ABS Demonstrators

Using the wiring diagram for your vehicle, find the ABS wheel speed sensor pin out connections to the ECU on the wiring diagram and the demonstrator. Record which ECU wires go to which speed sensors.

Mazda 323 JM BA

Left Front ECU pin# 2O and 2P

Left Rear ECU pin# 2R and 2Q

Right Front ECU pin# 2N and 2M

Right Rear ECU pin# 2S and 2T

By looking at the wiring diagram, what type of speed sensor is this?

Inductive. 4 Channel/ Each wheel has it's own individual sensor. And each sensor only has 2 wires.

Describe how it works

The wheel sensor is placed approximately 1mm above a sensor rotor. As the wheel rotates the pulse ring rotates with it. As each pulse ring passes under the sensor, a small voltage pulse is induced in the sensor. The pulses are sent as input signals to ECU. (ref Ed May and Les Simpson Automotive Mechanics Volume 2)

Locate an oscilloscope. Turn it on and set it up to be fully operational. What oscilloscope are you using.

DIGITECH Dual-Channel Oscilloscope QC1992

Record a Waveform for each wheel speed sensor in the boxes below. Note voltage per division and time for per division for each. Please don't keep the ABS units on very long because it drains the batteries. All Graphs will be in 5V per division on the Y-Axis, and .2 ms on the X-Axis.

Which wheel is this?

Left Front

Which wheel is this?

Left Rear

Which wheel is this?

Right Front

Which wheel is this?

Right Rear


Are all the waveforms exactly the same? Yes/ No. Discuss what are the differences, and what can cause the differences between the waveforms.

No, the front and rear sensor waveforms were different. The front left and right sensors gave me a amplitude reading of .6V whilst the rear left and right sensors were .4V, but the frequency for all sensors were both .38ms. I first thought they could be different because they had different numbers of teeth for the front and rear rotor teeth. So I checked, but both rotors had 44 teeth. I then measured the resistance for each wheel sensor and recorded .430k for the front wheels and .890k for the rear. I then asked myself how the amplitude could increase with the frequency remaining the same. I checked the air gap of all sensors and they were all within specifications. With all my results from my tests I had to conclude the greater resistance in the rear wheels was why the amplitude was greater for the front wheels, considering the wheels were rotating at the exact same speed.

With the wheel speed sensors spinning, measure AC volts with a multimeter and record here.

Left Front:

3.17V

Left Rear:

2.68V

Right Front:

4.22V

Right Rear:

2.85V

Can a multimeter be as accurate as in finding problems with the wheel speed sensors as an oscilloscope? Yes/ No

No

Discuss what the oscilloscope could find that the multimeter could not find and why.

The oscilloscope can give you a visual reading of what the sensor is picking up. If there is a chip or dent on the rotor tooth, the oscilloscope will display a break or distorted line indicating where the fault is. An oscilloscope also can measure frequency and amplitude at the same time, while on a multimeter you have to record your result and then change settings to record something else.