Modern light systems have the ability to warn the driver that a circuit failure has occurred with a Malfunction Indicator Lamp. The driver can then take the car to the workshop for diagnosis and repair.
The owner of one such vehicle was certain that he faced an expensive repair when the lighting circuit MIL illuminated on his Vauxhall, but when he checked the lights he could not find the one not working.
A quick code read suggested a fault with the rear left brake light circuit. However the brake lamps appeared to be working when the pedal was pressed. The light cluster was accessed and the lamps checked. The nearside brake lamp had indeed failed.
This led to a number of questions from the puzzled driver.
So how did the car know?
Why are all the lamps the same?
Why did the lamp appear to work when the pedal was pressed?
The answers are linked to how modern lighting circuits are controlled. The use of vehicle networks has reduced the number of fuses and relays by as much as half. But every circuit must have a form of protection, in the case of modern lighting it is the control unit that monitors the current drawn by circuits. It can then switch the circuit off if it detects a fault. It can then elect to use another lamp and circuit to replace the faulty one. In this case the brake light circuit was inoperative so the side light circuit was used to perform the task of the brake light. This is possible as the lights are controlled using a pulse width modulated signal. The brake lights require full intensity so have a pulse width of 100% but the side lights only require around 25% of the 21Watts available. By switching the circuit 278 times a second the 25% duty cycle is seen by the lamp as a 3Volt supply. (This is what you would read on a multi-meter) The 21 Watt lamp illuminates at around a quarter of its intensity, or 5 Watts the same as a tail light. The means the same lamp can perform the task of tail and brake lights. Using just one lamp makes economic sense as it reduces inventory.
The control unit sends a signal to the lamps and checks the circuit before it is used. Here the brake light circuit is being monitored every 10 seconds. The voltage is pulsed so fast that the filament does not even start to glow. This is how the control unit can detect a fault before the circuit is used.
Using an oscilloscope you can check the current draw during the circuit check. Here the time base has been reduced to 1ms per division. The red trace shown indicates a current of 9 Amps on a brake light circuit. This has to be a fault. Or is it?
Once the lamp is switched on (100% duty cycle) the current measured is around 1.7 Amps. This is normal for a 21 Watt lamp.
Watts law current = power/voltage
21/12 = 1.75 Amps.
Same lamp same circuit. Different current draw, this is because once the lamp heats up and starts to glow its resistance increases. Try it for yourself with a multi-meter, measure the resistance of a cold 21 Watt lamp. Then use Ohms law to check to expected current flow.
Remember Amps = Voltage/resistance.
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