Working with the Motor Tester


Experienced specialists will tell in a heartbeat.

Yes, that’s right: the sporadic defect. This means any defect that can be caused for any reason and manifests occasionally rather than continuously. Often, the defect does not reveal itself during the visit to an automobile repair shop. What steps should be made in the diagnosis? What should be done? Which element should be replaced? These are not the simplest questions.

However, sporadic defects can be found. To do so, it’s best to use the most interesting diagnostic instrument – the motor tester. We should connect motor tester probes to sensors or electric circuits of the control system we are interested in, start the recording process, and wait for the defect to manifest itself “in all its glory”. Then we should stop taken measurements and analyze the oscillogram obtained.

This is how a failure was detected on a VAZ 2110 with a 1.6 L V8 21114 motor equipped with a Yanvar 7 [January 7] control system. The problem was that the engine might die at any time.

After it died, it could be easily started again and run as if nothing had happened. That’s all right if it happens when the car is standing still, but driving such a car is not only inconvenient: it’s simply dangerous. Jumping ahead, let’s just say that the failure was plainly trivial, but detecting it was a real challenge.

Well, welcome to the job of being a diagnostician.

It is absolutely clear that for an engine to operate normally it requires fuel, reliable sparking, and compression in the cylinders. The latter certainly can’t be lost sporadically, so let us consider fuel supply and ignition systems.

As both these systems receive control signals from the engine control unit, the very first thing that occurred to us was to connect the scanner and evaluate the data stream parameters.

Let’s connect a Scanmatic scanner.

First of all, we are interested in the speed and injection time. If they are present when the defect appears, then the control unit “sees rotation” and tries to open the injectors. Whether it succeeds or not is another issue, but the point is whether the unit tries to do it.

Soon we discovered that right until the very end the scanner displayed the speed, the ignition advance angle, and the injection time. Well, we failed to take the fortress by storm. Let’s proceed to a siege.

We will use a USB Autoscope III motor tester, better known as a Postolovsky oscillograph.

Let us first consider the ignition system. As you know, on this engine there is a DIS-type ignition system with two coils located within the same housing.

The coil control switches and current monitoring circuits are located inside the ECU. The coil unit connector has three outputs: +12V is supplied to one of them from the onboard power system when the ignition switches on. The other two are primary coil outputs to be switched to the ground inside the ECU. By connecting motor tester probes to these three outputs, we can monitor the coil power supply and primary voltage.

Thus we will discover if the defect causing the sudden stopping of the motor is related to the ignition system.

So we connect channel 4 of the oscillograph (green-colour oscillogram) to the power output, channel 5 (red) to the primary circuit of cylinders 1-4 and channel 6 (violet) to the primary circuit of cylinders 2-3. We start the engine and wait. Hurray, it died! Now, we need to carefully study the oscillogram obtained and find out if the engine is stopping due to a fault in the ignition system.

“Something’s wrong”. This is the first thought to come to mind when looking at the oscillogram. And really, while the primary voltage waveform looks academic and arouses no suspicions, the supply voltage has obvious significant dips. Let’s change the oscillogram’s scale and examine it more closely.

The supply voltage on the coils (green oscillogram) is 13.3 V. The small waves are a consequence of the alternator running – this is not a defect. But when energy accumulates in the coils, the supply voltage starts monotonically decreasing to 8.8 V! In other words, no less than four and a half volts are getting lost somewhere. Obviously, this is a problem.

Remember how current flows through the ignition coil. As in any inductance, it does not appear instantly, but rather rises smoothly to achieve a steady-state value after a while:

Does the current waveform remind you the waveform of an on-coil supply voltage?

Of course, it does. When comparing the oscillograms of the supply voltage on coils and the primary voltage, for instance, of cylinders 2-3, it is quite evident that, when the increasing current goes through the coil (when it accumulates), the supply voltage on the coils is decreases monotonically in the same way.

What does this mean?

The coil current flows through the power supply circuit, doesn’t it? According to Ohm's law, this means that this current is creating a voltage drop across some parasitic resistance. Simply speaking, there is a very poor contact in the coil power supply circuit. Please look at the picture.

If there are no problems in the coil supply circuit, the voltage waveform on coils will be close to a straight line. If there is a poor contact in the circuit (the same as parasitic resistance Rpar), the voltage drop Upar occurs as per Ohm's law when the current flows through this resistance. Those lost 4.5 V. As a result, we see voltage dips on the oscillogram at the time of energy accumulation in the coils rather than an almost straight line.

But the most interesting thing is this:

The time when the engine dies. You can see that the coil supply voltage dropped down to 6 V, and thus energy accumulation and the ignition system ceased functioning. The control unit continued connecting coils to the ground, causing an even larger voltage drop. Well, this problem is almost solved.

Now, let us examine a diagram of the Yanvar 7 system.

It is the same as the Bosch 7.9.7 unit diagram. Based on it, coils are supplied directly from the ignition switch via several connectors. This means we need to check the wiring from the switch to the coils. The problems may possibly be related to the proper contact assembly of the ignition switch.

We will not keep you in suspense for long: at the very first glance under the dash, we discovered a toggle switch that had been installed in the coil supply circuit by some Mr. Fix It. It seemed to be some kind of antitheft system. Once the toggle switch was removed, the supply voltage oscillogram became normal, and the engine stopped dying suddenly.

It should be noted that a somewhat similar defect was described in the article “Mysterious Lanos”, but it dealt with a poor ground contact on the ignition module.

It is time to summarize. The conclusion is simple: using a motor tester and properly analyzing oscillograms based on an understanding of how the system operates and the fundaments of electricity allow us to detect almost any defects – even man-made defects, which are often the most difficult to find.