Probing the mysteries of ABS

Probing the mysteries of ABS

Bach, Jeff

Using a current probe in tandem with your scope can bring you into a whole new realm of ABS diagnostics.

In 1993, 43% of the cars on the road were equipped with ABS. Within a couple of years, that percentage had climbed to almost 80%. As a result, most of the vehicles you’ll see in the next few years that are coming off warranty will have antilock brakes on board.

In the past, many shops avoided ABS work like the plague. And with good reason. These systems have traditionally been expensive to fix and often difficult to diagnose. Most are modularized, meaning not many parts are separately serviceable. For example, a bad inlet solenoid often requires replacement of the entire valve block assembly. Some systems have the main and pump relays built into the pump motor assembly. So if something as simple as a relay goes bad, the complete assembly must be replaced as a unit. Ouch! Because of this design feature, it’s vitally important that you correctly diagnose the proper defective component before making that expensive call to your parts supplier.

System Operation

All antilock systems operate in a similar fashion. An Electronic Brake Control Module (EBCM) self diagnoses the system while also serving as the command post for system activation. There’s also a hydraulic pump and motor assembly with an accumulator to build and store system pressure, a reservoir, an inlet and outlet solenoid for each wheel to control hydraulic pressure to the appropriate caliper piston, plus wheel speed sensors to notify the EBCM of abrupt changes in hub velocity.

One of the more popular ABS systems uses a controller made by ATE Electronic Brake Systems. I’ve found this system on everything from Jaguars to Lincolns to my specialty, Cadillacs. It’s also the system that gives many techs some of their worst ABS headaches.

As many of you know, not all scan tools can access, read and clear trouble codes on ABS systems. Still, I find it hard to send work away due to technical lockout. Actually, I can’t do it. This rare malady is known in our profession as

I’m now using a holistic alternative medicine to combat challengeitis. It’s known as scopenproben-using a current probe with my scope-and it’s been working well for me. Actually, it’s a miracle drug.

This test method was born out of the frustration I encountered while going through a bout of challengeitis with systems using the ATE controller, before I obtained a scan tool that could read and clear ABS codes on Caddys. I now actually prefer this approach to the scan tool method.

It Starts at the Wheels

Wheel speed sensors can cause troublesome intermittent problems on ABS systems, in part because a bad signal from a speed sensor can go undetected by the EBCM. Note that wheel speed sensors are tested statically for circuit integrity, as well as dynamically for signal strength. The waveform in Fig. 1 shows a normal pattern of a healthy wheel speed sensor.

As you’re probably aware, a wheel speed sensor is nothing more than a magnetic reluctor that acts as a small AC generator. As a result, its signal. should increase in strength as the hub speed increases. Fig. 2 on page 27 shows this effect. This is the same speed sensor shown in Fig. 1, but with the wheel rotating at a faster rate.

The EBCM also looks for proper frequency and amplitude of the signal at 4 mph. The waveform shown in Fig. 3 on page 27 is of a speed sensor that was setting a code due to low voltage output. But since the output was outside the voltage threshold established in the software, it was an easy thing for the EBCM to spot.

Not so in Fig. 4. Here, no code set because the sensor was still operating and producing a sufficient voltage. But notice the noise on the signal, as indicated by the hash. This car was equipped with traction control as well as antilock brakes, and the EBCM was interpreting this noise as increased wheel speed. As a result, it kept applying the brake for this particular wheel. The customer complaint was that the car lacked power. In addition, the “Traction Active” message was appearing on the driver information center, and you could feel the brakes pulsing.

The cause of the noisy signal was a rusty scale that had built up on the wheel speed sensor magnet, and would come and go at will. I captured this waveform during one of its worst occurrences.

The ATE controller does a selfcheck of the solenoids at key on, as well as a 4-mph rolling-test of the wheel speed sensors. It can set a code and turn on the ABS light as soon as the key is turned, even before system pressure gets a chance to build. Once a code is stored, the EBCM shuts the system down during that key cycle.

The controller accomplishes this self test by sending out a sequence of pulses to the inlet and outlet solenoids, then monitoring the current level at specifically timed intervals. Using a lab scope and current probe connected to the feed wires for the solenoids allows you to see exactly what the EBCM sees. Fig. 5 on page 28 is a current wave

form of the initiation sequence of a normally operating ATE system during the self test. This pattern was obtained using a very low-resolution current probe. The spikes you see here are actually the test pulses the EBCM sends to the inlet and outlet valves to check for opens or shorts.

Fig. 6 is a waveform of the inductive current curve from a brake valve solenoid during normal operation. The inductive curve is simply the time a solenoid takes to reach full current, once energized.

Notice that it takes this 6-ohm solenoid just 8.4 milliseconds (mS) to reach full saturation at 1.88 amps, and only 2.5mS to fully open, as evidenced by the hump that appears in the first third of the waveform-what I call the “Seagull Effect.” This takes place when an inductor (such as a relay plunger) moves through a magnetic field. This movement can be easily traced with a low-resolution current probe.

Fig. 7 shows to scale the portion of the solenoid current that’s actually used for the test pulse by the EBCM. Keep in mind that I used the low scale (1mV per ImA) on the current probe to get the test pulses, while using the 100mV per-amp scale for the solenoid current shown in Fig. 6.

By sending out this test pulse and monitoring the current level, the controller can determine whether or not the solenoid is charging at the correct time at that particular voltage level. If the solenoid has a resistance problem, the current will either get too high too fast, or not get to its limit soon enough. Either way, it will set a code for that particular solenoid.

Many of the problems I encounter with the ATE system aie either in the solenoids themselves or with bad solder joints on the mylar strip that connects them to the harness. In the latter case, you can get complaints of the ABS light coming on intermittentlyusually in the morning, or when it’s cool-and then be all right after the underhood temp increases.

Using the EBCM’s test pulses, it’s easy to see when you have a problem with one of the inlet or outlet valve solenoids. The waveform in Fig. 8 on the opposite page shows a code being set for the right front inlet valve solenoid at key on, as evidenced by the missing signal.

Notice that it took just a little over 65mS at key on for the EBCM to go through the self-test, recognize the fault, set the code, then shut down the system.

Each of the valve solenoids on the ATE system has a place in the pulse line, and a subsequent code if it’s missing or if the current level is too high. The waveform in Fig. 9 shows a normal test pulse sequence. It begins with the left front outlet valve solenoid current, then runs continuously while the key is on. After the initial seven pulses take place, there’s a 70mS pause, then the left front inlet valve begins pulsing with each of the other inlet valve solenoids (see Fig. 10).

Fig. 11 shows a code 42 being set for the right front outlet valve. The missing pulse (arrow) causes the EBCM to set the code, then shut down the antilock system. The larger spike at the end of the waveform is caused by the EBCM pulsing all the solenoids at once. This pulse is seen only when a code sets, just before system shutdown.

Fig. 12 is indicative of a code 51-a problem with the right rear inlet valve. To set this test up, you need only clamp your low-current probe around the two feed wires for the solenoid block, set your scope, then turn on the key. It takes less than two minutes, including opening the hood.

The missing pulse test is fast and accurate. However, I must warn you, scopenproben, while an effective treatment for challengeitis, has been known to cause enjoyment and amusement. If taken in large doses, it may be habitforming. It should be taken in moderation; remember, you’re at work.

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Copyright Hearst Business Publishing Jun 2000

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