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Bad Sensors ABS Systems

Delco VI ABS has been in production for almost a decade now, which is amazing considering how fast ABS technology keeps changing. Introduced in 1991 as an option on Saturn, Buick Skylar, Oldsmobile Cutlass Calais and Pontiac Grand Am models, it soon became standard equipment on most General Motors front-wheel drive passenger cars. But its days are numbered.

Last year, Delphi’s new DBC 7 system was introduced as the replacement for the aging Delco VI system on 1999 Buick Regal and Century and Chevrolet Tracker. For model year 2000, DBC 7 replaced Delco VI on Chevy Impala, Monte Carlo, Malibu, Cavalier and Silhouette. Within another year or two, Delco VI will be history as far as new GM cars are concerned. Even so, we’ll be working on Delco VI ABS for many years to come.

Compared to some other ABS systems, Delco VI is technician friendly. It’s an "add-on" or "nonintegral" type of anti-lock brake system with a conventional master brake cylinder and power booster. Individual system components can be replaced separately and cost less than many other systems.

Delco VI ABS has four wheel speed sensors. The front ones are mounted on the steering knuckle assembly. The sensors are nonadjustable so there’s no air gap adjustment to make if a sensor has to be replaced. The sensor rings for the front wheel speed sensors are on the outside of the outboard constant velocity joints — which means the CV joint has to be replaced if the sensor ring has been damaged. Even a slight nick on the sensor ring can sometimes be enough to produce a bad wheel speed signal.

The sensors are magnetic and tend to attract metallic debris that can come from the semimetallic brake linings and rotors. Debris on the sensor tip can produce erratic wheel speed readings and set a fault code. The cure? Remove and clean the wheel speed sensors. The wheel speed sensors are plastic rather than stainless steel, so handle carefully. Never pound on one of these sensors or attempt to force it into place. If the sensor doesn’t slide right into place, check for obstructions. The plastic sensors do not need to be coated with a corrosion-inhibiting lubricant prior to installation.

The wheel sensors are connected to the main wiring harness by a jumper harness made of highly flexible, twisted-pair wiring. The wires are twisted to reduce radio interference. Their flexibility also allows to handle the motions of the suspension. If the sensor wires are broken, badly corroded (a common problem), loose or damaged, replace them rather than trying to repair them. Soldering, splicing or taping can come loose as the wires flex with the suspension. And never, ever use a butt connector to splice low voltage circuit wires together because the repair won’t last. Resistance will buildup, causing a loss of signal strength.

The rear wheel speed sensors and sensor rings are located inside the rear wheel bearing and hub assemblies, and can only be replaced as a complete assembly. Rear sensors are also nonadjustable.

The brain of the Delco VI ABS system is the electronic controller. You’ll find it located on the firewall in the engine compartment. Inside the controller is a quad driver module that controls the operation of the warning lights and enable relay. It also helps protect the controller against damage in the event of accidental probing.

The controller monitors wheel speed through the four wheel speed sensors during braking, and modulates braking action as needed to prevent wheel lockup. A switch on the brake pedal signals the controller when the brakes are being applied.

Power to the ABS system is provided through the "ABS enable relay" when the ignition is turned on. The relay is located under the hood in the relay cluster. The enable relay switches power to the DC motors, isolation solenoids and electromagnetic brakes.

Two warning lights are used: an amber ABS light that signals the driver when the ABS self-diagnostics has detected a fault that may also cause the controller to disable the ABS system; and a red BRAKE light that comes on when the parking brake is applied or if the hydraulic pressure has been lost in either the primary or secondary side of the brake system.

Attached to the master cylinder is the Delco VI ABS hydraulic modulator and motor pack assembly. The modulator is held by two banjo bolts at the master cylinder’s two upper outlet ports and two transfer tubes at the two lower outlet ports.

An important service point to note here is that the two lower transfer tubes and O-rings must be replaced if the master cylinder and modulator are disconnected from one another. This is necessary to prevent leaks that could cause the brakes to fail.

The modulator assembly consists of fluid chambers for all four brakes, two isolation solenoid valves (one for each front brake), four check balls, a motor pack containing three bidirectional, direct-current motors with electromagnetic brakes and/or expansion spring brakes, three ball-screw assemblies, four pistons, a gear drive set and gear cover. There’s no isolation solenoid valve for the rear brakes because the rear brakes only handle about 20 percent of the vehicle’s braking effort.

The modulator motor pack, isolation solenoid valves, gear cover and individual gears are all serviceable parts and can be replaced separately.

The most important difference between Delco VI and other ABS systems is the method by which brake pressure is modulated during anti-lock braking. On most other four-wheel ABS systems, pressure is held in a given brake circuit by closing the isolation solenoid valve. An outlet solenoid valve is then opened to reduce pressure in the circuit to prevent wheel lockup. Pressure is then reapplied by opening the isolation solenoid valve to the affected wheel circuit. The hold, release and reapply cycle is then repeated as long as needed to prevent wheel lockup.

