Mysterious Lanos

A call from a car repair colleague promised an interesting job. It appeared he had been trying to repair a DAEWOO Lanos with a 1.5-litre motor for a long while, but he had failed to do anything.

The problem was that the car’s engine died from time to time at idling speed; it could also spontaneously make a rpm dip for one or two seconds and then restore its rpms; and the car twitched when driven.

The fellow did everything he could: he replaced the fuel pump, the ignition module, spark plugs and high-voltage wires. Moreover, he tried to install an engine control unit from another car – nothing helped solve the problem. Well, that only makes it more interesting.

WHAT DO WE HAVE AT THE START?

A Lanos. 1.5-litre motor, Euro 2, distributed injection, DIS-type ignition, ITMS-6 control system. We are not going to be so linear and we won’t start replacing everything. Instead, we’ll try to apply diagnostic equipment and logic.

We connect a scanner to start with.

When idling, all of the engine’s parameters are quite appropriate, but, most importantly, the lambda probe is actively working and the fuel supply correction factor indicates that the mixture composition is fine. This result is quite predictable, because short-term sporadic failures in motor operation cannot appear due to fuel supply problems. The system’s response was very slow. So replacing the fuel pump right off was in vain.

But work with the scanner allowed us to make the first important conclusion: when the motor has trouble running, the ECU clearly displays the rpms and the injection time. This means the problem is not related to synchronization. In other words, as diagnosticians say, “the ECU sees the turning”.

But that’s something. Next, we take a good old MotoDoc II, which has been serving faithfully in our shop for many years. We connect probes to the high-voltage wires and start the engine. The result was not long in coming – here it is:

High voltage is lost when the engine fails. And it is not just lost, but horrible noises appear as well, as if it is not a pulse but rather some interference being supplied to the ignition module input. It occurs to us that there is contact chatter or a problem in electrical wiring from the ECU to the ignition module. Anticipating a swift victory, we connect the motor tester’s multipurpose probes to module inputs and start recording an oscillogram:

The result is puzzling. The pulse waveforms are certainly not changing at the times of failure; the pulses are arriving from the ECU in the same steady manner. What does this mean? Is the module faulty? But it was already replaced without effect. By the way, the ignition module from the VAZ can be connected to such a motor on a one-to-one basis, which makes the task easier.

Let us think. The problem seems to be due to high-voltage failures. There are control pulses on the module. What else is there? Of course, power and ground. We connect the multipurpose proble to the ground output, start the engine, and see the following oscillogram:

That’s it. The problem is solved.

The lack of proper contact between the car’s ground and the ignition module is obvious.

Let us explain the oscillogram.

As soon as the control pulse comes from the ECU, the ignition module starts charging one of the coils, but quite a considerable current is flowing over it. Due to a parasitic resistance, the voltage appears at the module ground output and this voltage’s behaviour is absolutely unpredictable.

In our case, it increased to 1.2…1.5 volt. When the voltage is too high (the ground contact is very poor), the coil fails to accumulate energy sufficient for spark gap breakdown and spark burning, which leads to failures in engine operation.

Here’s another picture:

You can see quite clearly in the picture that, when the ground is lost, the high voltage on the coil also disappears. Here is this segment in an expanded view:

The voltage at the ignition module ground output (white oscillogram) at the time of failure reaches 5.5 V.

Ultimately, to make sure that our conclusions are correct, we read the signal on one of injectors at the same time:

It is quite evident that when the defect manifests itself, the signal on the injector (blue oscillogram) is present. Therefore, there are no complaints about the fuel supply system, while the defect’s cause is a poor ground at the ignition module connector.

Generally, it is very simple to check the ground quality using a MotoDoc II (III) motor tester. One end of the multipurpose probe should be connected to the ground in question, the second one – to the positive battery terminal; we set the 16-volt limit and turn on the recorder. Or another type of synchronization as desired. Then we start the motor and watch the voltage behaviour.

If everything is fine, the line will be straight and smooth. If not, at the moment the load occurs on the circuit in question, the voltage will drop, sometimes very significantly – by several volts. Alternatively, you can connect to the ground in question and the negative terminal, as I did. Then the presence of any voltage will indicate a poor contact.

Eliminating the defect consisted of soldering an additional wire from the ignition module connector directly to the battery’s thick negative wire; fortunately, they are only centimetres away from each other on this motor.

CONCLUSION

If you seriously commit to diagnostics, acquire good diagnostic equipment. It will make your work easier and save you the trouble of replacing each and every part.