Kia ABS to the Max
Robo Tech was called to a shop on a 2004 KIA, with a 2.5 liter engine. This car had a specific
problem with the Anti-lock Braking System or A B S. It looked like a problem that could
be solved easily, because it always occurred when the speed of the car was above 60 miles
per hour. The problem showed itself by the A B S warning light that went ON, every time
the car came above 60 miles per hour. This makes it easier to trace down the root cause
of the issue.
This KIA's A B S system could be read out with a scan tool, and also with the use of
flashing codes. The A B S warning light indicated that there was a fault with the left wheel
sensor, so the fault appeared to be resolved easily. The wheel speed sensor was replaced
by the shop and the car was ready to go. But nothing was further from the truth! The A
B S warning light still went on when the car got above 60 miles per hour. The newly installed
wheel speed sensor was also replaced again, thinking that this new sensor also was defective.
The distance between the sensor and the metal teeth is very important, because a larger
distance between those two parts produces a smaller magnetic change and thus producing
a lower voltage. In this case the margin should be between 0.5 and 3 sixteenth of an inch.
I knew that, but the shop never checked.
I started my diagnostic by grabing my scope, since the scan tool is not meant to test wheel
speed sensor signals. And, we knew that it wasn't the wheel sensor to beging with. I
started capturing the signal of the A B S wheel sensor at a speed of 30 miles an hour.
The peak to peak voltage of this alternating current was approximately 3 volts. Typically
for this signal is the offset in the signal. The signal is not alternating around 0 volt,
but just above 0.8 volts or so. This offset can be measured when the wheel is not rotating.
Nowadays this offset voltage is used for diagnostic reasons, so that the Electronic Control Module
can better recognize a circuit problem.
In situations like this, I use a function in the scope, which continuously measures
the input signals and captures the signal for transfers to the computer. The large internal
memory allows for a long measurement that contains lots of information. Later on even
a spreadsheet software, like excell, can be used to plot the data. After the long data
capture, I examined the waveform.
Then, what a shock! Immediately it got clear to me what the problem was!
Very prominent in the scope capture are two strange anomalies in the signal, at two separate
places. Since these anomalies appear in a fixed pattern, it cannot be a problem with
the electronics, but the cause had to be at the mechanical side of the car. The repetition
is caused by the constantly reappearing issue. After concluding that it had to be a mechanical
problem with the A B S tone ring, the ring was examined.
Wow, right away I had found the problem! The screen capture clearly shows the broken teeth
on the disk.
The only question that remained was why the fault was only reported, when the speed was
above 60 miles per hour. The answer to this question was quickly found, when the signal
was measured at a higher speed. It showed that the amplitude drops when the broken teeth
passes the sensor. At low speed, the peak to peak amplitude was 1.5 Volts. In the same
situation, but then at 60 miles per hour, the voltage dropped to below 1.0 volt. This
drop in amplitude is certainly caused by the broken teeth.
The A B S computer sets a threshold limit on the voltage of the signal coming from the
A B S wheel sensors. This is a fixed value, where the signal always must go above such
pre-programmed value. The rule of thumb is 2 volts peak to peak. It is also possible
that this limit is set dynamically, depending on the speed of the car. Such values are always
unknown and not published by the manufacturer. The A B S tone ring was replaced with a new
axle and the issue was solved. This case explores the importance of signal recognition threshold,
which is dependant on the tech's experience to recognize
the issue.