Open-load diagnostics

[Diver300]

I've just got an aftermarket LED centre brake light working on a Fiat by adding a 1 kOhm resistor in parallel.

With the brake light off, the car was testing the circuit for continuity which was making the LED light flash with very dim flashes. If the lamp wasn't detected by the car, that brake light would not be energised when the pedal was pressed. There was no warning to the driver of the malfunction.

Once the 1 kOhm resistor was fitted, the small current that caused the dim flashes didn't result in enough voltage to illuminate the lamp, the car detected the resistor, and the lamp illuminated when the brake pedal was pressed.

The original lamp was made of eight tiny 2.4 W bulbs. Even one of those bulbs would have had far less than 1 kOhm resistance, so the car would not have been able to tell the difference between one bulb working and all eight working.

Can anyone tell my what use it is that the circuit is disabled if an open circuit is detected? I can't see any scenario when anything is improved by disabling the circuit after an open circuit is detected. If the lamp is open circuit, there is no harm in powering it when the brake pedal is pressed. If there is a short circuit to +ve, which probably gives the same readings to the car, then the lamp will be on all the time, and there's no harm in the normal circuit trying to turn it on when the pedal is pressed.

Is this a case of overzealous fault detection?

 

[rjenkinsgb]

Is that light on a tailgate that has plunger connections, rather than a flexible cable?

That's the only scenario I can think of that makes sense, so if the tailgate is open to carry a large load, the brake light is not seen from inappropriate angles??
(But if the plungers are disconnected, the light does not work anyway - detection for another system or facility, possibly).

 

[Diver300]

It's one with a flexible cable.

I suppose if there were one with plungers, turning it off if disconnected would reduce the damage if something else then shorted the pads that the plungers press on. However, if there was a short, the system wouldn't notice a fault until the light was powered on an there would be overcurrent.