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Home Brakes Diagnosis of Faulty ABS Systems

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ABS diagnosis typically starts with accessing the associated DTCs and then applying the appropriate flowchart. There will be times when this type of approach will not be possible, or the occasions when it doesn’t yield positive results. When this is the case, it is always helpful to have an understanding of how the systems and components work so you can develop generic testing methods.

Before discussing these generic methods it’s important to understand that you should generally follow the DTC-to-flowchart method mentioned. This is especially true if you are unfamiliar with the system being diagnosed. This method will typically lead to finding the cause of the problem, while at the same time keep you out of trouble. The generic methods we are going to discuss can be used in conjunction with the published methods or as stand-alone techniques.

Figure 1

Having access to the system’s wiring diagram is essential when performing any type of ABS diagnosis. The wiring diagram will allow you to break out the specific circuit being tested. Some systems provide a specification chart that gives values for the various electrical components. One such chart is shown in Figure 1. Armed with this chart and the wiring diagram a technician can pretty much diagnose every component without the use of a flowchart.

Wheel-Speed Sensors
If you are into ABS diagnosis, you might say “if you have seen one speed sensor, then you have seen them all.” Right? Wrong. The speed sensor that we have all grown up with since the introduction of ABS has evolved.

Since the introduction of ABS in the United States, all systems have used what is known as the variable-reluctance type speed sensor. A typical variable-reluctance speed sensor is shown in Figure 2. It is a simple part consisting of a coil of fine wire wrapped around a magnetic core. Each end of the coil terminates at the sensor connector. Wherever you find a speed sensor, you will find a tone ring.

A speed sensor can be thought of as a mini-AC generator. The speed sensor is mounted so that the magnetic core is a small distance from the teeth of the tone ring. This distance is known as the air gap (See Figure 3). The air gap is fixed on most WSS, but can be adjusted on some older systems.

The speed sensor produces a small AC voltage that alternates between positive and negative. This output is represented with what is known as a sine wave. The AC voltage starts from a point of zero volts and proceeds to its maximum output as the tone ring tooth passes the magnetic core. The signal decreases back to zero and then continues to a negative voltage as the trailing edge of the tooth leaves the magnetic core. As the gap or “no tooth area” passes the magnetic core, the voltage goes back to zero. This represents one cycle.

The strength of the signal is variable. The high and low points of the sine wave will increase and decrease as the magnitude of the AC voltage increases. The number of cycles that an AC voltage completes in one second is called the frequency of the signal and is represented in Hertz (HZ).

The output of the speed sensor can be affected by the following factors:

  • Air gap;

  • Size and strength of magnetic core;

  • Number of windings in coil;

  • Speed of the tone ring;

  • Winding efficiency; and

  • Total circuit resistance.

The only two factors that can have outside influence are the air gap and speed of the tone ring. Variable-reluctance speed sensors are also called “passive speed sensors.” The EBCM internally converts the frequency and amplitude of the analog signal into digital information that the EBCM can use to perform ABS braking.

Passive speed sensors have a major disadvantage. The signal output at low speeds becomes too low quality to provide an accurate signal to the ECU. It is for this reason that the “active speed sensor” was developed. Active speed sensors come in two types, magneto-resistive type and Hall Effect Sensor.

Unlike the variable-reluctance type sensor, the magneto-resistive sensors and Hall Effect Sensors cannot generate their own power. Instead, they rely on an external power supply in the form of 12VDC from the ECU.

Magneto-Resistive Bridge Sensor
Inside the sensor is a small integrated circuit containing a magneto-resistive bridge (See Figure 4). The magneto-resistive bridge changes resistance due to the relationship of the tone wheel and magnetic field surrounding the sensor. The sensor’s electronic circuitry modifies and amplifies the varying resistance into a DC square wave signal. As the tone wheel rotates and shifts the magnetic field, the sensor changes the voltage and current levels on the signal circuit to the ECU.

Since the magneto-resistive sensor outputs a DC signal, it provides two voltages to the ECU depending on whether a tooth is passing or not. The voltage to the ECU will be 0.90 volts when a no-tooth condition exists and 1.65 volts when a tooth is aligned with the sensor. A current flow through the signal circuit is maintained for diagnostic reasons. The current will be at 7 milliamp at the 0.90-volt output and 12 milliamp at the 1.65-volt level.

The ECU uses the DC signal to determine the speed of the wheel and since the output of the sensor is independent of the wheel’s speed, accurate readings are provided all the way to zero wheel speed. This improvement in accuracy accounts for increased ABS performance.

