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[GastronomicKleptomaniac]

In releasing my inner chav (and because I had some laid around the workshop), I popped a pair of LED stop/ tail bulbs in the back of the Omega. Not a cheap eBay pair either, They look OK and work well, only problem now is that the high level brakelight is very very dimly lit.
 Since it's only happened after putting the bulbs in, I think I can rule out a bad earth - some casual Googling suggests it's because they have a very low current draw and so some of it is escaping to the high light?

 Is it a simple case of popping a resistor or two in somewhere along the line, or is it something more complex and sinister? I'm not great at electrickery, but I can follow basic instructions...

[TheBoy]

whack the bulbs back in to check if that cures it.

if it does, leave them in

[GastronomicKleptomaniac]

Thought you might say that.

[TheBoy]

Worth doing the check, just to ensure there isn't a problem with standard setup.

[GastronomicKleptomaniac]

Normal bulbs back in, all back working fine. I shall be leaving them alone.

It did the same in my old car, until a suitable amount of glowering and swearing made it behave itself.

[Chris_H]

Normal bulbs back in, all back working fine. I shall be leaving them alone.

It did the same in my old car, until a suitable amount of glowering and swearing made it behave itself.

That's spooky.  You don't say if you have an early car or a late one (late ones seem to have LED high-level stop light).

LED lamps normally take less current which is why some indicator replacements have a resistor to increase the drain and keep the flasher unit happy.

I can only think that the Check Control circuitry is messing up the current distribution.  The wiring diagram for late models (in Haynes  ) is bizarre.

[Dave DND]

Only the indicators ever need the resister to load up the flasher relay, not stop/tail lights.

I suspect that these may be of fairly poor quality, and that the diodes inside are not actually blocking any rougue voltages, but instead are actually allowing them to filter into other parts of the circuit. Similar sort of effect as to when an earth goes and all the bulbs light up when one is operated

[Kevin Wood]

There will be some leakage current through the bulbs when off, as stated, due to the check control testing for filament conductivity and also because the brake light signal is shared with other systems in the car (engine and gearbox ECU, ABS, Cruise control, etc.) and these probably each have a pull-up on the input so they "fail safe" if the brake light wiring to them is severed. A few milliamps of leakage through a pair of 21W incandescent bulbs is of no consequence, but it wall cause LEDs to glow.

The solution?
What TheBoy said!

 

[Chris_H]

There will be some leakage current through the bulbs when off, as stated, due to the check control testing for filament conductivity and also because the brake light signal is shared with other systems in the car (engine and gearbox ECU, ABS, Cruise control, etc.) and these probably each have a pull-up on the input so they "fail safe" if the brake light wiring to them is severed. A few milliamps of leakage through a pair of 21W incandescent bulbs is of no consequence, but it wall cause LEDs to glow.

The solution?
What TheBoy said!

 

Reading your response makes me re-read the OP!  I had interpreted it as the high-level light lit dimly when the brake lights were on. Not so it seems;  'tis on permanently.

In that case a ballast resistor across the LED stop lamps would put it out.  The voltage across the LED chain has risen above the switch-on threshold.

And I have to review my reasoning on why Omegas don't blow exterior lamps very often.  I thought it was due to the series resistor but in fact it is likely to be because the filaments are pre-heated.

[Kevin Wood]

In that case a ballast resistor across the LED stop lamps would put it out.  The voltage across the LED chain has risen above the switch-on threshold.

True enough, if you're determined to make it work.

And I have to review my reasoning on why Omegas don't blow exterior lamps very often.  I thought it was due to the series resistor but in fact it is likely to be because the filaments are pre-heated.

Hmm. Not sure you'd get enough current to pre-heat them, but it's an interesting thought.

[GastronomicKleptomaniac]

It's an ex-plod with a TID, is the check control still there and checking itself, just not wired into my display?

I am sticking with the original lamps, to be quite honest you can't tell the difference with them being textured taillamps.