Checking the ABS hydraulic unit with a tester
1. Raise the car with a jack and install safety racks under it in specially designed places, or install the wheels to be checked on the drums of the brake stand.
Attention:
- The surfaces of the drum of the brake tester and the tires of the vehicle wheels must be dry.
- When testing the front brakes, apply the parking brake on the vehicle.
- When testing the rear brakes, block the front wheels.
Connector harness of the ABS electronic control unit.
Table. Checking at the connector of the wiring harness of the ABS electronic control unit.
2. Release the parking brake lever and check the resistance to rotation (moment of resistance) on every wheel.
Note: When using a brake tester, record the measured values of the resistance force on each wheel.
3. Turn the key in the ignition switch to position "OFF" (OFF) and connect the tester to the diagnostic socket.
Attention: before connecting (or disconnection) tester, turn the ignition key to position "OFF" (OFF).
4. After making sure that the automatic transmission selector is in position "N", start the engine.
5. With function "ACTUATOR TEST" forcibly activate the brake actuator for each wheel.
Note:
- During the check, the ABS warning lamp will be on and the ABS control will be disabled.
- Checking the actuators is not possible if the emergency mode of the ABS system is activated.
6. Turn the wheel by hand and check the change in braking force with the brake pedal depressed (pressing force is sufficient to lock the wheel).
Note: when using a brake tester, depress the brake pedal until the braking force reaches the indicated values, then check whether the braking force decreases when the components of the ABS hydraulic unit are forced on to the values measured in step 2.
Rated value:
- Front wheel - 785 - 981 N
- Rear wheel —— 588 - 784 N
7. Analysis of the test results.
A) In normal condition, the braking force release should occur 3 seconds after the wheel locks (see picture).
b) When checking according to paragraphs. 1 - 2: if the wheel does not lock when the brake pedal is pressed, then either the brake system pipes are clogged (item 1), or hydraulic block channels (item 2).
V) When checking #03-#4, if the braking force does not decrease, then either the brake pipes are not properly connected to the hydraulic unit or the ABS solenoid valves inside the hydraulic unit are not working properly.
8. After completing the test, turn the key in the ignition switch to position "OFF" (OFF) and disconnect the tester.
Checking the signal voltage of the wheel speed sensor
1. Raise the car with a lift (hang the wheels) and release the parking brake.
2. Disconnect the connector from the ABS ECU and connect the test harness to it.
3. Rotate the wheel at a speed of approximately 0.5 - 1.0 revolutions per second, measure the signal output voltage at the connector on the wiring harness side using a multimeter or oscilloscope (see table "The conclusions of the ABS electronic control unit for checking sensors").
Table. The conclusions of the ABS electronic control unit for checking sensors.
Output voltage during measurement:
- multimeter - 42 mV or more
- oscilloscope (signal amplitude) - 120 mV or more
5. If the output voltage is less than the specified values, then the causes of the malfunction may be:
- A) Wheel speed sensor.
- b) Excessive clearance between wheel speed sensor pole piece and speed sensor rotor.
Checking the wheel speed sensor waveform with an oscilloscope
Verification procedure
Set the transfer case lever to the position "2H". Start the engine and rotate the rear wheels into first gear. Turn the front wheels manually, keeping a constant speed.
Note:
- - Before connecting the oscilloscope, check the presence of contact in the wires and sensor connector.
- - The waveform test can also be performed directly on a moving vehicle.
- - At a low speed, the output voltage will be small. Accordingly, it will increase with increasing speed.
Parameters to pay attention to when observing a signal
1. If the oscillation amplitude is very small or equal to zero, then either a faulty wheel speed sensor (replace sensor), or the gap between the rotor and the pole piece is not correct (adjust clearance).
2. If the oscillation amplitude deviation is too strong (minimum: amplitude equal to 100 mV or more is not a malfunction), then there is either a large eccentricity or a significant runout of the wheel hub (replace wheel hub).
ABS electronic control unit connector.
Table. Voltage at the terminals of the connector of the ABS electronic control unit.
3. If there is interference or distortion of the waveform, then it is possible:
- A) Open circuit in the sensor (replace sensor).
- b) Break in wiring (fix or replace).
- V) Incorrect sensor installation (install the sensor correctly).
- G) Damaged or broken sensor rotor tooth (replace the rotor).
Note: as the wheel speed sensor wiring moves according to the travel of the front or rear suspension, there may be a lack of contact in the circuit (wire break) when driving on gravel roads, and when driving on roads with a normal surface, the wheel speed sensor will function normally. Therefore, when measuring the output waveform (signal voltage) sensors simulate similar road conditions (rough road) by wiggling the sensor wiring.