Hall Effect Sensor
The reference voltage is supplied to an internal semiconductor in the active wheel-speed sensor called a Hall Effect Sensor (See Figure 5). The Hall Effect Sensor creates a magnetic field around the sensor. The tone ring interrupts the magnetic field as the wheel spins. When a tooth passes near the Hall Effect Sensor, the signal voltage output toggles from low to high, creating a square wave D/C output.

The frequency of the D/C square wave signal output increases with wheel speed, but does not increase in amplitude. The EBCM uses the frequency to interpret wheel speed for ABS operation.

The advantage of a digital active wheel-speed sensor is that the signal input is much more accurate. Because of the increased signal accuracy, the ABS system can react faster to wheel slip. Also, the speed at which ABS can be activated can occur at slower vehicle speeds.

Passive Wheel-Speed Sensor Diagnosis
Wheel-speed sensors are still the most common of all ABS failures. These failures can be classified into two types – static and dynamic. Static failures will generally include sensor coil failures and wiring problems. Dynamic failures are usually associated with signal quality problems.

Visual Check
The visual inspection is an important step and should not be overlooked when diagnosing any ABS component. When performing a visual inspection for speed sensors, check the sensor mount, sensor tip (if exposed), pigtail and connector (See Figure 6). If the tone ring is exposed, perform a tooth-by-tooth check looking for damaged teeth or a cracked tone ring (See Figure 7).

The sensor-to-harness connector should be checked for problems. These problems can be in the form of corrosion, water intrusion or pin problems. The ECU connector should also be checked as part of the visual check, paying close attention for any pins that have been pushed out.

Although not actually visual, I am going to include a wheel bearing check in this section. Any speed sensor mounted at the wheel relies on the tightness of the wheel bearing to maintain the air gap. A loose wheel bearing can allow the air gap to fluctuate, which can lead to either a DTC or false activation.

Air Gap Check
While not usually possible due to the design of most speed sensors, the air gap should be checked where possible (See Figure 8). Only a few vehicles provide specifications for air gap with the range usually being from .020″ to .070″. Again, if in question, compare side to side and, if significantly different, check to see if the cause can be determined. When checking air gap, you should always use a brass feeler gauge to prevent the possibility of altering the magnetic properties of the tone ring or sensor.

Wiggle Test (Static & Dynamic)
The wiggle test is just what it says it is. The wiring and connectors going to the sensor are wiggled in an attempt to duplicate the problem. The wiggle test is usually performed in conjunction with intermittent problems and can be used during both static and dynamic testing procedures.

Salt-Water Test
Moisture intrusion is a common problem with speed sensors. This test involves spraying the speed sensor, wiring and connector with a weak salt-water solution to see if it has an effect on the test results. The typical mixture uses a 5% salt-water solution. Mix it and use a spray bottle to douse the components being tested. The salt-water test can be used in both static and dynamic testing.

Resistance measurement (Static)
This test is one of the core passive speed sensor tests. It checks the integrity of the sensor’s coil and pigtail, if equipped. The typical acceptable range for resistance of a sensor is between 500 and 3,000 ohms. Some sensors provide a resistance chart based on temperature. If you do not know what the specification is, compare it to the other side. It should be noted that when given a range for acceptable values, it is sometimes acceptable to be slightly over or under the min/max numbers. Many a technician has changed a sensor because it was 50 ohms out of spec only to find out it didn’t solve the problem.

The passive sensor’s resistance isn’t the only thing to check. Once the sensor itself is checked, the integrity of the wiring harness to the ECU should be checked. Connect the sensor to the wiring harness, locate the appropriate terminals at the ECU connector and check the resistance (See Figure 9). If out of spec, or the reading indicates an open condition, the cause will have to be identified.

Short to Ground (Static)
Passive speed sensors should be isolated from the vehicle’s chassis. If a short to ground exists, it will most likely result in a DTC. To check a passive sensor for a short to ground condition, check the resistance between each sensor terminal and a good ground. Most sensor tests will give a cut-off for good/bad. Ideally you would expect an infinite reading. If not sure, compare the readings to the other terminal on the same sensor and to the sensor on the opposite side.

The wiring harness from the sensor to the ECU should also be checked for a short to ground in the same manner as described above.

Sensor Output (Dynamic)
As mentioned before, passive speed sensors are mini-AC generators and are tested for acceptable output. Most flowcharts suggest spinning the wheel by hand while monitoring the AC output of the sensor. They usually provide a cutoff point ranging from 100mV to 500mV. This method can be difficult to perform and may not provide the best of results.

If your shop is equipped with an on-the-car lathe, you can use it to spin the wheel being tested at a constant rate. The consistency of this will allow not only a better measure of the output, but will also allow you to check for tone ring damage if you have a scope available. Spinning the wheel by hand makes it difficult, if not impossible, to spot a glitch in the signal caused by tone ring damage.

