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Dim rear brake and tail lights on Mk3 GT6


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I have an issue with dim rear brake and tail lights on my Mk3 GT6

With the car engine off I have:

12.03 v across the battery terminals, across the battery +ve and the earth contact, across the battery +ve and the engine block

When the brake pedal is pressed for both brake light feeds, I'm getting 6.9v across the +ve green / purple brake light line feed to the brake lights and a direct ground to the -ve battery terminal, however, one light is brighter than the other.

I have replaced the brake switch, both brake light bulb holders, both bulbs

I have a night dimming relay. 

Any suggestions on what the issues is, and how to resolve it?

Any suggestions greatly received. 

Many thanks






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Just disconnect the earth lead from the NDR that renders it OFF

you can delete the relay join all like colours together to totally eliminate it

the rear lamp claw holders are a nightmare of mix and not matched you will likely have a mixed bag of these some earht via the claw fitting of the bulb holder   some will have a dedicated earth wire fitted to the claw holder this along with the single filament and double filament all get mixed up as do the bulbs that get fitted 

same on Spitfire with claw fitting lamps  needs a careful look to check all the holders are being earthed via wire /claw/the lamp housing/the body 

then check you have twin filament bulbs with offset pins in the stop tail  and single filament in flasher and reverse

i wish i had a £ for all the ones ive faced that are a complete  mess 

the claw fittings are not inhibited they all fit any hole you fit them too so the whole thing becomes a easy nightmare

of dull weak hopelessness 



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Much as Pete's said

  1. Just pull off the earth wire from the night dimming relay and it will never dim and takes one PITA out of the equation.
  2. The bulb holders often 'fail' internally.  They have a copper 'element' that provides the bulb side (earth) contact.  This then gets sandwiched between the plastic of the bulb holder and the steel 'spring ring' that secures the bulb holder into the light fitting.  All fine when it's brand new but 50 years later with a bit of moisture and you can get corrosion/electrolitic action between the 2 metals leading to a high resistance, or more often infinite resistance and a non-working light.  You can carefully peel the bulb holder apart, clean things up, squeeze things back together - but only ever a short term fix.  You can hack in your own earth wire, though the 'neatest' solution are to swap to the bulb holders from some of the other Triumphs that actually have a dedicated earth post on them, rather than relying on the spring clip to earth.  Long time ago I switched mine so I forget what but at a guess either one of the TRs or one of the saloons.
  3. But start with some simple tests:
    1. Compare the dark side between 'brakes on' and 'sidelights on'.  If the bulb's brighter with the sidelights on than the brakes probably just the bulb in the wrong way around or the brake/sidelight wires connected to the wrong sides of the bulb holder.
    2. Rig up some fly leads so you can power the bulb outside the car/bulb holder.  They should be the same, both each bulb powered from the same side and also side-to-side.  If not try running a new wide to the dark side.
    3. Assuming both bulbs/wires are fine repeat swapping bulbs and holders from side to side.  If the dark side switches it's a bulb holder issue.
    4. If the same side stays dark with the bulb holders wired up but NOT pushed into the light fitting try running a wire from the spring clip ring to a good earth.  If the bulbs are now bright the issue is the earthing of the light fitting to the car body - which from memory is just through the mounding bolts.  Easy option is to get run a dedicated earth from the light fitting.
    5. If none of that works, umm, pass!
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There's obviously a high resistance joint somewhere in either the +ve feed or the earth return line. Disconnect the battery and resistance check each side. once you have determined if it is feed or return, find the joints in the line and recheck the resistance at each point until you locate the bad joint. Testing for voltage isn't a good way to fault find this type of snag as most meters have very high resistance themselves and consequently will mask the error. Somewhere in the dashed bits is a high resistance:

BATT +ve ----- loom ------ Light Switch ----- loom ----- filament -----loom ----- EARTH -ve

Voltage checking may give erroneous readings as you may have an effective circuit like this:

12 volts BATT +ve ----- Light Switch ----- FAULTY CONNECTOR (3 ohms?) ----- Filament (3 ohms) ------ EARTH -ve

The 12 volts splits equally across the 3 ohm faulty connector and the 3 ohm filament so you only get 6 volts at the filament. Putting a meter in place of the filament you get:

12 volts BATT +ve ----- Light Switch ----- FAULTY CONNECTOR (3 ohms?) ----- Meter (20,000 ohms) ----- EARTH -ve

3 ohms of faulty connector is insignificant compared to the 20,000 ohms of the meter which will read nearly 12 volts.



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