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Frustrating earth issue


AlanT
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Hi All,

The niggles continue on my Spit 1500 resto. I have an earth leak to the green/purple wire that goes to the brake lights and brake switch. This means that the tail lights can operate with no earth connected! Of course bigger problems if brake operated (short).
I’ve tested the brake switch by directly joining the wires to see if the earth leak remains. Switch isn’t the cause. Nor are the bulb holders as have the leak when testing the wires directly whilst disconnected.

Any thoughts? I’ve got a testing meter so have been checking what I can. 

Appreciate help. It’s one of those ‘hard to track’ things that drives us all nuts! 

Not sure what to test next. Thanks.

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Have you inspected the loom all the way back? A very common cause of this sort of thing is the loom rubbing against bodywork* where it passes through a hole, and cutting the insulation off.

 

* other sources of abrasion are available. I had a VW Passat with a bad misfire caused by the engine loom rubbing on the engine earth strap

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You may be right. Thinking about this logically again, if the brakes are off (switch breaking circuit) then the short to earth must be between the switch and bulb, not in the circuit before the switch. 
Whole interior fitted…will do more testing this evening. 
 

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and never discount the mix and match rear lamp claw/bulb holders and the ability to fit and mix up single and twin filament bulbs 

and the extra fun of some claws earth via the claw and lamp housing and some need a seperate earth wire to a smll lucar terminal

these all get seriously mixed up and cause wonderful shorts and christmas tree illuminations at the rear 

the fact that tail/stop bulbs have offset bayonet pins doesnt stop holders and bulbs getting in the wrong places 

it happens a lot on this design of rear lamp used on many a triumph

Pete

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I think the problem is the idiot. Please help! Left hand tail lamp has three connections-black (earth-middle connector on holder), red (tail light), green/purple (brake) - see pic.

Right hand has two connections - red (tail) and green/purple (brake). Earth from the claw. Despite being out of the claw holder, the tail light is on. Brake light operated turns right hand side out.

Plainly I’ve wired something wrong. Probably on left side? 

Thanks 

Alan

One thing I didn’t realise/seems odd. I always thought the outside Bayonet of the bulb was negative, the two bottom terminals positive, but to different filaments. But putting the two bottom onto a battery works the sidelight. Seems weird? A short? 

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12 hours ago, AlanT said:

I always thought the outside Bayonet of the bulb was negative, the two bottom terminals positive, but to different filaments. But putting the two bottom onto a battery works the sidelight. Seems weird? A short?

If the earth is not connected to anything, then a battery between the tail and stop filament terminals is connected to both filaments in series (the shared earth is just the middle of the two). Since the tail light is lower wattage than the stop, it's also higher resistance, so most of the voltage is dropped across it, and it's the one that lights up. So not weird at all - fully expected normal behaviour.

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Thanks. Really appreciate the explanation. Told you so electrics don’t get on with me! Took another look last night. It was indeed a frayed wire where it comes through the wheel! Now sorted.

thanks again.

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Bulbs and bulb holders come in many varieties. Originally, they were all plated brass or bronze. Nearly all are poorly plated steel now. The double filament bulbs used on Triumphs have bayonet pins opposite one another but at a different heights and must be the correct way round for Brake/sidelights. There are later double bulbs with the bayonet pins NOT opposite one another. Unwary 'Fitters' jam them in incorrectly and this can cause shorting out, blown fuses or burned wiring. Occasionally, they light up either too bright or too dim.

Just be careful when selecting replacements. They will not be included in the selection booklets in Halfords etc. They may fit mechanically, but they will not operate properly.

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