SERVICING DELCO ABS VI BY JEFF MASTERMAN While radically different in design, Delco s ABS VI is one of the most technician-friendly antilock brake systems on the market. GM s ABS VI system has been used for a number of years on many of their lighter vehicles. There s also a version that provides traction control. ABS VI is one of the more technician-friendly braking systems in use. There are many DTCs available, and the scan tool plays an important role in diagnosis and service. This article takes a look at some of the more useful diagnostic functions that are built into this system. 40 June 2001
Photos: Jeff Masterman June 2001 41
piston in the displacement chamber. This effectively reduces the apply pressure to the brake on the slipping wheel. Once the affected wheel picks up speed that equals that of the others, the motor reapplies hydraulic pressure to the circuit under control and the system is ready for another go-round. This wheel speed sensor data graph indicates that the right front sensor frequency dropped to zero during a normal stop. This can cause unwanted antilock operation and reduced braking performance on dry surfaces. Diagnosis GM recommends that diagnosis on ABS VI begin with a Diagnostic System Check. Start with a preliminary inspection of the master cylinder fluid reservoir to make sure the brake fluid level is correct. While you re in the area, check for contamination, leaks and wiring damage. Next, inspect the brake components of all four wheels to ensure that no drag exists and that the conventional braking system is operating properly. Also check for worn or damaged wheel bearings that may cause a wheel to wobble All antilock brake systems control wheel slip during braking by modulating hydraulic fluid pressure to the wheels. ABS VI uses a unique method of performing this task motors that vary the displacement of fluid chambers. The hydraulic modulator assembly mounts to the side of the master cylinder and is placed in line with the calipers and wheel cylinders. ABS VI is capable of both removing and applying braking force. It s a three-channel design in that it controls the front wheels separately and the rear wheels as a pair. When the Electronic Brake Control Module (EBCM) senses wheel slippage from one of the wheel speed sensors, it isolates that hydraulic circuit from the master cylinder, then increases the displacement by lowering a A scan tool must be used to release the tension on the motors before removing the hydraulic modulator, motor pack or master cylinder from a Delco VI system. 42 June 2001
The pistons must be rehomed with a scan tool before the hydraulic modulator or wheels can be bled on a Delco VI system. Any conventional bleeder can be used. and affect brake operation. Make sure the wheel speed sensors and sensor wiring are in good shape. This is a common source of illumination of the Brake warning lamp. The sensors should be solidly attached and the wiring should not show signs of rub-through or other damage. Now connect your scan tool to see if any DTCs are displayed. If multiple codes are present, select DTC History to determine which malfunction occurred most recently. Diagnose and repair this malfunction first. After recording the DTCs, clear them and see if any reappear. If a DTC does reappear, it s a hard fault and should be repaired first. Be aware that a test drive may be required to reset a code. You can use the snapshot mode of your scanner to capture data when the DTC does set. If normal driving doesn t set a code, you may need to perform an ABS stop on a surface such as gravel to recreate the conditions. Be careful during the test drive! Your scan tool has additional features that will help make diagnosing this system easier. For instance, you can monitor system data while manually controlling certain key components to perform the following specialized tests and procedures: Motor Test (Apply/Release). Each motor on ABS VI can be switched on for up to 5 seconds. During the test, current is controlled at 10 amps in Forward and 6 amps in Reverse. After operating the motors, current information will be stored and can be reviewed to determine if the motors are operating properly. There ll be a slight pedal drop and rise during the test. Here s how to perform it: With your scanner connected, select the motor you want to test. Next, pump the brake pedal to deplete vacuum from the power booster. Now depress and hold the pedal, then release the motor with the scan tool. The brake pedal should move smoothly toward the floor. Erratic pedal movement here may indicate an intermittent electrical These electric motors are part of a pack used to actuate the displacement pistons. The device on top of each outside motor is a brake. 44 June 2001
