A customer of GMTO was confronted with a problem of a KIA Carnaval. This car had a specific problem with the ABS system. It looked like a problem that could easily be solved, because the problem always occurred when the speed of the car was above the 130 kilometers an hour. The problem showed itself by the ABS-warning light that went on every time the car came above the 130 kilometers an hour. This makes it normally easier to trace down the cause of the problem. The KIA can be read out with serial reading instruments and also with use of flashing codes. The ABS-warning light indicated that there was a fault with the left wheel sensor, so the fault looked to be easily solved. The sensor was replaced and the car was ready to go. But that wasn’t the fact. Again the ABS-warning light still went on when the car got above the 130 kilometers an hour. The new installed sensor was also replaced thinking that this sensor also was broken. Distributors of this parts must get mad from this needless actions, because they are the ones who get all the guarantee claims.

The wheel sensors of ABS-systems work with a familiar principle that is also used by crankshaft and camshaft sensors. The rotating part, in this case the wheel, has a disc with metal teeth. The sensor is equipped with a coil and a permanent magnet. A coil has the characteristic feature that is produces a voltage when a moving magnetic field is present. A wheel that is standing still does have a permanent magnetic field, but because it is not moving, it does not affect the coil. In the case of the wheel sensor it depends on the presence, or absence, of metal in front of the sensor. If there is no metal in front of the sensor the magnetic force lines from the permanent magnet will go from north to south in a specific way. These magnetic force lines also go through the coil windings. With a rotating wheel the metal tooth will come in front of the sensor in the course of time. The magnetic force lines then completely change and also in the coil of the sensor. This change in the magnetic field produces, only once, a voltage. We assume that this is a positive voltage. This voltage is thus produced in the transition from non-metal to metal in front of the sensor. If the metal stays in front of the sensor (wheel is standing still), the voltage drops to 0 volt. In the opposite case a voltage will be generated if the wheel keeps rotating and the metal tooth is no longer in front of the sensor. Then a negative voltage pulse is produced in the coil of the sensor, because the magnetism is returning back into its previous state. When the wheel is rotating the cams shall alternate faster. This produces a continuous change in voltage. The distance between the sensor and the metal cams is very important, because a greater distance between those two parts produces a smaller magnetic change and so producing less voltage. In this case the margin is between 0.1 and 1.5 millimeter.

Sensor signal

After installing the second sensor, the ABS warning light still went on. A universal multimeter was used to measure the generated voltage (always set the meter to AC) and to check the wiring using the Ohm meter setting. Both measurements did not give any reasons to worry and a mystery was born. The car was then brought to GMTO for investigation. In this case, GMTO started directly with an oscilloscope, because reading the fault memory and examining the retrieved data was already done by the garage and offered no solution to the problem.

In cases like this, GMTO specialists always use a special functionality of the ATS scope for measuring the signals. Displaying the signal directly is a nice feature, but a lot of the data is lost because much time is needed for measuring, retrieving the data and displaying the data on the screen. The ATS has, with every measurement, a function available called “Continuous Measuring”. With this function the signal is measured with a high sample speed and stored in a large memory buffer. The signal is only transfered to the PC and display when the mechanic triggers the scope. This causes no loss of data and a 100% correct diagnosis is made. After doing these measurements it got clear for GMTO what the problem was. See Picture 2.

Picture 2: ABS wheel sensor signal

Very typical in the scope picture are two, at two places, strange abnormalities in the signal. Because of that, these abnormalities occur in fixed pattern it can't be an problem with the electronics, but the cause of the problem should be found at the mechanical side of the car. The regularity is in the looping of the ABS disk teeth. The signal also showed a little bit of oscillation on the top of it. But that is seen more often and is caused by the teeth, on the ABS disk, which are not complete round. After this conclusion the ABS disk was examined and there the problem was. Picture 3 clearly shows the broken teeth on the disk.

Picture 3: Defective teeth on ABS disk

The only question that remained was why the fault was only reported when the speed was above the 130 kilometres an hour. The answer to this question was quickly found when the signal was measured at this high speed, see picture 3. It shows that the signal changes drop in amplitude when the broken teeth pass the senor. The difference when the defective teeth pass the sensor was at a low speed 1.5 volt peak to peak. In the same situation, but then at 130 kilometers an hour, the voltage already dropped to 1.0 volt. This change in voltage is certainly caused by the the defect of the teeth on the ABS disk.

Picture 4: Drop in signal voltage

The ABS ECU sets a limit on the voltage of the signal coming from the ABS wheel sensors. This can be a fixed value, where the signal always must be above or under a defined value, but also with use of software. The defined values depend on the speed of the car. So it can be that at a lower speed the limits are less strict then at a high speed. In this case it is clear that the ABS ECU did not recognize some of the teeth’s at a alternating voltage that was not above the 1 volt. Therefore an irregularity was found at the disk. The ECU generated a fault code and switched off the ABS (light goes on). The ABS system can not function properly if faultily information is received from the sensor which increases the possibility for making a wrong brake decision. The broken teeth were repaired with liquid metal and the problem was solved. In a later stage, the teeth disk was replaced by a new one.

R.H.M. Metzelaar
www.gmto.com