OK, I am aware of how the brake light circuit works. Brake light switch has two elements. One circuit completes a circuit to operate lear brake lights whilst other breaks a loop to ECU.

Failure of other sets up fault condition.

I've now got tired of a drop off in car lamp quality so I'm gradually replacing all non halogen lamps with LED. 

So I replaced both brake lamps with LED, and. LO, I got the flashing Glo plug alarm.

First step, go back to pre change. After a few minutes, alarm had reset.

Next question is that with one lamp failed, no alarm is given.

So has anyone any idea of current fail threshold, as if I want to use LED brake lamps, then I will have to provide a shunt on the LED and need to work out resistance value and wattage.

Measure the resistance of the existing incandescent bulbs (I am too zonked to work it out for you) and then choose a shunt resistor say 10% less, dont forget the current rating, a 1/4 watt resistor will probably fry! Also think about heat dissipation, if you choose one with double the required current rating or a wire wound Alu cased heatsink one then it will be less of an issue.

LED lamps may well be even less reliable. 

Thanks JR, my thoughts, but I hoped someone would know.

I know that no alarm comes up with one incandescent lamp out, so I'd suggest that a minimum of 21w ( approx 2A @ 12v) will do. To limit current to 2A @ 12v, I'll need a resistor of circa 6 ohms @ (2x2x6) = 24w.

Seems like a rather large resistor, so I might look at an electronic solution/ put up with status quo.

If my electronic calcs are still working.

Pete, LED are far more reliable. Rail signal heads have replaced incandescent for LED and lamps in other safety critical systems for a lot of years.

 

LEDs can be far more reliable, but aftermarket automotive ones are extremely variable in quality.

Loads of posts on here about multiple failures in people determined to mess about with suchlike.

3 minutes ago, VWD said:

Pete, LED are far more reliable. Rail signal heads have replaced incandescent for LED and lamps in other safety critical systems for a lot of years.

 

LED car bulbs from China can be rubbish.

I always keep to a maxim from an old electronics mag( replacing Hong Kong with China)

All things from Hong Kong, all too often tend to go wrong. I'd sooner pay a bit more for UK electronics than chance Chinese.

13 minutes ago, Breezy_Pete said:

LEDs can be far more reliable, but aftermarket automotive ones are extremely variable in quality.

Loads of posts on here about multiple failures in people determined to mess about with suchlike.

I've kitted mine out with LEDs from same place, as given good reviews on here. No problems to date. 

13 minutes ago, VWD said:

I always keep to a maxim from an old electronics mag( replacing Hong Kong with China)

All things from Hong Kong, all too often tend to go wrong. I'd sooner pay a bit more for UK electronics than chance Chinese.

Where did you buy your time machine? I would like one too!