02GF74
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 posted on 7/1/08 at 10:30 AM |
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Fixed it !!! but how did it ever work before??!?!?
Those who have been paying attention will recall a problem I had with my head lamps. Moving mk2 esc stalk trhough its 3 positon would light:
1. off (ok)
2. side (ok)
3. main (bad = should be dipped)
toggle flasher should flash main but these were already on and doing this blew the fuse 
Now I had tidied up the wiring and had been meticulous as well as testing as I went along and all was well ... until I drove it then it went pear
shaped.
Been putting off looking into it as the wiring (not done my me) is a complete f*** up but started at the relay for the main lamps.
Quick look on underside showed the standard connection mumbers 80,85,86 whatever so a quick look in VWP identified the relay and contacts and
showed there was a snubber diode across the coil.
The alam bells started to chime, hmmmm, I wonder if the coil is wired back to front so that the doide is shorting out the fuse? Seemed a logical
thing to try so I reversed the wires, and guess what, it works!!
I had a look at how it was wired, see linky, to check that I had not messed up wiring the relay and it seems to have always been like that ... so how
the heck did it ever work?!?!!?!
TIPif you are faced with a blowing fuse and need to have the circuit complete, albeit momentarily in order to diagnose, then don't waste
money on a new fuse but use a couple of filaments of mains cable wrapped round the fuse.
But beware of weird stuff happening, if the filaments aren't making proper contact - I thought I had another problem when the relay would click
at random moments as well as the lamps coming on all by themselves 5 minutes after everyting was turned off - VERY SPOOKY!!!
see relay on the right - wired the same right?
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RazMan
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| posted on 7/1/08 at 11:50 AM |
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You could alway use a resettable fuse when fault finding - it saves letting the smoke out of the wires 
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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02GF74
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| posted on 7/1/08 at 01:30 PM |
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resettable fuse, good one that but didn;t have one at hand and cannot be cheparthen some bits of mans cable.
bulb, hmmm, if that blos, theat would work out more ££ then fuse surely?
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MkIndy7
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| posted on 7/1/08 at 05:27 PM |
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quote:
Originally posted by RazMan
You could alway use a resettable fuse when fault finding - it saves letting the smoke out of the wires
I agree Raz, I found these in Maplins, they near fit well enough into a normal blade type fuse box for testing purposes.
Linky
My only complaint is that you can't instantly tell when they've tripped, or reset either. I wire up a little bulb to the outgoing leg and
then attatch that the the battery -ve or the chassis somewhere.
Certainly alot less hassle when trying to find a niggely fault or trying something new.
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RazMan
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| posted on 7/1/08 at 07:19 PM |
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I think I see what you mean Rob, but a filament bulb increases in resistance when it glows, so if you replace a fuse with a bulb, you will get naff
all current flowing through it to power up the circuit it supplies, making fault finding a tad awkward.
MKIndy7, That Maplins auto breaker is an excellent idea. I believe you can get another type which has an internal LED which glows when the breaker is
tripped - probably expensive though.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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MkIndy7
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| posted on 7/1/08 at 07:44 PM |
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I was thinking of getting a 12v 3mm Green LED like 1 of these,
Link:
And drilling it into the top or the side of the case if there wasn't enough room as a power on \ run light.
You would still need an -ve connection to it somewhere though to make the LED light while there was power going through the breaker, I doubt you could
make something light when it has tripped with out making it unnescaerily complicated.
Presumably as there auto re-setting there is no display for when there tripped or not, its just a bi-metalic connection inside that when the current
goes over X amps they move apart (thermal overload)
On the mechanical "trip once" type its usually the reset lever/button that moves and is either red on the underside or uncovers a window
with red behind.
*There also not only very good for testing but for keeping in the glove box as emergency spares, as should you have a niggely electrical fault, that
would have taken out multiple fuses these could get you home.*
[Edited on 7/1/08 by MkIndy7]
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RazMan
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| posted on 7/1/08 at 10:44 PM |
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oooh looky here and
here
[Edited on 7-1-08 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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MkIndy7
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| posted on 7/1/08 at 11:57 PM |
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Cool, I wonder how they do that then?
Unless there's something else magic in there, a battery or something that makes it light up when there's no power going through the
fuse.
I know how its done in big electrical pannels with hold off relays etc but am still mistified if it can be done without a connection to a -ve
somewhere 
They do both look like a disposable option still though, typical of the last link the Yanks, can't even tell if a piece of fuse wire in a clear
window is blown until a light tells them so  .. Ok it might make it marginally easier but still.
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02GF74
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| posted on 8/1/08 at 09:56 AM |
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looky here too
quote:
Originally posted by MkIndy7
Cool, I wonder how they do that then?
Unless there's something else magic in there, a battery or something that makes it light up when there's no power going through the
fuse.
there is no magic.
you have 2 circuits in parrallel, LED + its resistor is wired across the fuse.
12 V --- fuse ------- load ----- 0v
|--------- LED ----|
when fuse is intact, voltage across it is 0V so LED is off.
when fuse has blown, there will be voltage across the LED plus 2 resistors in series.
Inevitably the load will be of much lower resistance (load draing several amps) vs LED resitor (LED draing few milliamps) so that the LED will
light.
this fuse will not will not work well or at all if the resistance of the Load is high > 100s ohm but in those cases you wouldn't need a
resistor. Only example at the mo'I can think of is a clock.
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MkIndy7
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| posted on 8/1/08 at 05:54 PM |
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Ah, so it uses the potential difference between the full 12V still unblown live side and the now "dead" side of circuits 0v returning tot
it ( the earth path from other things that were on the circuit like bulbs etc) to light the bulb,
but the resistor is big enough not to let any current through to make the circuit "live again"
Or if its an LED then then being a Diode it also won't make the circuit backfeed and live again.
Hopefully if read carfully that makes sence and is correct lol.
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02GF74
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| posted on 10/1/08 at 10:13 AM |
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no the circuit is live when the fuse has blown - but the current is limited by the LED's drop resistor, probalby around 10 mA. It is unlikley
that that would cause damage. Compate that to the current that would flow in a short circuit, whcih is limited by the characterisitcs of thecar
battery and can be typically several 100s of amps - enough to trun your loom into toast very quickly!
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tks
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| posted on 12/1/08 at 06:24 PM |
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The bulb idea would work.
but only if you connect in parallel with that fuse.
in event of the filament in the fuse breaking, the energy will be drawn accros the bulb. the bulb won't blow because its self limiting.
if you have a 100% short the light will be bright if you have another kind of problem it wond be that bright + you are still passing current through
some wires wich may don't like the bulb current.
the led principle is the same.
the resistor limits the current to the led to the needed 17ma the led needs.
if the fuse in place it never cant lit because acroos the led there isn't 12volts just 0volts... in the vent of the fuse breaking/melting there
will be a voltage different wich will toggle on the led.
remember that putting things in series with your loom never ever can make/repair your loom. you need to see the fuse like kind of bridge/jumper taking
out the in series positioned component.
@02GFxxxx
There is only a couple of possibilities wy it first did hold and then lateron not.
1) the diode was dodgy and broke down due to overvoltage(spikes) and vibrations..
2) you turned around the wires of the relais and when you used the light it broke down.
The above comments are always meant to be from the above persons perspective.
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