With Delco VI, brake pressure is modulated during anti-lock braking by moving a small piston up and down inside a fluid chamber. To understand how this works, let’s take a closer look at the modulator and motor pack assembly.

The modulator is divided into three circuits. The right and left front brakes are controlled individually, while the rear brakes are controlled as a pair. For each of the front brake circuits, there’s an isolation solenoid valve, a check ball, a ball screw and piston, DC motor, gear drive and electromagnetic motor brake (EMB). The EMBs are disc-style brakes located on top of the front motor assemblies. When no voltage is applied to the EMB, a plate and spring pushes down against the pads keeping the motor from turning. When voltage is sent to the EMB, the electromagnet pulls up on the plate disengaging the brake. The motor is now free to turn.

During normal braking, fluid pressure from the master cylinder passes through the modulator and on to all four brakes. The ball screw and piston in each circuit is at its highest or "home" position. This holds the check ball open so fluid can go through the upper passageways to the brakes. The ball screws and pistons for the front brake circuits are locked in this position by the deenergized electromagnetic brakes that prevents the DC motors from turning. The two solenoid valves for the front brakes are also open when deenergized, which allows fluid to go through their passageways to the front brakes in the event of a failure of either the isolation solenoid or ball-screw assembly.

Each rear brake circuit also has its own piston chamber, but the two pistons share a common ball screw and motor. During normal braking, the ball screw for the rear circuit is also at its highest or "home" position. This holds each check ball for the rear brakes open so fluid can pass through to each rear brake. An expansion spring brake (ESB) prevents the rear motor from turning during normal braking. The ESB will apply braking action to the rear motor assembly when current to the motor is removed. The ESB has no direct electrical connection to the controller. It is a mechanical brake that works similar to a window-crank mechanism or overturning clutch.

The system has four wheel speed sensors, remember, so it uses what’s called the "select low" principle of operation. When the rear wheels are slowing at different rates, the ABS controller looks at the inputs from both rear wheel speed sensors and determines which wheel is slowing or decelerating the fastest. The one with the lowest speed sensor input is the one that’s most apt to lockup, which initiates anti-lock braking.

When the ABS controller detects that a front or rear wheel is about to lockup, it goes into action. If a front brake is involved, the first step is to energize the right or left isolation solenoid valve. This blocks the flow of fluid pressure through the isolation solenoid passage to the calipers. At the same time, the electromagnetic motor brake is energized to free up the motor and ball screw so they can turn. The motor will then draw the piston down by turning the ball screw, allowing the check ball to seat and isolate the brake circuit. This prevents any additional brake pressure from reaching the caliper.

As the motor continues to turn backwards, the ball screw and piston move lower. This allows the volume area on top of the piston chamber to receive the corresponding drop in fluid pressure away from the caliper to prevent wheel lockup.

Pressure is then held or "maintained" when the force exerted by the motor against the ball screw and piston equals that in the brake circuit itself. When this point is reached, the piston stops moving downward and pressure is held steady. The controller provides current to the motor to accomplish this function.

Pressure can then be reapplied as needed by running the piston back up. To do this, the controller increases current to the motor. This turns the ball screw the opposite direction and reverses the direction of piston travel. This is the pressure increase phase. As the piston moves back up, it pushes fluid back into the caliper line and increases pressure at the brake. The piston can be moved upward and downward in the modulator bore and held in position at any point by applying current to the motor assembly.

When anti-lock braking is no longer needed, the motor is commanded to return the ball screw to its uppermost or "home" position. The EMB is deenergized and holds the motor, keeping it in "home" position. The piston then returns to the top of the chamber and unseats the check ball, reopening that passageway for the brake fluid. The isolation solenoid valve is also deenergized and opens the second passageway to the calipers.

Now let’s look at what happens in the rear brake circuit during anti-lock braking. When anti-lock braking is initiated, the brake system is no longer split diagonally. The rear wheels are controlled together. The controller commands the rear motor to draw down the ball screw. The rear ball screw now turns, allowing both rear pistons to back down the bore of the modulator. This seats the check balls for the rear brake circuits and isolates the lines from the master cylinder.

At this point, the apply and release phases are much the same as that for the front brakes. The motor allows the pistons to back off and reduce pressure at the rear brakes to prevent lockup. When pressure needs to be reapplied, the motor reverses the ball screw and forces the pistons back up, sending more pressure to the rear brakes.

The up and down cycling of the pistons in the Delco VI system is a somewhat "softer" approach to anti-lock braking than a solenoid-actuated ABS system, so the driver feels less pedal feedback. But it’s important to note that this system cannot increase brake pressure on its own above that which the driver’s foot provides through the master cylinder. Remember, there’s no high pressure pump or accumulator in the Delco VI system.

