Ignition stays on and melted brown wire to headlight switch

[BaulyCars]

Hi everyone, 

Need some guidance on an electrical fault ('74 TR6)

Topmost fuse was blown, and when replaced the engine stays running once key turned off. Pulling that fuse stops the engine. Replacing it turns on ignition. 

The brown wire which runs from the battery positive terminal to the headlight pull switch (wire highlighted below) is badly melted. 

With or without the fuse in, the tail lights don't work (but brake lights work, and headlights work).

Horn stays on (may not be related, but betting it is).       

I fear re-connecting the battery with the fuse in place as it hasn't prevented the melted wire so far.

Thoughts on likely suspects and best approach?

   

Kind Regards

Toby

Edited December 17, 2023 by BaulyCars

[RogerH]

Hi Toby,

 you need to isolate various circuits.

Start by disconnecting both wires coming out of the switch  RG & U.  - does the brown wire melt!!

Connect  RG and see what happens - if Ok disconnect ERG & Connect U.

 

Work slowly and logically

 

Roger

[BaulyCars]

Thanks, Roger, 

That brown wire runs right through the harness, am thinking I should unplug the melted brown from the switch and run a new length of wire in its place (same route, from the switch back to the battery) as a 'test wire' in case it melts again.

 

 

  

[RogerH]

Hi Toby,

 when that brown wire melts it usually takes out neighbouring wires.

Indeed route a new wire outside of the loom.   When (not if) you find the fault you can test ALL systems to make sure everything is working.

 

Roger

[RobH]

2 hours ago, BaulyCars said:

That brown wire runs right through the harness, am thinking I should unplug the melted brown from the switch and run a new length of wire in its place

From your description the burning might have done more than just melting the insulation on that brown wire - it sounds as though something live is shorting to something else (probably on the 'green' circuits?) for the ignition and horn to act like that, so who knows what other hidden damage has been done to the rest of the wires? 

Of course you also need to find out why the wire burned. Something in the lighting circuit must have shorted to earth, and that may also have damaged the light switch since it must have carried the fault current.  Pity Triumph did not see fit to put a fuse in the main lighting feed - that circuit is unprotected. 

 

[Jonny TR6]

Given the age of the wiring and potential time chasing down the issue, is a new loom sensible ? Not sure on the cost, as mine was luckily changed by the PO, however, from what I’ve read they’re easier to install than the originals (being thin walled) and come with modern relays and connectors.

Edited December 17, 2023 by Jonny TR6

[RobH]

16 minutes ago, Jonny TR6 said:

is a new loom sensible ?

That would be my feeling if the damage isn't just very local.  Otherwise it's taking a bit of a gamble IMO.

[BaulyCars]

Thanks guys. This is on a restored car, with a new loom, so I’ll try to chase out the melted wire from the loom. 

This is the switch. 

The only wire that shows any damage is the brown - the blue and red-green appear entirely  normal. 

I don’t want to risk replicating the short again, so as not to lose the loom. 

One observation is that the brake lights were working even when I took the bulb receptical completely out of the light binnacle, meaning that the bulb was surely getting a ground from the other live red wire? Does that mean the red is shorted to ground somewhere along its length? 

[RogerH]

Hi Toby,

 that is not a TR6 light switch.

It is used  as a TR4 light switch or a TR4A two speed wiper switch or TR6 heater switch.

It may well work but shouldn't be there

 

Roger

 

[roy53]

so i guess it depends where that loom came from.

If you strip back the outer black covering from both ends of the brown you should have a indication of how much damage has been done.easy to re rap.

You still have to find why it happened.

Roy

[harrytr5]

If it was me I would change the loom for a new one.When pulling out the old loom which was not that old and stripping off the insulation you would be surprised how burnt the other cables were surrounding

The brown cable.

No contest,just change out for a new one.

Far quicker than chasing you tail.

Regards Harry

 

 

 

[RobH]

You would have to unwrap the whole length to see what internal damage there is affecting the other wires. If the exposed part of the brown wire got that hot, the bit inside the loom with no air cooling has been hotter and will have cooked the insulation of the adjacent wires.    

