where

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#6 rodomo

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Posted 04 August 2008 - 06:20 PM

Tip:
The quicker the fuse blows, the closer the short is to the fuse.

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#7 _torbirdie_

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Posted 04 August 2008 - 06:48 PM

Tip:
The quicker the fuse blows, the closer the short is to the fuse.

I dont think there is any logical reason for that to be true, but willing to be enlightened.

Actually, a bit puzzled here, obviously you have a fuse blowing, but the headlights are not protected by a fuse , unless you count the main fuseable link or have modified the wiring so there is one.
The parkers are on a fuse, but the headlamps should still work if this fuse goes, if they dont it means that somehow you have wired it up with the output of the parker fuse going to the headlight terminal.

If the headlamps still go, it means your method of connecting the wiring is possibly okay and there is just a short in the park light circuit somewhere.
Removing all the globes is a good start.

Id look at firstly disconnecting the park lights wires from the switch while keeping the fused supply connected. If it blows when you having those disconnected, then the problem is the switch or how you have it wired.
Failing that, you have to look for shorts in each wire to each lamp to suss out

Edited by torbirdie, 04 August 2008 - 06:49 PM.

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#8 orangeLJ

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Posted 04 August 2008 - 06:53 PM

my headlights have a fuse in the LJ.....

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#9 rodomo

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Posted 04 August 2008 - 06:56 PM

Tip:
The quicker the fuse blows, the closer the short is to the fuse.

The longer the wire is to the short, the longer the fuse takes to heat up and pop.

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#10 _SS Hatchback_

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Posted 04 August 2008 - 07:17 PM

Try disconnecting the high/low beam switching relay which should be mounted on the guard , that may be another problem.

Other than if there is more than one wire coming from the dead side of the fuse then you may have to disconnect one of them at a time until it doesnt blow anymore leaving you with the wire that you mayhave to trace and find the short.

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#11 REDA9X

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Posted 04 August 2008 - 07:54 PM

Take out the headlight assemblies complete with the backing, insulate the back of them from hitting the body and refit them then turn them on and see if the fuse blows.

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#12 _torbirdie_

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Posted 05 August 2008 - 10:42 AM

The longer the wire is to the short, the longer the fuse takes to heat up and pop.

I suppose so, due to a longer run of wire that the resistance will be higher and therefore the short circuit current will be slightly less, though with normal wire the difference would be probably measured in microseconds.

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#13 _SS Hatchback_

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Posted 05 August 2008 - 08:45 PM

any progress??

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#14 rodomo

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Posted 05 August 2008 - 08:59 PM

I suppose so, due to a longer run of wire that the resistance will be higher and therefore the short circuit current will be slightly less, though with normal wire the difference would be probably measured in microseconds.

It's visable if you watch the fuse as you fit it.
You sound like you might be a sparkie? Surely you have noticed?

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#15 _SS Hatchback_

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Posted 06 August 2008 - 06:26 PM

first disconnect your headlights, your switch, high/low beam relay, and you already have looked at your fuseboard so its not touching anywhere. Put a fuse in , if it blows there isnt much but your wiring left and unfortunately you need to start pulling apart your wiring searching first in areas of sharp turns or rubbing against metal. If it doesnt blow then add things into the circuit until it does blow then thats the problem. Cant really do much more until we hear some feedback. I think whether it blows in 1 sec or 2 seconds is irrelevant the short still has to be found. Have fun !!

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#16 Peter UC

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Posted 06 August 2008 - 08:51 PM

Sorry I haven't been on the forums the last couple of days, and I haven't been able to look at the car more as yet. However the wires are all disconnected from the headlight relay, the parkers are disconnected on the plug under the bonnet, the taillights are disconnected under the dash and I have all the wiring untaped and I checked all the wiring and I can't find the short. The next step I think would be to disconnect the parker wire from the switch and try again. I think I will need to go to the shop for more fuses . . .

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#17 _Chris M_

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Posted 06 August 2008 - 09:44 PM

I suppose so, due to a longer run of wire that the resistance will be higher and therefore the short circuit current will be slightly less, though with normal wire the difference would be probably measured in microseconds.

Kind of on topic, it's about impedance as well as resistance. Inductive impedance (which is like resistance to AC) goes up with length(same as resistance). Difference is, inductive impedance also goes up with frequency. The instant where the fuse is put in (or a switch turns on) is equivalent to a really high frequency, so that's why length has more of an effect than you might think. Now back to the bourbon for me...

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#18 Dangerous

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Posted 07 August 2008 - 11:03 AM

First things first. NO Torana has a fuse in the fusebox for the headlights. The nearest thing is the main fusable link near the starter motor, which is designed to burn out in the case of a large overload which is not on a fuse protected part of the wiring. All Toranas (and All Holdens of the period, and probably most other cars as well) have a thermal breaker built into the headlight switch, where in the instance of a short circuit or a large current draw for the headlights, it opens and closes rapidly, cutting the current down, but still allowing the hedalights to work dimly. This is much safer that driving along a country road in the middle of the night, suddenly popping a fuse and losing all headlights instantly.

Peter, I'm betting your problem is either a trailer plug connection, or the remains of the carnage caused when someone has hooked one up previously, or my first choice would be an aftermarket CD radio which has night illumination fitted, and someone has squashed the wiring for the night illumination against the metal of the dash when they've fitted the radio.