The dynamic measure of the sensor can take place directly off the sensor connector or back at the ECU connector. If the measurement is taken at the ECU, it covers both the sensor and wiring harness. The wiggle and salt-water test can be incorporated into this test if necessary.

The sensor’s output as seen by the ECU can be monitored with a scan tool if the system allows live readings. This is helpful when trying to see if all sensors are providing equal readings. Depending on the refresh rate, this technique will not always spot glitches or intermittent problems, but it is still a useful technique.

In some cases, it may be necessary to monitor more than one sensor during a test drive. If you have a multi-channel scope, the best method to accomplish this is by back probing at the ECU connector. This way you are seeing the signal the ECU sees.

Active Speed Sensor Diagnosis
Active speed sensors cannot be diagnosed in the same way as passive speed sensors. The only similarities they share are the visual, wiggle and salt water tests. The resistance of the sensor cannot be measured directly due to the additional circuitry. If you try, you will generally get something near infinite in value. As a result, active speed sensors must be checked dynamically.

So the question becomes how to identify if you are working on a passive or active speed sensor? The answer to this question is not straightforward. Active speed sensors were first used in the late 1990’s, so you shouldn’t have to worry about anything earlier. One method is to look up the vehicle in Mitchell 1 or ALLDATA and check one of the sensor flowcharts. Another method would be to check it for resistance and if you get a near infinite reading, check it for a power wire. This can be done by back probing both wires at the speed sensor connector and then with the key on, check each wire for DC voltage (See Figure 10). If it is an active sensor, the power wire will provide a reading in excess of 10VDC.

If you have identified the power wire, this will make the remaining wire your signal wire. There are two methods to use to diagnose active speed sensors. The first involves checking for the DC output of the sensor on the signal wire. This is done by taking the other test lead to a known good ground and rotating the wheel. If the sensor is functioning properly, you will get a clean DC square wave, as shown in Figure 11.

The other method is to check the current output of the sensor. To perform a current check, the ammeter must be connected in series in the signal circuit. The current on the sensor signal circuit will toggle from high to low as the wheel is spun very slowly, as seen in Figure 12. It is important to be sure the digital multimeter has an intact internal ammeter fuse to avoid DTCs during the diagnostic procedure. If a wheel speed sensor DTC is set during a functional wheel speed test, the wheel-speed sensor will no longer output a signal.

Pump/Motor Diagnosis
While no longer used, there are still integral ABS systems on the road. Integral ABS was the most common type of system in the late 1980’s to early 1990’s. Integral ABS systems incorporate the master cylinder, power assist and ABS into one unit.

These systems use high-pressure brake fluid to provide power assist. Using a high-pressure pump/motor, which provides pressurized fluid to a high-pressure accumulator, does this. Depressing the brake pedal releases some of this high-pressure fluid into what is typically called a power piston that provides the power assist.

These integral ABS systems can suffer failures relating to this sub-system. These failures will usually involve one of the following scenarios:

  1. Pump runs too long or continuously;

  2. Pump doesn’t run at all; or

  3. Pump runs too often.

The published diagnosis for these types of problems will generally involve installing a pressure gauge between the accumulator and housing to monitor the pressures during different stages of operation. Most shops are not equipped with this tool so to get around this there are some generic tests that can be done.

When the pump runs too long or continuously, it is generally accompanied by a hard pedal and both warning lights on. The causes of this will usually be a bad pump, a restriction in the plumbing to the pump or a defective power piston. To diagnose this problem, follow these steps:

  1. Check system for external leaks. No leaks, go to step 2.

  2. With key off, remove feed hose from pump and check for free flow of fluid. If there is free flow, re-install hose and go to step 3. Restricted flow means you must identify point of restriction.

  3. Remove pressure hose from pump at far end, insert hose into container and then turn key on. Properly functioning pump should provide strong flow of fluid. If fluid flow is good, go to step 4. If not, replace pump.

  4. Reconnect pressure hose with key on and observe fluid in reservoir. If fluid in reservoir is noticeably flowing, the power piston is bypassing and the booster/master assembly will have to be replaced.

    If the pump doesn’t run at all, it could either be a power supply problem, a bad motor or a pressure switch. This problem will be accompanied by a hard pedal and both lights on. Use the following steps to determine the cause:

  5. Ignition off, depressurize the accumulator by pumping pedal until it is hard. Disconnect pump motor connector and check for battery voltage with the key in the run position. If battery voltage is found, the motor is bad. If no voltage is found, go to step 2.