connection within the motor. With the brake pedal still depressed to the floor, now use your scanner to apply the same motor. The brake pedal should smoothly rise back to the top of its travel until the piston has reached the top of its bore (home). Repeat the test for the remaining motors. Solenoid Test (On/Off). The solenoids can be tested by first releasing the motor for the circuit you want to test, then turning on the solenoid. When you stomp on the brake pedal, it should be very high and firm. If the pedal goes nearly to the floor, the solenoid may be leaking or not fully closing, and/or the check ball may be leaking. While holding pressure on the pedal, turn the solenoid off with your scanner. The brake pedal should drop immediately. Hydraulic Control Test. This test is used to verify the brake system s ability to apply, as well as release, hold and reapply hydraulic pressure. With all wheels off the ground, select the channel to be tested, then firmly depress the brake pedal. It should remain high and firm. At this point, have an assistant attempt to rotate the wheel(s) under test. If it doesn t rotate, that channel is working properly. With the brake pedal still depressed, press the correct key on your scan tool to start the hydraulic test. The brake pedal should remain high and firm. With the pedal still depressed, have your assistant attempt to rotate the wheel(s). It should rotate with little or no resistance, indicating a proper release function. After an 18-second delay, brake pressure will be applied and a slight bump may be felt in the pedal. Now your assistant shouldn t be able to rotate the wheel(s). Motor Test. Each motor can be tested once the motor pack is removed from the hydraulic modulator. Gear Tension Relief Sequence. When the displacement pistons are in their normal braking (upper) position, each motor will be loaded to ensure that the pistons are held firmly at the top of their travel. This tension, however, can make motor pack separation difficult. To avoid injury or damage to the gears, the gear tension June 2001 45
relief sequence briefly reverses each motor and eliminates any problem. You should always perform the gear tension relief sequence prior to removing the hydraulic modulator or the motor pack from the vehicle. This includes the replacement of a master cylinder. Relay Test. The voltage available to the EBCM while turning the Electronic Brake Control Relay on and off is monitored during this test. When the relay is on, the voltage displayed on your scanner should be battery voltage. When the relay is turned off, the voltage should drop below 5 volts. Note that the voltage won t drop all the way to zero when the relay contacts are open due to a circuit in the EBCM. Voltage Load Test. The ABS VI system uses a lot of current when operating, so it should be tested under load. This test turns on several components to loadtest the vehicle s electrical system. It s useful in diagnosing low-voltage malfunctions or intermittent EBCM operation. Using the scan tool, you can monitor both the ignition supply to the ABS and battery voltage during the test. Rehome Function. The motor rehome function will return all of the hydraulic modulator pistons to their normal braking, or upper, position. This allows all fluid paths within the modulator to be open so the modulator can be properly bled. The motor rehome function must be used prior to bleeding the brake system or fluid will not pass through the modulator. Note that the rehome function cannot be performed if any current DTCs are stored. If codes are present, repairs must be made and the codes cleared. DTC Info There s some useful information in the DTC History function of the scan tool that will provide clues as to how recently and how often a DTC set. This information is available for the first five codes, as well as the last code stored. Here s what the History function can give you: DTC number. Number of occurrences. Number of drive cycles since the first occurrence. 46 June 2001
Solenoids such as this are used to isolate the front wheels during ABS braking. Be sure to open a bleeder screw at the wheel before pushing in a caliper piston for pad replacement, so dirty fluid doesn t flush back through them. Number of drive cycles since the last occurrence. The first five DTCs are stored in the order they occur. This order can then be used to determine if a previous malfunction is the root cause of a more recent one. For example, a DTC that set only once over a large number of drive cycles will be difficult to find. On the other hand, if the code set on the last drive cycle, it s probably current. Another example is a DTC with an occurrence count of 15 out of 30 cycles, with a first occurrence of 30 and a last occurrence of 15. From this we can infer that a repair was made 15 cycles ago, but the codes were not erased, since the fault did not reoccur. Other Tidbits Wheel speed sensor signal dropout can cause unnecessary ABS operation, as well as reduced braking performance on hightraction surfaces. If the Delco VI EBCM sees a zero frequency signal from a wheel speed sensor, it will release the brake to that wheel. The recording on page 42 is data grabbed during several stops on a dry road. Notice that the right front wheel speed sensor (RFWS) signal goes to zero on the last stop, indicating a problem. Always remember to open the bleeder screw at the wheel before pushing back the caliper pistons for lining replacement. Failure to do so can allow dirty fluid to enter the hydraulic modulator and cause big-time problems. One last point: Make sure to test-drive your scan tool on an ABS VI system before you need to rely on it for a difficult diagnosis. You may be surprised at how many helpful features you ll find. Remember, trying to learn the diagnostic capabilities of the system and your tool at the same time can lead to high stress levels. And who needs that! Visit www.motor.com to download a free copy of this article. Copies are also available by sending $3 for each copy to: Fulfillment Dept., MOTOR Magazine, 5600 Crooks Rd., Troy, MI 48098. 48 June 2001