ABS Repair From the Inside Out

Jan. 1, 2020
Working with an Anti-lock Braking System (ABS) in the body shop demands knowledge of the system basics.
Working with an Anti-lock Braking System (ABS) in the body shop demands knowledge of the system basics. 
Carefully inspect the hoses attached to the modulator unit. Lines to this Jeep Cherokee unit may deform if the unit moved due to impact forces.
Carefully inspect wheel sensor wires and harnesses. Ensure that all harness stabilizing brackets are intact and the harness remains free of abrasions during lock-to-lock steering operations.
This ABS wheel sensor counts teeth to detect the speed of a rotating ring attached to the axle or wheel hub. Carefully inspect the ring and sensor for collision damage. Check sensor gap. Look for dented teeth or a cracked ring, which can play havoc with proper ABS operation.
This Ford 4WD sports a rear differential sensor on top of the housing to detect rear-wheel speed.
Typical of RWAL, this Dodge van contains modulation valves nested between the combination valve and master cylinder. Remember, RWAL only helps prevent rear-wheel lockup and wheel hop. RWAL, while assisting steering during hard braking, is not a four-wheel brake control system.ABS is a control system that is poised and ready to help maintain vehicle traction during braking. During operation, ABS senses the wheel speeds of the vehicle. If skidding occurs during braking, the ABS dictates corrective action by making changes in the brake system’s hydraulic pressure.Two styles of ABS are installed beneath the body of most modern vehicles. The first type uses very high hydraulic pressure to assist the driver in obtaining sure, safe stopping. The second is an add-on ABS pressure control, which tops off a traditional vacuum-operated power brake system. But this is where the simplicity ends. ABS is not merely an effortless system that helps a motorist stop his or her vehicle. It’s a series of intertwined steps, each affecting the next, that together provide the ability to control the stopping capability of a vehicle. First and foremost, ABS helps keep motorists safe and in control of their cars. This purpose, in and of itself, makes the importance of ABS maintenance and repair rather obvious. But to complete a truly thorough repair job, you must first have a truly in-depth understanding of the ins and outs of today’s ABS. Integral ABS
Usually, high-pressure ABS—typically called the “integral” system—gets boost power from a pump, not unlike older hydraulically boosted power brake systems. Now used on vehicles for nearly a decade, this ABS contains an integrally built pump assembly, hydraulic pump pressure switch, pressure accumulator, fluid level sensor and solenoid valve block assembly.
Typically, when you turn the vehicle’s ignition key with these systems, a power relay wakes the ABS system’s electronic brake control module. The pump responds to pressure switching and builds high pressure for power-assisted brake functions. Remember, this high-pressure system is not a vacuum-boosted power brake system. In these systems, as the high-pressure unit, the ABS pump maintains the high pressures sent to a control valve. When the driver applies the brakes, brake pedal linkage activates the main control valve. The control valve thus supplies hydraulic pressure to the brakes.
The pump also provides pressure to an accumulator. The accumulator is a nitrogen-charged pressure storage unit. System pressure works on an accumulator’s internal diaphragm. The diaphragm moves away from the hydraulic pressure and compresses nitrogen gas that is trapped behind it. Once it’s charged, if the pump pressure fails, a warning light is illuminated. The pressurized nitrogen then supplies emergency pressure available for several brake pedal applications.There are several variations of this ABS system. The first generation design uses a separate pressure modulator assembly and brake master cylinder. The second generation incorporates a master cylinder and hydraulic modulator assembly into one unit.
The ABS computer senses wheel speeds during braking and lessens the braking pressure to a skidding wheel to maintain a vehicle’s stability. This control comes via two-, three- or four-sensor circuit varieties. Depending on its design, the modulator assembly consists either of two, three, or four rapidly switching solenoid valves. Some have two accumulator chambers (one for each hydraulic brake circuit) and a fluid return pump. The modulator valve maintains or reduces the hydraulic pressure to the brake calipers independent of the pressure in the tandem master cylinder. Remember, the modulator valve does not increase the pressure above that transmitted to the master cylinder, nor can it apply the brakes by itself.
Note: For safety reasons, do not attempt repairs to a modulator valve. Only relays may be replaced. If a problem occurs in the modulator, replace the unit. Also, never loosen any screws on the modulator assembly. Once loosened, it’s virtually impossible to properly re-tighten.Add-On ABS