As for lengths of wire and proximity to fuses being a clue, it's not really. The fuse itself is a clue. If the fusible link inside the fuse is splattered all over the inside of the glass of the fuse and the glass is grey/white or silver (or cracked), then you can bet it's a pretty big short, especially if its happening with large rated fuses. If the centre of the fusible link inside the fuse is simply broken or a small section is melted and missing, it's more like a short somewhere near (but over) the rating of the fuse. A fuse that blows quickly and violently is just an indication of how bad the short is, not where it may be. It's amazing how much current can flow through even very small diameter wires and bits of corroded plugs or globe holders.

To save on fuses, especially if it's a big short, I'd suggest the following. Put all your wiring back together, and connect a 12 volt light bulb (eg a small dash light or indicator bulb - no larger) between the two contacts of the fuse (don't put a fuse in). Because you obviously have a short somewhere between the fuse output and the earth of the car, the light globe will glow until the short is removed. Wire the globe in, see it light up (presuming you still have the short), and remove connections and looms, globes, etc one by one until the globe goes out. The last thing you moved/removed will most likely be the culprit. The beauty of using a small 12 volt globe is that it's easy to see when it's lit, and also it limits the short circuit current to a safe level which should not harm any of the wiring no matter how long it takes you to trace the short.

Good luck. Let us know how you go.

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#19 _torbirdie_

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Posted 07 August 2008 - 11:50 AM

Kind of on topic, it's about impedance as well as resistance. Inductive impedance (which is like resistance to AC) goes up with length(same as resistance). Difference is, inductive impedance also goes up with frequency. The instant where the fuse is put in (or a switch turns on) is equivalent to a really high frequency, so that's why length has more of an effect than you might think. Now back to the bourbon for me...

Chris, certainly inductance is a factor if we have coils of wire, but im thinking that is not the case...

If we put identical fuses at opposite ends of a large length of wire and short out one end, then the current that flows through each fuse will be the same(according to Kirchoff's law), and there is no reason why the fuse further away would take longer to blow than the one closest to the short.

edit:(as dangerous has said ^ its more the nature of the short that will depend on how long it takes, and this can be seen by the remains of what is in the glass holder)

Edited by torbirdie, 07 August 2008 - 11:55 AM.

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#20 REDA9X

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Posted 07 August 2008 - 06:53 PM

I've had this problem before, as I said earlier try insulating the bodies of the headlights from the body of the car, some tape will do it. If this stops it from blowing the fuse you have an earthing problem. You say you are building the car, have you done paint work recently? Have a good look at your earths, but try the insulation first as it's a quick easy test.

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#21 _SS Hatchback_

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Posted 07 August 2008 - 09:00 PM

I've had this problem before, as I said earlier try insulating the bodies of the headlights from the body of the car, some tape will do it. If this stops it from blowing the fuse you have an earthing problem. You say you are building the car, have you done paint work recently? Have a good look at your earths, but try the insulation first as it's a quick easy test.

From what he was saying before , i dont think he has any wires connected to the headlights at all so it wont really help him much.

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#22 _Brewster_

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Posted 07 August 2008 - 09:18 PM

It might be something as silly as the high beam, low beam switch. But who knows, trying to deduct an electrical fault via correspondance is pretty tricky! :tease:

Edited by Brewster, 07 August 2008 - 09:19 PM.

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#23 _Chris M_

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Posted 07 August 2008 - 09:18 PM

Chris, certainly inductance is a factor if we have coils of wire, but im thinking that is not the case...

edit:(as dangerous has said ^ its more the nature of the short that will depend on how long it takes, and this can be seen by the remains of what is in the glass holder)

Torbirdie, even straight wire has some inductance. I agree about Kirchoffs current law, but it's the length of wire in the path to ground that we're looking at; it doesn't really matter where in that wire the fuse is positioned. Just putting a reason behind Rodomo's statement. I can do some calcs for you if you really want. :mellow:

PM me if you want more details or I'm if not being clear (or I'm babbling).

Either way, Danegerous has a great, simple idea with the light globe. I'm thinking he is the one with the practical solutions ( not me) :spoton:

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#24 _torbirdie_

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Posted 07 August 2008 - 09:43 PM

Torbirdie, even straight wire has some inductance. I agree about Kirchoffs current law, but it's the length of wire in the path to ground that we're looking at; it doesn't really matter where in that wire the fuse is positioned. Just putting a reason behind Rodomo's statement. I can do some calcs for you if you really want. PM me if you want more details or I'm if not being clear (or I'm babbling).

Either way, Danegerous has a great, simple idea with the light globe. I'm thinking he is the one with the practical solutions ( not me)

I dont think the rest of the forum needs to hear more about ^ , so have sent a pm.

You are right, dangerous has some ideas/solutions: light bulb is good, maybe even a 12V buzzer, so that it can be heard/not heard when poking around giggling the wires in the boot etc.

Edited by torbirdie, 07 August 2008 - 09:55 PM.

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#25 REDA9X

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Posted 07 August 2008 - 10:00 PM

From what he was saying before , i dont think he has any wires connected to the headlights at all so it wont really help him much.

The park lights are in the headlights, he says it blows a fuse as soon as he turns the park lights on. If however they are disconnected and still blowing a fuse when he turns them on, then in that case it won't help. It's got to be something simple, all the theorys and laws are really going a bit far for this. BTW, welcome back DEVILSADVOCATE AKA TORBIRDIE.