Diagnosing ABS And Vehicle Stability Control Issues
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Diagnosing ABS And Vehicle Stability Control Issues

These driver aids are empowered by control unit monitor inputs from the wheel speed sensors, steering angle sensor and yaw sensor. Using these inputs, the control unit makes decisions and sends output commands to the ABS modulator and throttle body to help the car follow the course that is expected by the driver.

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This month’s article is going to focus on Nissan’s ABS and vehicle stability systems, also known as Active Trace Control or Electronic Stability Control (ESC). These driver aids are empowered by control unit monitor inputs from the wheel speed sensors, steering angle sensor and yaw sensor. Using these inputs, the control unit makes decisions and sends output commands to the ABS modulator and throttle body to help the car follow the course that is expected by the driver. In simple terms, if the vehicle should start to slide as it goes around a corner, the yaw sensor reports the condition and the control unit applies brake pressure to the appropriate wheels to help the driver maintain control.

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I always like to include a case study when I do these articles, but in the framework of Nissan ABS and vehicle stability systems, we really don’t see too many specific issues. So, instead of focusing on certain models, we’ll talk about some of the common brake-related problems you may see and discuss how to go about diagnosing them.

Access to the ABS controller can be a challenge.

One of the primary brake-related issues you will see is that the ABS lamp and slip lamp are illuminated, indicating a fault. Like so many other problems we handle, the key to accurate diagnosis is having an enhanced scanner that will allow you to get into the system to retrieve fault codes and the appropriate data so you can see what the control unit is seeing and how it’s responding to those inputs.

I like to do a full scan on the initial diagnosis. It doesn’t take very long and will give you a good overview of the vehicle, some insight into module communication and the ability to find a related code in a system you wouldn’t have chosen to look at in the first place. We have also seen more than a couple of Nissans that needed nothing more than a steering position sensor reset to extinguish the lamp after the battery went dead.

It is no surprise that rust and corrosion can play a role in ABS problems.

It is essential that your next step in diagnosing any issue or code should be to check your service info for TSBs. This is especially true with Nissan ABS issues. While we see very few problems with the system, there are some models that have had problems that were addressed in various TSBs, so always use available information.

Be careful not to damage the ABS sensor when removing it. If it is rusted in, make sure a replacement is available.

If you’re faced with a wheel speed code, go to the data option and choose the wheel speed parameters; initially, I’ll look at the speeds in digital format, looking for a zero reading. If you should have no speed input at one wheel, don’t jump to conclusions that the sensor is bad — like any fault code, it is system specific, not part specific. In this case, be sure to inspect the wiring harness going to the sensor for breaks or a rub-through spot. If everything looks OK with the wiring, it doesn’t take long to check the output signal at the sensor, but you will need a scope or graphing meter.

On pre-2010 cars, with the sensor unplugged, connect the leads of an AC voltmeter to the sensor connecter. Turning the wheel by hand, you should see about 250 mv. If you have the signal there, your next step is to confirm the signal at the ABS control unit. Later-model cars use an active wheel speed sensor that needs a voltage signal to operate. To confirm the sensors are getting the voltage from the ABS controller, unplug the sensor and check for voltage between the positive sensor signal wire and the negative low reference wire. With the ignition on, there should be battery voltage.

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If not, check the integrity of the ground circuit by grounding the meter to the chassis. If you’re not getting the expected results, perform the same test at the control unit.

If you are faced with the complaint of ABS overactivity where the driver is coming to a normal stop and the ABS kicks in, note that a code may or may not be set. Many times, the customer will complain of the pedal shuttering. This is why it is so important that the proper questions be asked when the appointment is made or when the vehicle is checked in. If the only information the service writer gets is that the pedal shutters, time could be wasted chasing your tail without ever pinpointing the problem. Worse yet, the car will likely be returned since the customer’s problem will not have been addressed.

Always push the contaminated, dirty fluid into the bleed bottle, rather than back into the ABS modulator. The last step of any brake service should be a fluid flush.

Diagnosing this problem can be a bit of a challenge since the system is actually doing what it is supposed to be doing — preventing the slowing wheel from locking up. The problem we have to identify is which wheel is providing false information to the control unit that it is about to lock up.

Take care to route the sensor wire correctly to prevent damage.

Again, we’ll be looking at the wheel speed data on the scan tool for this test. I prefer to look at the signal in graph form. Ideally, you can get four separate graphs so all the wheel speeds can be compared. If possible, I always bring an assistant to observe the scanner while road testing to catch the sensor dropout. In some cases, the offending sensor can be spotted while driving slowly in a parking lot. All of the sensor speeds should match when traveling straight, so if one is lagging, you have found your culprit.

We have identified bad sensors as the cause of this problem (as well as sensors that didn’t have the proper air gap) resulting from them being moved by rust buildup around the sensor mounting area, poor work habits with reinstallation and contamination of the sensor rotor. Nissan, like many other manufacturers, uses a sensor rotor (as they call it) that is integrated in the wheel bearing. When removing the sensor for inspection, take a look down the mounting hole for any signs of damage or corrosion that could be affecting its operation.

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CORE BRAKE SYSTEM

While we have been talking about ABS, it’s important to keep in mind that in order for this system to work as designed, the basic brake system needs to be in top-operating condition. This means it is critically important that good, solid work habits are employed when performing any brake service.

The sensor on the brake booster monitors available vacuum.

The control valves in the ABS modulator rely on good, clean brake fluid and have little tolerance for dirt. That’s why our first step in any brake job is to free up and open the bleeders before the pistons are pushed back into the calipers so we can catch all of that dirty fluid in the bleed bottle rather than it being pushed backward through the modulator. This step also lets you know that all of the bleeders will open so the fluid can be flushed as the last step of the brake job.

These same work habits should carry over to pad installation. Be sure to clean and lubricate all of the hardware, paying particular attention to the slider hardware. Remember, ABS and vehicle stability systems depend on a brake system that is performing as designed. It is our job to ensure that it is up to the task.

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