The cabin part of the loom is probably scrap.

Electrically the red wires don't go anywhere near the brake lights whose wires are green.  

 

Edited December 18, 2023 by RobH

[BlueTR3A-5EKT]

Any help here?

https://trf.zeni.net/TR6greenbook/36.php

 

Is your car post 1974?   Then RKC960 should be the main loom    https://www.autosparks.co.uk/triumph-tr6-main-wiring-harness-9
 

 

 

Edited December 18, 2023 by BlueTR3A-5EKT

[BaulyCars]

34 minutes ago, RogerH said:

Hi Toby,

 that is not a TR6 light switch.

It is used  as a TR4 light switch or a TR4A two speed wiper switch or TR6 heater switch.

It may well work but shouldn't be there

 

Roger

 

It's the same switch as the heater switch, with the interior/illumination knob on it, which they've wired as the headlight switch. It was working OK apparently. 

The usual rocker switch to the side of the steering wheel is un-used.

 

 

   

[RogerH]

Hi Toby,

 that switch BHA4578 should do the job but for side & head lights.  Make sure the correct contacts are used.

 

Roger

[RobH]

8 minutes ago, RogerH said:

Make sure the correct contacts are used.

Power in (brown wire) Pin1 or Pin 7.   [pins 1 and 7 are always common]

Feed to sidelights (red/green wire) Pin 4

Feed to headlights (blue wire) Pin 8

[BaulyCars]

Brown is on pin 4

Blue on 8

RG on 7

Might that be an issue? 

The owner reports that everything was working perfectly. 

[RobH]

Yes, connected  that way will work too. 

The pattern is:

OFF 1,6,7 connected 

position1.      1,4,6,7 connected

position2.     1,4,7, 8 connected

None of the pins should connect to the switch body. 

Looking again at your photo the only thing that could have caused that burning is a short to earth from the brown wire, either direct to the tag or through the switch body. The tag does look discoloured at that end.   The fault current obviously did not flow through either of the other two wires.

 

 

 

Edited December 18, 2023 by RobH

[Z320]

 I‘ve never seen such a switch on my 4 A.

Good to see is the copper terminal has glowed,

possibly one of the both not visible too, making a short circut to the dashboard frame.

Just an idea.

Ciao, Marco 

[stuart]

Looks like a 4a wiper switch been used , if its in the centre dash console its very easy for one of the terminals to touch the metal backing dash if it twists slightly. Basically whoever fitted the loom probably used that as the original clear hooters switches didnt work, either way its a bit of a bodge. I would be fitting a new loom and if the original light switches are US then fit the readily available later Lucas ones. Better safe than having a fire.

Stuart.

[BaulyCars]

Thanks for all the help, bit stumped. 

Certainly looks like only the brown wire shorted, so can't be a short in either of the blue or red/green circuits. 

And yet I'm confused as to the related ignition remaining on when you turn the key off. That could be unrelated and its own issue maybe. 

The rear lights/brake light holders have the usual extra ground cable mods/repairs, which I initially thought might be causing a short somewhere, but you'd think the wires would fry from those holders back to the switch too. 

I'm pulling the dash to get eyes on the loom, see whether the brown has made a mess throughout - that brown seems only to run through one branch of the loom. 

Shame to swap the loom out as its practically new.     

[RobH]

Just now, BaulyCars said:

And yet I'm confused as to the related ignition remaining on when you turn the key off. That could be unrelated and its own issue maybe. 

 

It's probably because the insulation has melted in the loom and the bare wires are touching together so bypassing the ignition switch. That is probably what blew the fuse too. 

[BaulyCars]

Yeah quite possibly.

Annoying the fuses don’t blow before the wires get so damaged. 

[RobH]

As I explained before, there is no fuse protecting that circuit.  Triumph didn't put one in the headlamp feed though there is one for the sidelights.  

[Jonny TR6]

Can you get an endoscope/phone camera on the back of the mounting point to see if there's a contact point ? Every time I've earthed a screwdriver in the house when working live there's an arc and the screwdriver virtually welds itself to whatever I've earthed against, leaving a tell tale mark.