The scan tool, when plugged into the DLC, becomes part of the vehicle's electronic system. The scan tool can perform the following functions:

In general the scan tool has five test modes for diagnosing the ABS.

Use the following scan tool ABS tests in order to assist in diagnosing the ABS malfunction.

Motor Test (Apply/Release)

Each motor can be manually turned ON for up to 5 seconds, at a current of 10 amps in forward (pressure apply) or 6 amps in reverse (pressure release). After running the motors, the command and feedback currents will be stored in the snapshot buffer. This information can be reviewed to determine if the motors are operating properly.

The test below describes how the front motors will normally operate. The rear motor can be tested in a similar manner, except there will be only a slight pedal drop (bump) and rise during the test.

1. Turn the ignition switch to the OFF position.
2. Install a scan tool.
3. Turn the ignition switch to the ON position.
4. Select SPECIAL FUNCTIONS, MANUAL CONTROL.
5. Pump brake pedal until firm to deplete vacuum from power booster.
6. Depress and hold brake pedal.
7. Release the motor to be tested. The brake pedal should move smoothly toward the floor. As the pedal drops, the feedback current should momentarily drop to only a few amps (indicating motor movement) and then become equal to command current (6 amps). This indicates that the motor is no longer moving since the piston has reached the bottom of the bore. Rough or jumpy pedal movement indicates an intermittent electrical connection within the motor.
8. With the brake pedal still depressed and at the floor, apply the same motor. The brake pedal should now smoothly rise back to the top of its travel. The feedback current should momentarily drop to a few amps, then quickly increase to the command current (10 amps). This indicates the motor is no longer moving since the piston has reached the top of the bore (home position).

Solenoid Test (ON/OFF)

1. Turn the ignition switch to the OFF position.
2. Install a scan tool.
3. Turn the ignition switch to the ON position.
4. Select SPECIAL FUNCTIONS, MANUAL CONTROL.
5. Release the motor for the channel being tested.
6. Select the solenoid for the channel being tested.
7. With no brake pedal pressure applied, turn the solenoid ON.
8. Depress the brake pedal. It should be very high and firm.

If the pedal goes nearly to the floor, the solenoid is leaking or not closing and/or the check ball is leaking.

Important: After performing step 9, bleed the brake system. Refer to Hydraulic Brake System Bleeding in Hydraulic Brakes.

9. With brake pressure still applied, turn the solenoid OFF. The brake pedal should drop immediately. This verifies normal solenoid operation. If the pedal does not drop and the motor is moving, the solenoid is stuck ON or OFF. To verify which solenoid is stuck, apply the motor for the channel being tested.

•	If the brake pedal rises, the solenoid is stuck OFF (open) or the check ball is leaking.

•	If the pedal drops suddenly, the solenoid is stuck ON (closed).

•	If the pedal (while depressing) slowly sinks toward the floor or if it rises during the apply of the motor, physically switch the two solenoids and repeat the test for the channel in question.

-	If the pedal continues to sink, the check ball is leaking. Replace the Brake Modulator. Refer to Brake Pressure Modulator Valve Assembly Replacement .

-	If the test now works properly, test the other channel. If the pedal sinks or rises, replace the solenoid in this channel. Refer to Brake Solenoid Valve Replacement .

-	If the test fails for the same channel, the check valve is leaking. Replace the Brake Modulator. Refer to Brake Pressure Modulator Valve Assembly Replacement .

Hydraulic Control Test

This test verifies a base brake apply and ABS release, hold, and apply functions. During testing, if a problem is encountered, the scan tool provides information on the problem.

1. Turn the ignition switch to the OFF position.
2. Raise the vehicle such that all wheels to be tested are off the ground.
3. Install a scan tool.
4. Turn the ignition switch to the ON position.
5. Select SPECIAL FUNCTIONS, HYDRAULIC CONTROL.
6. Select the channel to be tested. When testing the rear brakes, both rear wheels should rotate and not rotate together at the various points in the test. This is true because both rear brake pressures are controlled together.
7. Firmly depress brake pedal. Pedal should remain high and firm.
8. Have an assistant attempt to rotate the wheel being tested. If the wheel does not rotate, the base brake apply is working properly.
9. With the brake pedal still depressed, start the test. The brake pedal should remain high and firm. If the brake pedal moves toward the floor, verify that the solenoid electrical connectors are connected to the correct solenoids.
10. With the brake pedal still depressed, have the assistant attempt to rotate the wheel. The wheel should rotate with little or no resistance for the full 18 seconds of the test.
11. After 18 seconds, brake pressure will be applied and a slight bump may be felt in the pedal. This is normal. The assistant should not be able to rotate the wheel at this point.

Motor Test

Once the motor pack has been separated from the hydraulic modulator, this test will help determine if the motor pack is operating properly.

The scan tool will command the motors in one direction, then the other.

No Gear Movement

After the motor pack has been removed, rotate each gear by hand on the hydraulic modulator. The front gears (non-center gears) should be able to be rotated approximately 8.75 full turns, lock to lock.

If the gear does not turn freely, or at least 8.5 turns are not possible, replace the hydraulic modulator. The rear gear should rotate approximately 3.75 turns. If the middle gear does not turn freely, or at least 3.5 turns are not possible, replace the hydraulic modulator.

Refer to Brake Pressure Modulator Valve Assembly Replacement .

Gear Tension Relief Sequence

When the displacement cylinder pistons are in their upmost home position, each motor has prevailing torque due to the force necessary to ensure each piston is held firmly at the top of its travel. This torque results in gear tension, or force on each gear that makes motor pack separation difficult. In order to avoid injury or damage to the gears, the gear tension relief function briefly reverses each motor to eliminate the prevailing torque.

Always perform the gear tension relief function prior to removing the hydraulic modulator or motor pack from the vehicle.

Relay Test

This test allows monitoring the voltage available to the EBCM while turning the Electronic Brake Control Relay ON and OFF.

When the relay is commanded ON, the voltage should be equal to battery voltage. When the relay is OFF, the voltage should drop below 5 volts.

Important: Voltage will not drop to zero when the relay contacts are open due to capacitors in the EBCM. If voltage drops below 5 volts, the relay is operating properly.

Voltage Load Test

The ABS VI system can draw significant amounts of current when operating. This test turns ON many of the system components to load-test the vehicle's electrical system.

If intermittent EBCM operation or low voltage malfunctions are occurring, this test will allow you to monitor two separate power circuits for ABS: ignition and battery. If only one of these two inputs drops below 10 volts during testing, a high resistance may be present in that power feed circuit.

Rehome Test

Important: The motor rehome function cannot be performed if any current DTCs are present. If current DTCs are present, the vehicle must be repaired and DTCs cleared before performing the motor rehome function.

The motor rehome function will return all of the hydraulic modulator pistons to their upmost home position. This allows all fluid paths within the modulator to be open so that the modulator can be properly bled. The motor rehome function must ALWAYS be used prior to bleeding the brake system.