The maximum cycling frequency of this system is about half the rate of some ABS systems — but is still sufficient to prevent wheel lockup under most circumstances.

Keep in mind that basic brake system service on vehicles equipped with the Delco VI system is the same as a vehicle that’s not equipped with ABS. Most brake problems are not ABS related. So don’t blame the anti-lock system if the brakes are noisy or pulling.

A slipping motor pack, though, can cause a soft pedal in Delco VI systems. If the motor pack doesn’t hold when brake pressure is applied, the modulator piston can be pushed down inside its chamber. One way to check for this condition is to remove the bottom cover on the motor pack. Have a helper push on the brake pedal. If you feel the gears move, the unit needs to be replaced. Also if fluid is leaking from the unit, it needs replaced.

The Delco VI system has no flash codes, so you’ll need a scan tool to access fault codes if the ABS warning light is on. You’ll also need a scan tool to clear codes from memory (disconnecting the battery or ABS module fuse won’t do it).

The self-diagnostic capabilities of the ABS-VI system include five separate diagnostic modes. Checks include the controller itself, isolation solenoids, DC motors, wheel speed sensors, enable relay, brake switch, EMBs and wiring harness.

If a fault is detected, the controller will either flash or illuminate the ABS warning light. A flashing light indicates a problem that does not affect the operation of the system and ABS remains active. But a continuous ABS warning light indicates a more serious problem that automatically disables the ABS system. If the red BRAKE warning light is also on, it signals a hydraulic problem in the brake system.

Scan tool capabilities differ, but with the right tool, you can access the following:

• Data inputs — Display data from individual wheel speed sensors, the vehicle speed sensor, battery voltage, the ABS warning light, brake switch, solenoid status, amp readings for each of the three motors, electromagnetic brake status, enable relay status and brake telltale status;

• Code history — This mode provides a brief history of up to five trouble codes as well as the last one set. For each of the first five codes that are logged, which are stored in the order in which they occurred, the controller keeps track of the number of failure occurrences and the number of drive cycles since the failure first and last occurred (a drive cycle occurs when the key is turned on and off and the vehicle is driven faster than 10 mph);

• ABS snapshot mode — Captures system data to help detect intermittent ABS problems that occur while driving; and

• ABS function tests — Requires bidirectional scan tool capability. This mode includes both automatic and manual tests of individual components in the Delco VI system. Test options include manually controlling all motors, the two isolation solenoids, the enable relay and the electromagnetic brakes, light tests, a relay test, a battery voltage test, a system identification check, a "gear-tension relief" procedure, plus an automatic diagnostic routine that tests the mechanical operation of the modulator and motor pack.

The gear-tension relief procedure must be used when replacing the motor pack assembly on the modulator to prevent damage or injury that might otherwise occur if you attempt to remove the gears without relieving tension.

When checking fault codes (see chart), read current trouble codes first, then history codes. Refer to the appropriate diagnostic flow chart in the service manual for instructions on how to proceed. The usual procedure is to start with the lowest number code first if more than one code is present. If no codes or obvious mechanical problems are found, run the automated modular test using the scan tool to see if the fault will set a code. If the problem is intermittent, test drive the car and use the "snapshot" diagnostic mode to capture and analyze any data that might help reveal the fault.

Before the brakes can be bled, the front and rear displacement pistons in the modulator must be in the top most or "home" position with their check balls unseated. If the pistons are not in this position, they will not allow the brakes to be bled.

The two ways to home the pistons are with a scan tool (which is the preferred technique) or with a special manual procedure. If using a scan tool, select the "ABS Test" mode, then the "manual control" or "motor rehome" option. The motors in the modulator can now be commanded to drive the pistons to their home positions. The motors will not return to their home positions, however, if the ABS warning light is on or if there are any fault codes present in the system. All faults must first be repaired and all codes cleared before the pistons can be homed.

Once the pistons have been homed, bleed the modulator starting with the rear bleeder screw first. Attach a length of clear tubing to the rear bleeder screw, and submerge the other end in a clear container partially filled with brake fluid. Open the bleeder screw one more turn and apply pedal pressure. Close the bleeder screw when the pedal bottoms out. Repeat until no bubbles are in the fluid. Then repeat this procedure for the front bleeder screw on the modulator. Once the modulator has been bled, the brake lines can be bled manually or with pressure equipment in the following sequence: right rear, left rear, right front, left front.

To bleed the system without a scan tool, start the engine and allow it to run for at least 10 seconds, making sure the ABS warning light is off. Do not touch the brake pedal during this time. This will allow the ABS system to initialize itself. Make sure the ABS warning light goes out after three seconds (if it remains on, further diagnosis with a scan tool will be necessary before the system can be bled.) Turn the ignition off and repeat this procedure a second time. Turn the ignition off and bleed the modulator and brake lines using the same procedure described above.

Use only DOT 3 brake fluid. Do not use DOT 5 silicone brake fluid in this or any other ABS system.

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