Another response to brake slide may involve using an add-on rotary motor ABS control unit. Coined the “ABS-IV” by General Motors Corp. (GM) at its introduction, all necessary pressure adjustment occurs in an add-on pressure modulation unit. Added to a normal, vacuum-assisted power brake, the modulator contains all of the hardware for pressure cutoff and modulation. In the GM unit, housed within the upper section of the modulator are electromagnetic solenoids and check balls for circuit isolation. Below these are three rotary electric motor armatures, three motor-driven jack screws and six motor rotation control devices.
The GM modulator uses a cutoff solenoid for each front disc brake circuit. This solenoid does the job of cutting off the pressure to the brakes on the front wheels when necessary. In the modulator, within the isolated circuit, an accumulator-like chamber and jackscrew stand ready to spring into action. Only the front wheels have individual motors and jackscrew chambers. Although a similar circuit design is present for rear wheel control, and the rear sensing system has two sensors (one for each wheel), only one motor and jackscrew chamber controls both rear wheels. GM also opts for only check balls to perform rear circuit isolation. No solenoid is used in the rear wheel circuit.When the ABS computer senses wheel lock, the computer cuts off the brake supply pressure. Once cutoff occurs, pressure modulation begins. One of the three jackscrews in the modulator unit serves to lower the isolated circuit pressure. The wheel’s service brake releases slightly and the sliding wheel begins to regain traction.This cycling happens until the ABS is no longer needed or when the driver releases the brake pedal.Two-Wheel ABS
Rear Wheel Anti-Lock (RWAL) is most commonly used in trucks to reduce occurrences of rear wheel lockup and wheel hop during severe braking conditions. This traditionally occurs during hard stops on slippery roads with a small load on board.
Receiving master cylinder pressure through the brake system combination valve, the control assembly consists of a dump valve and an isolation valve. The RWAL computer braking pressure regulation occurs in this control valve assembly. The dump valve releases pressure into an accumulator and an isolation valve regulates rear brake pressure. The computer allows the control valves to maintain the applied hydraulic pressure, pulse-regulate the pressure, or dump the hydraulic pressure completely. RWAL combines predictable steering with the maximum braking traction available.
Sensor Circuits
Note that various locations exist for wheel sensors. Some vehicles have a sensor at each of the front wheels and only a common driveshaft or differential sensor for the rear wheels. For example, a solo rear sensor normally resides at the propeller shaft’s differential yoke, as on some early Lincoln, high-pressure Teves ABS vehicles. Others may have a single rear sensor counting revolutions at the rear differential ring gear, as in the Ford Expedition.
Common in the operation of all systems, however, is that each sensor electrically counts the teeth of circular rings that rotate beneath them. Driven proportionally to wheel speed, the teeth disturb the magnetic inductance of the sensor as they pass beneath the sensor pickup. In a wheel assembly, you’ll find the sensor rings are pressed to the rotating wheel and disc rotor assembly, the driveshaft/differential, or housed inside the spindle/bearing assembly of certain vehicles.In operation, if the sensor signal detects that the wheel speed suddenly drops below the expected deceleration program built into the computer, pressures cycle rapidly. The cycling modifies hydraulic braking pressure as needed to maintain the tire rolling and limit wheel lockup.On some vehicles, as the ABS begins to modify braking application, the driver feels a pulsing in the brake pedal. On others you’d get a much less dramatic pedal reaction, and you may hardly realize the ABS is at work.
In today’s luxury vehicles, you may also find an added feature called the Traction Control System (TCS). In some TCS-equipped vehicles, wheel spin by a wheel under power is detected by the brake sensor, and the brake applies pressure to counter the condition. These TCS-equipped vehicles thus provide a positive traction ability.
Along with the standard checks, there are some service precautions to observe when dealing with ABS. These include the following:
If you are using an arc welder, unplug the computer system.During paint work, the computer should only be subjected to a maximum temperature of 95
About the Author

TeeJay Beam

TeeJay Beam has experience as a tool and die machinist, a body shop technician and a mechanical technican. He served as a teacher of automobile technology for a number of years and continues to write for the industry in various mediums, including ASE study guides.

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