Serious electrical short, TR4a

[qkingston]

Thanks Dic, I tend to agree. Having looked further behind the dash I was able to see several additional areas of melted loom + insulation on the permanent 12v cable. I have decided that the risk to having the loom compromised isn't worth it, so a new loom was ordered from Autosparks this morning; the pain is the lead time can be up to 10 weeks so that's going to write off a fair chunk of the season, hey ho

David

[Waldi]

Hi David,
maybe you can install by-pass wiring with an additional fuse to temporarily replace the defective wiring) after you have (hopefully) found the cause while waiting on the new loom. This will give assurance it will not re-occur with the new loom, and you can drive over the next couple of months, maybe even up to next winter?

Installing a loom nicely will take some time too.

Just a thought.

Waldi

Edited April 21 by Waldi

[Z320]

Hi,

what I would do is to check the diameters of the cables of the now worn loom - and also on the new one.

The cable to light switch and from there the cables to all lights are in a bundle and along the steering column through the dashboard.

With too small diameters they will heat up each other pretty much.

Ciao, Marco 

[qkingston]

Thanks guys, helpful advice. Having built the car from scratch I know exactly what's involved. I never thought the running of the 12v permanent live in that bundle of cables which is then jammed between the 2 halves of the steering column clamp was a particularly great idea!

David

[Z320]

David,

this is why my earlier question is if you always drive with light and H4.

The loom was not made in the 60-ies (from Triumph maybe never) to do this).

It was getting pretty warm as long as I did this…

With smaller cable this could be a problem.

Ciao, Marco 

[stuart]

2 hours ago, Z320 said:

David,

this is why my earlier question is if you always drive with light and H4.

The loom was not made in the 60-ies (from Triumph maybe never) to do this).

It was getting pretty warm as long as I did this…

With smaller cable this could be a problem.

Ciao, Marco 

Unfortunately you cant really go by the thickness of the cable outer wall as lots of looms these days are built with modern "Thinwall" wiring, it takes the same current but the insulation is thinner.

Stuart.

[PodOne]

Hi David

Given the work involved to repair it you could consider removing it and using it as a pattern to make your own loom with as many fuses and relays as you want and with better terminations with separate earths for each circuit rather than using the body for earths . I did this for my TR6 after finding the old loom had being on fire in a few places and repaired badly plus the cost of a new loom was too rich for me.

The modern wires carry more current for the same diameter but as Stuart says thinner insulation. The overall thickness of my loom wasn't too thick in the end even with uprated cable.  It's just a long process to undertake but you get what you want.

Andy

[qkingston]

Hi Andy, indeed the thought did cross my mind, but in the end I've bitten the bullet and ordered a new loom from Autosparks while I continue dismantling the interior to find the root cause of the failure

David 

[Z320]

Hi, 

I remember a German supplier,

he offers a listing of old and new cable diameters (outside).

But sadly the wire diameters are not listed at the TR workshop manual and David has not the original one to measure.

If I remember correctly the German supplier also offers a listing of recommended wire diameters related to the current.

I‘m a bit bussy, perhaps somebody browses the website from for www.Kabel-Schmidt.de

Ciao, Marco 

Edited April 22 by Z320

[Z320]

Found it, that was easy!

https://www.kabel-schmidt.de/tipps-zur-kabelauswahl
 

[RogerH]

Well hunted down Marco.

 

Roger

[Z320]

With some phantasy you all should be able read it?

 

[keith1948]

Adding fuses is not the answer. The problem lies in the fact that the full power to the headlights goes through the lighting switch and the wiring runs up under the steering column. If you fit the Moss headlamp relay kit then less power goes through the light switch wiring. It also makes the headlamps brighter. (Standard sealed beams). Here are links to the 4A wiring diagram and the Moss kit instructions.

http://www.advanceautowire.com/tr24a.pdf

https://www.moss-europe.co.uk/media/pdf/117-515_instructions.pdf

I have fitted this kit on my 4A some years ago and it has worked fine for some years. The car also has the original loom from 1965. I think the wires used then were slightly larger cross section than the modern looms but I stand to be corrected. Little bits of the wiring have been replaced as and when required.

Keith

p.s. I have never been a fan of wrapping all the wires together in a loom. As you have discovered one faulty wire becomes several when a fault develops. I live with it on the TR.

