Blowing Fuses

Landlover99 · Jul 28, 2018

Hi all,

Just tidying-up a few loose ends on my 2003 Defender TD5 before I put it up for sale.
I'm finding that the n/s headlight is blowing 10A fuses whenever it's put on main beam. The fuses blow right away so it looks like a near or total short-circuit. Any ideas as to what could be causing this issue or how to locate the source of the problem?

Ta!

Kwakerman · Jul 28, 2018

Landlover99 said:
Hi all,

Just tidying-up a few loose ends on my 2003 Defender TD5 before I put it up for sale.
I'm finding that the n/s headlight is blowing 10A fuses whenever it's put on main beam. The fuses blow right away so it looks like a near or total short-circuit. Any ideas as to what could be causing this issue or how to locate the source of the problem?

Ta!

Where the headlight wiring runs along the left wing by the heater ducting is a common place for cables to chafe on the sharp edges

Oldseadog · Jul 28, 2018

You say that the '10A fuses' are blowing.
Does that mean a single 10A fuse blows each time it is switched to full beam?
IIRC each side has individual main and dip fuses, so if they are all blowing simultaneously, then something serious must be causing a fault in the wiring.

tottot · Jul 28, 2018

Remove the bulb and see if it still blows.

Landlover99 · Jul 28, 2018

Kwakerman said:
Where the headlight wiring runs along the left wing by the heater ducting is a common place for cables to chafe on the sharp edges

Well, that could certainly account for it! Thanks.

Landlover99 · Jul 28, 2018

Oldseadog said:
You say that the '10A fuses' are blowing.
Does that mean a single 10A fuse blows each time it is switched to full beam?

Yes, the fuse responsible for n/s mainbeam blows just as you say.

Landlover99 · Jul 28, 2018

tottot said:
Remove the bulb and see if it still blows.

It's not the bulb. Checking the resistance between the contacts at the empty bulb socket with the headlights all turned off shows a very very low reading; as low as my meter will go. I'd have expected it to read infinite resistance if it were in good order.

Oldseadog · Jul 28, 2018

A low reading or a low resistance?
Zero, or a couple of Ohms resistance generally indicates a short to somewhere.
Find that, and fix it, then problem solved.
As Kwakerman said, start at the obvious places around the heater box and either run a new bit of wire, or repair the existing, which might be easy to find where it has been chafed through.

Landlover99 · Jul 28, 2018

Oldseadog said:
A low reading or a low resistance?
Zero, or a couple of Ohms resistance generally indicates a short to somewhere.
Find that, and fix it, then problem solved.
As Kwakerman said, start at the obvious places around the heater box and either run a new bit of wire, or repair the existing, which might be easy to find where it has been chafed through.

Low resistance; indestinguishable from short circuit, so it looks like the chaffed wire hypothesis may well be the culpret. Rotten weather here today so it'll have to wait a little bit longer before I can see to it...

Mcclegend · Jul 30, 2018

If you take the bulb out and it blows straight away you know the issue is from the switch to bulb holder.

I’d check the switch first they really are crap, high current switch with moving scraping contacts.

Landlover99 · Aug 1, 2018

I've done a few more checks. After pulling apart the plug & socket at the bulkhead to isolate the headlamp bulb socket from the relays, fuses and switchgear, the main headlamp terminal at the headlamp end is still shorted to earth. Oddly enough, the mainbeam n/s (the problem one) has an additional wire crimped to it. A plain white one. The mainbeam lead is blue and grey and in the Haynes manual, there is no mention made of the additional white wire (which seems to be an original factory fitting) so I have no idea what its function is. The short to earth could be in either of these two wires. Then weather stopped play.

Kwakerman · Aug 1, 2018

Landlover99 said:
I've done a few more checks. After pulling apart the plug & socket at the bulkhead to isolate the headlamp bulb socket from the relays, fuses and switchgear, the main headlamp terminal at the headlamp end is still shorted to earth. Oddly enough, the mainbeam n/s (the problem one) has an additional wire crimped to it. A plain white one. The mainbeam lead is blue and grey and in the Haynes manual, there is no mention made of the additional white wire (which seems to be an original factory fitting) so I have no idea what its function is. The short to earth could be in either of these two wires. Then weather stopped play.

Looking at the wiring diagram there doesn't seem to be a second wire, this is from the 1999 TD5 diagram and the LH main beam wire just heads back to fuse 18

Turktowner · Aug 1, 2018

Mines a 300. Started with the main beam then the indicator. The loom passes inside the wing over the arch and the wires had chafed over the years on the inside of the wing causing the short. Wiggling the loom moved it off the wing and gave temporary relief. Have to remove the plastic arch liner to get at it properly, the plastic rivets are a pain but is not difficult. The replacements from Halfodds have a screw head and easy to fit..
Cheers

Landlover99 · Aug 2, 2018

Well I can now confirm that the source of the issue was this mysterious white wire. It seems to have been there since manufacture but I can't be certain. Once I separated the grey/blue mainbeam wire from the white wire the problem went away. The white wire by itself reads close to short-circuit to ground. I suspected the function of the white wire is to illuminate the main beam on tell-tale on the instrument cluster, but that is still working fine (maybe there's a white wire on the rhs mainbeam headlight that's still working fine for this purpose I don't know). Anyway, white wire to blame. Just to be certain I reconnected it when the main beam was on and it instantly blew the same fuse. So there we go; snip it out of circuit and all's sorted.

Kwakerman · Aug 2, 2018

Landlover99 said:
Well I can now confirm that the source of the issue was this mysterious white wire. It seems to have been there since manufacture but I can't be certain. Once I separated the grey/blue mainbeam wire from the white wire the problem went away. The white wire by itself reads close to short-circuit to ground. I suspected the function of the white wire is to illuminate the main beam on tell-tale on the instrument cluster, but that is still working fine (maybe there's a white wire on the rhs mainbeam headlight that's still working fine for this purpose I don't know). Anyway, white wire to blame. Just to be certain I reconnected it when the main beam was on and it instantly blew the same fuse. So there we go; snip it out of circuit and all's sorted.

May have been there to control additional spot lights in the past?

Landlover99 · Aug 2, 2018

Er, yes, maybe. In fact there are additional spotlights (2 sets in fact - see my avatar). I've never used 'em, though, so... I'll look into it. In fact I have an old pal of mine from Sussex coming over this weekend and he's run his own Landrover business for nearly 50 years. If he doesn't know, no one will!

Blowing Fuses

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