  6. The pump/motor is provided power through a relay and pressure switch. Typically the pressure switch completes the ground circuit to the relay, which energizes the pump/motor. The pressure switch should be closed with the accumulator discharged and go to an open condition when charged. The first thing to check for is continuity across the pressure switch. With the accumulator discharged, if there is no continuity, the pressure switch is defective. If there is continuity, go to step 3.

  7. If there is continuity across the pressure switch, verify that the pressure switch is providing the proper feed. Most pressure switches complete the ground to the pump/motor relay so verify there is a good ground reaching the pressure switch. If there is a good ground, the problem is most likely a relay problem. This is covered as a stand-alone item later in this article.

If the pump runs after only a couple of brake applications, the cause is most likely a bad accumulator. The accumulator is a storage vessel for high-pressure brake fluid. This fluid is used for power assist and during an ABS stop. Use the following steps to determine if the accumulator has failed:

  1. With the key off pump pedal until it is hard to depressurize accumulator. With a properly functioning accumulator this should take from 10 to 50 applications.

  2. Turn key on and time how long pump takes to turn off. A failed accumulator will cause the pump to run longer than normal. Usually the pump should not take any longer than 45 seconds to charge the accumulator. Once the pump has stopped, go to step 3.

  3. With the key on and pump off, apply and release the brake pedal as in a normal stop counting the times until the pump/motor kicks back on. A failed accumulator will cause the pump/motor to kick back on anywhere from one to three applications.

Relay Diagnosis
Relays are used to provide power to components such as the pump/motor and ECU. A relay consists of an electromagnet that is used to open or close a pair of contact points (See Figure 13). In ABS systems, they are generally open until energized. Power is supplied through a fused circuit. Some relays are called “enable relays” because they are energized when the ignition is turned on and provide the operating power for the system. Other relays are used to provide the power for the pump/motor to function.

Pump/motor relays on integral ABS systems are external to the unit while some non-integral ABS systems incorporate the relay into the ECU, as shown in Figure 13. This arrangement increases repair costs because a failed $20 dollar relay will require the entire ECU or EHCU to be replaced. To diagnose an external relay, use the steps below (Use Figure 14 for reference):

  1. Perform a quick check by feeling the relay when the ignition is turned to the run position. If the relay is felt to click then the coil isworking. If this is the case go to step 5. If no clicking is felt, go to step 2.

  2. Typically the relay’s hot side will be energized when the ignition is turned on. In the example schematic shown this is the brown wire at terminal 2 of the relay. With the ignition on check for 12VDC at terminal 2 of the relay. If 12VDC is present then go to next step, if no voltage is found then check the fuse and wiring to the relay.

  3. Verify the ground side of the relay is receiving a good ground. On pump/motor relays the ground is typically supplied by the pressure switch. Check the resistance between the gray/red wire (terminal 3) and chassis ground with the key off and accumulator depressurized. If no continuity is seen the pressure switch circuit will have to be diagnosed. If continuity is seen then go to next step.

  4. Measure the resistance across the relay coil. If the coil shows anopen then the relay needs to be replaced.

  5. Use jumper wires to power the relay coil while checking the contact terminals for continuity. On the schematic in Figure 18 this would terminals 2 and 5. With the relay coil energized there should be continuity between terminals 1 and 4. If no continuity is seen contacts are defective and the relay need replaced. If continuity is seen then go to next step.

  6. Check for power feed to the relay circuit. The relay provides the pump/motor it’s operating power once the relay contacts close. Check to verify that power is reaching the relay. In the example this would be terminal 4, the red wire. If 12VDC is found here then the pump/motor circuit will have to be diagnosed. If there is no voltage then the cause will have to be identified.

HCU Diagnosis
Generally, you will not be performing any generic diagnosis on the HCU (hydraulic control unit). HCU faults will usually be accompanied by DTCs and should be diagnosed using the appropriate flowchart.

ECU Diagnosis
The majority of the time there are no direct tests performed on the ECU. The ECU is usually diagnosed by a process of elimination. If everything else that could cause the problem has been eliminated, but the problem still exists, it is usually the ECU.

The generic tests that can be performed on the ECU involve verifying that the power and ground is reaching the ECU. Proper ABS operation depends on the ECU being properly grounded and receiving the correct operating power. Using the wiring diagram, locate the power and ground circuits and perform continuity tests for the ground circuits and voltage tests for the power circuits.

I’ll close by restating what I said earlier. The majority of ABS diagnosis will be performed using the DTCs as a starting point and then the appropriate flowcharts. The above tests are not meant to circumvent this process. They are meant to enhance or supplement it.

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Bill Williams

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