ABS Diagnostic Tips

Today’s Anti-Lock Braking Systems can Present Challenges

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Modern technology has added multiple systems to the old conventional hydraulic brake system. How they are integrated, what they do and how do we prevent a system failure will be the topic at hand. 

One of the worst feelings in the world is when you’re driving your vehicle and you experience a malfunction in the braking system, whether it’s a warning light on your dash or a braking application that just doesn’t feel right. Modern technology has added multiple systems to the old conventional hydraulic brake system. How they are integrated, what they do and how do we prevent a system failure will be the topic at hand. 

Back in 1978, the initial form of the anti-lock brake system that we know today was introduced on a W116 Mercedes-Benz S-Class. It became the first ‘four-wheel multi-channel anti-lock system’ that would soon revolutionize the automotive industry. ABS works by releasing and then reapplying or ‘pumping’ the brakes to a wheel or wheels in heavy braking situations. When a lockup is detected, the ABS pump motor applies the brakes, hundreds of times per second. This stops the wheel or wheels from skidding and helps keep the driver in control of the vehicle. The anti-lock braking system helps you steer in emergencies by restoring traction to your tires. This helps prevent wheels from locking up – possibly allowing the driver to steer to safety. Stopping a vehicle in a hurry on a slippery road can be very challenging. Anti-lock braking systems take a lot of the challenge out of this sometimes nerve-wracking event. In fact, on slippery surfaces, even professional drivers can’t stop as quickly without ABS as an average driver can with ABS.

The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. There are four main components to an ABS system: speed sensors (Figure 1), a pump, valves and an electronic control module.

Let’s look at it more closely. The speed sensors in the system are the reporting components that let the electronic control module know what each wheel is doing. The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases, in the rear differential, provide this information. The valves, which are located in the hydraulic unit, normally have three positions. In position one, the valve is open; pressure from the master cylinder is passed right through to the brake. In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder. In position three, the valve releases some of the pressure from the brake. The pump is used to release pressure from the brakes, but there has to be some way to regain that pressure. That is what the pump does.

When a valve reduces the pressure in a line, the pump is there to get the pressure back up. The EBCM (electronic brake control module) is the component that makes the decisions based on the braking event and applies the necessary action. This is just the basic ABS system. Fast forward to modern times wherein more systems have been added to work with the ABS systems.

Here is an example of a 2019 Chevrolet Traverse (Figure 2). This vehicle has several brake system enhancements that work with the ABS. This vehicle is equipped with a TRW EBC460 brake system. The electronic brake control module and the brake pressure modulator are serviced separately. The brake pressure modulator uses a four-circuit configuration to control hydraulic pressure to each wheel independently. Depending on options, the following vehicle performance enhancement systems are provided: Traction Control, Stability Control, Dynamic Rear Proportioning, Hill Descent Control System, Hill Hold Start, Assist Cornering Brake Control, Hydraulic Brake Assist, Optimized Hydraulic Braking, System Intelligent Brake Assist, Trailer Brake Control System and Trailer Sway Control system. As you can see with this system, network diagnostics (Figure 3) could play a big part in diagnosing an ABS fault. 

When working on an ABS with a customer concern, sometimes you can come across a strange symptom that could throw off your diagnostic thought process completely. Understanding the system and all the players involved will go a long way in performing a complete and accurate diagnosis. My friend, Bob Powell from Perron’s Automotive in East Longmeadow, Mass., came across a unique problem on a 2012 Chevrolet Traverse LTZ, which was a few years older than my example vehicle.

This vehicle has a 3.6L V6 and shares a lot of the same onboard systems as the newer Traverse. This vehicle was also equipped with the Hill Start Assist system or Hill Hold Start as it’s referred to on the newer model. The hill start assist allows the driver to launch the vehicle without a rollback while the driver is moving their foot from the brake pedal to the accelerator pedal. The EBCM calculates the brake pressure, which is needed to hold the vehicle on an incline and locks that pressure for a certain time by commanding the appropriate solenoid valves ON and OFF when the brake pedal is released. Hill-start assist is activated when the EBCM determines that the driver wishes to move the vehicle uphill, either backward or forwards.  Bob’s approach to the diagnosis of this vehicle was a learning experience and really makes you think about how the system operates. This vehicle has the Hill Start Assist system and the customer had an interesting complaint. If they stopped on a hill and then tried to go, the vehicle would get locked up. The engine would rev, but the vehicle didn’t want to move. They said the only way they could get it unlocked was to remove and replace the powertrain fuse. The first thing that was needed was a confirmation of the customer complaint. The vehicle seemed to operate normally when stopping and starting until you stopped on a hill. If the brake was held and released,  after 1-2 seconds the system would release normally, but when the brake pedal was released and he applied on the gas before the Hill Start had a chance to release, then suddenly the brakes would be locked and they wouldn’t release. While on the hill he could give it lots of throttle and it didn’t want to move. He even shifted to reverse and it still didn’t want to move. When it got stuck, it was STUCK!  So one thing he quickly realized is that for all intents and purposes, the system is really just a software program. There are no standalone “Hill Holder” components. It is a programming function that uses the existing components of the ABS/Traction system.