[RobH]

That mod is only to reduce voltage drop on the switch Keith - it adds no protection whatever to the existing wires.  Relays in the main lamp supply won't help you if the wires before the relay coils short to earth anywhere. There may be less current flowing there when all is OK  but a short circuit to earth will still pull all the current it can through those wires. Only a fuse can save the loom . 

 

 

Edited April 22 by RobH

[Kiwifrog]

5 hours ago, Z320 said:

With some phantasy you all should be able read it?

 

or magic google translate

https://www-kabel--schmidt-de.translate.goog/tipps-zur-kabelauswahl?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=fr&_x_tr_pto=wapp

[qkingston]

So I'm considering the in-line fuse option, Rob would you put the fuse between the starter solenoid connection and the supply wire to the lights? Autosparks tell me that the Brown/Blue is 28 strand of 0.3mm rated at 17.5a continuous, so would a 15a in-line fuse be appropriate?

All my lights are led, I have an alternator and no control box, just to clarify for those offering advice, but thank you, it is much appreciated. Once I can get the steering column clamp loosened off and the lighting wires pulled out I will have a better idea of where the original issue lay

David

Edited April 22 by qkingston

[Z320]

1 hour ago, Kiwifrog said:

or magic google translate

https://www-kabel--schmidt-de.translate.goog/tipps-zur-kabelauswahl?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=fr&_x_tr_pto=wapp

That’s cool!

[RobH]

Two things David.  The fuse should go as close as is reasonable to the power pick-off point and the starter solenoid is a good place only if you have no ammeter.  If you do have one, it will register the lighting current as a 'charge' which is incorrect, so in that case the pick-off should be directly where the alternator output cable connects.  

A  15 Amp fuse is fine - a fuse should be rated slightly lower than the cable it is protecting, though in this case since the lights draw a lot less current you could go even lower if you wanted. 

[qkingston]

Thanks Rob, no ammeter, voltmeter instead 

David

[Z320]

With the brown-blue cable direct from the starter solenoid it was clear he has no ammeter….

[qkingston]

I've now managed to remove the damaged loom and dismantle the steering column wiring/switches. I now believe that the original cause of the problem was that the indicator self-cancelling clip was not sitting flush to the steering column shaft and stick proud allowing it to come in contact with the 12v live/main beam contact on the lighting switch causing a short and cable melting along the Brown/Blue. Subsequently a melted section of the Brown/Blue under the dash happening to cross paths with, and was touching an earth wire to the body causing the earth insulation to melt and then to short out 12v live to the body leading to further extensive insulation melting and the white smoke that was visible, Interestingly a number of items of bare metal are showing signs of surface "rust" which is very localised and wasn't there before; is that part of the chemical reaction to the short circuit?

Many lessons

David

[stuart]

10 minutes ago, qkingston said:

I've now managed to remove the damaged loom and dismantle the steering column wiring/switches. I now believe that the original cause of the problem was that the indicator self-cancelling clip was not sitting flush to the steering column shaft and stick proud allowing it to come in contact with the 12v live/main beam contact on the lighting switch causing a short and cable melting along the Brown/Blue. Subsequently a melted section of the Brown/Blue under the dash happening to cross paths with, and was touching an earth wire to the body causing the earth insulation to melt and then to short out 12v live to the body leading to further extensive insulation melting and the white smoke that was visible, Interestingly a number of items of bare metal are showing signs of surface "rust" which is very localised and wasn't there before; is that part of the chemical reaction to the short circuit?

Many lessons

David

That actually looks as if the brown wire was routed wrongly when fitting the switch Im afraid as the wires should all face away from the switch backwards and the brown seems to have been snagged round forwards. The rust will be a result of getting hot and then damp air condensing on the parts as they cooled.

Stuart.

Edited April 26 by stuart

[qkingston]

Thanks Stuart, yes that's always possible although I would have said it was installed with the wires all pointing the same way. 

What is the best source for replacement RHD chrome stalk switches?

[stuart]

16 hours ago, qkingston said:

Thanks Stuart, yes that's always possible although I would have said it was installed with the wires all pointing the same way. 

What is the best source for replacement RHD chrome stalk switches?

You might struggle to find a chrome one but a lot of the supplied black stalks are chrome underneath.

Stuart.

[RogerH]

My last light switch was Chrome under black heat shrink.

I bought an OD switch recently that was also Chrome under black heat shrink.

 

Roger