Vectra Light failure

Modern cars have the ability to monitor circuits and report errors when they are detected.

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.

Lost your spark

Modern ignition systems have evolved from contact breaker or points systems. If you look carefully you can still see some of the DNA of these systems in the latest systems fitted today. So is it reasonable to assume that some of the tests performed on older ignition systems can still be performed on these modern systems?

The answer is yes and no. Some of the old tests can still be performed when there is suitable access but many of the tests focused on the High Tension or secondary side of the coil. However access to the high tension side is often only possible after removing the coil pack, you can use an extension between the coil and the plug to test the HT outputs. These tests can be performed with an oscilloscope or a spark tester.

Some (older) readers will remember measuring dwell angles. Testing the LT or primary circuit, has changed little since the days of points, a test lamp can still provide quick and effective proof of circuit integrity. However an oscilloscope can provide extra details that can lead to more effective diagnosis. With an oscilloscope it is possible to analyse the current draw as well as the control signal/voltage at the same time. The reason why this is so important becomes clear when you consider how the modern ignition system is controlled.

Modern ignition systems consist of various inputs, logic and outputs. Inputs are from sensors such as crankshaft position, (engine speed & position) Manifold absolute pressure, or Air Mass (load) and knock sensors (abnormal combustion).

The logic or ECU, crunches the numbers and selects the correct ignition advance, dwell period as well as monitoring the circuit for faults and providing the circuit protection.

The outputs are the low tension circuits, fault codes and malfunction indicator lamp. Or in the case of amplified coils a signal to switch the amplifier and often a conformation of ignition signal back to the ECU.

These control signals can be an internal function of the ECU or in the case of amplified coils a square waveform that is used to switch the primary coil. The on time of the coil is controlled by this signal. The output stage allows current to flow through the primary windings when the voltage is present and stops the flow of current when the voltage drops to 0V. This ‘dwell’ can be measured much like on the older systems. The yellow trace is the control signal and the blue trace is the current flow through the primary windings. 

The cursors are measuring the on time or dwell, in this case 3.16ms. A typical value for a running engine. Notice the coil has reached around 5.5 Amps. Then current in the circuit is limited. The primary coil windings have low resistances, between 0.2 and 0.8 Ohms. This allows a rapid build-up of current, and can reach 60 Amps if left unchecked in around 40ms. The current in the circuit is dependent upon the voltage so to ensure good saturation the ECU can compensate for low voltages. The chart shows the current build up in a coil of 0.2Ω for both 12 and 8 Volts.