Bob thought there must be some type of programming error but a thorough search did not turn up any software updates for this system. Service info says it uses the ABS solenoids to trap line pressure in the system. While watching the scan data he did not see any abnormal solenoid activity that relates to system operation. Bob went as far as hooking up a tech2, which is the GM factory scan tool, but the system functioned normally but he did not see any solenoid activation. When this thing was stuck, all four wheels were locked so he knew it had to be something common to all the wheels. The brake modulator seemed unlikely since all wheels were locked, but the Brake Pressure Sensor is integrated into the brake modulator, so it’s possible. He tried doing a YAW sensor relearn with no change in symptoms. Bob scrutinized and analyzed all the data inputs in the system and finally saw something that didn’t look right.

During normal operation when releasing the brake, the Brake Pressure Sensor PID would drop to zero volts. When the fault was happening he noticed the pressure PID showed a small voltage. Not much (.14v) but it was the only thing that looked out of place (Figure 4).

Stepping back, he had to analyze what the heck could be causing residual brake pressure. What he found was that the master cylinder piston was sometimes sticking in the bore and would not release completely. The residual pressure was causing the Hill Start system to stay engaged. It likely was waiting for the brake to be released but because there was still some brake pressure in the system, it was waiting for the brake pedal to be released. It waited and waited and waited. ... At first he couldn’t make sense of why it only stuck on a hill but then realized, it was sticking at other times too but only slightly. Since the hill start is not activated on level ground there was not enough residual pressure to notice in normal driving. It was also puzzling why it would only stick when he got off the brake and onto the gas before the hill holder released. That one is still a mystery. He suspected it had something to do with a combination of changing booster vacuum and torque etc. that exacerbated the sticking master cylinder piston. So the bottom line is that the master cylinder was the cause of the fault. With the master out and held in a vice, he stroked the piston and it was obvious that it was sticking in the bore. Also, the seal on the cap was a little swollen so it was suspected to have some contamination in the system. He replaced the master and bled the whole system and the problem was gone. 

Bob’s diagnostic thought process along with a keen eye on his scan data and having a thorough understanding of the system and components that came into play allowed him to catch this mystery problem. Sometimes you have to do a lot of thinking in and out of the box to be able to nail down a tough diagnostic problem.

The various components of an ABS are prone to contamination through many outside sources. Water is one of the worst types of contamination in a brake system.  Make sure that you store and secure your brake fluid.  Open caps can destroy a new bottle of brake fluid in no time.  If your system is contaminated you must flush the whole system.  Once contamination gets in the master cylinder, the hydraulic unit, and any other brake component especially components with internal seals, problems will arise.  Some of these components can be very expensive.  Not to mention a contaminated system will hinder the operation of the brake system as it’s designed. Other issues of the ABS can be on the electric side. The EBCM or electronic brake control module plays a big role in ABS operation. The wiring has to be routed correctly.  The cables going to the sensors have to be routed correctly and they need to be in the proper place so nothing interferes with any moving components. Also, suspension parts and wheel/brake parts can come in contact with the sensors. If that happens then problems will arise.  Make sure that all connectors are properly sealed and free from dirt and moisture. Check to make sure that the weather seal is on the connector so moisture and corrosion won’t intrude. Corrosion is an electronic component’s worst nightmare. When performing diagnostics on an ABS be sure to scan all the systems in the vehicle....just like you would when you are performing a pre-scan for a customer. Just scanning an ABS alone could lead you down the wrong diagnostic path.  Just like the vehicles we’ve mentioned, there are other systems that contribute to the ABS. If you don’t scan all the systems you quite possibly can miss something. Last but not least, don’t forget to check the vehicle’s electrical system. ABS problems will surface if the voltages aren’t correct. A quick electrical test of the battery, the charging system and the starting system will go a long way in preventing premature ABS failures.  When you perform a road test on a vehicle with the ABS, one of the things that are necessary is to hook up your scan tool and graph the wheel speed sensors (Figures 5 and 6). Look for any abnormal readings such as dropouts, incorrect vehicle speed and erratic sensor readings. My recommendation is either bring along a passenger to watch the scan tool while you drive or set your scan tool up to record a movie. When you’re at a safe spot, then review your recorded movie. In today’s traffic-congested roads it’s probably not best to drive and watch your scan tool at the same time.

When performing ABS diagnostics, it’s important that you have received the correct information from the customer about the complaint.  Having the correct tools and information system for the vehicle that you’re working on will make all the difference in the world. Working on any vehicle braking system is a critical matter of safety. You want your customers to be safe as well as protecting your shop’s liability. Stopping a 5,000 to 6,000 pound vehicle is mission-critical. There’s a lot of liability that needs to be thought of. Correctly performing the job and everything that it entails will put your customer’s mind at ease.   

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