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BMW Recognized for Engine-Building Excellence

BMW Group’s engine-building prowess was recognized with four wins at the latest International Engine of the Year Awards. The drive unit in the BMW i8 earned two class wins as well as being declared overall winner, with a further class win being garnered...

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Diagnostic Dilemma: The Case of the Missing Code

When doing mobile diagnostic work, no-code stalling complaints are a major part of your agenda. In most cases, the client shop is simply too busy to duplicate the failure or, in some cases, a long test drive will yield nothing in the way of useful...

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Secondary Ignition: The Art of Spark

What is a coil? From the beginning of the internal combustion engine, several different ignition systems have been used to create a high-energy spark. The most popular system, and the one that’s in use today, is a step-up coil. A coil is nothing...

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Regulators Launch Investigation Into Jeep Grand Cherokee Brake Defect

Federal regulators are investigating whether the automatic braking systems in some 2014 Jeep Grand Cherokees may be defective after receiving a number of complaints from motorists about a potentially dangerous glitch that caused their vehicles to come...

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Electronic Proportioning Valve: Doing More With Less Hardware

Anti-lock brake systems (ABS) and the HCU are replacing proportioning, combination and other valves to change the braking forces in the front and rear. This is called Electronic Brake Distribution (EBD) and it can dynamically change the proportioning...

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NHTSA’s GM Brake Line Corrosion Investigation: Reading Between the Brake Lines

There will be no recalls on some GM vehicles for brake line corrosion. Instead, we received an advisory from the National Highway and Traffic Safety Administration (NHTSA) about brake line inspection and car washes. What was not discussed was the corrosion...

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Customer Loyalty to Vehicle Brands Spells Ongoing Maintenance Opportunities

Vehicle owners’ allegiance to brands should give us all something to cheer about. This is especially true for those of you who service import vehicles. Nine import nameplates — Infiniti, Land Rover, Lexus, Mazda, Mitsubishi, Nissan, Porsche, Subaru...

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The Element of Trust And Its Impact on a Repair Scenario

Trust: It’s an important word, and it’s one I hear a lot behind the counter at the shop. “I know you’ll treat me right because I trust what you do,” is the general sentiment. But I’ve always wondered how deep that level of comfort goes...

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Servicing Mercedes-Benz AIRMATIC Suspensions

The Mercedes-Benz AIRMATIC suspension system was introduced in 1999 on the S-Class and has subsequently been used on the E-Class and most of the automaker’s SUVs. The system employs electronically controlled air springs that provide an ideal balance...

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Hunter's Social Media Contest Includes Trips to SEMA 2015

Hunter Engineering has launched a new, two-part social media contest that offers trips to SEMA 2015 and the chance to win a free SmartWeight Touch wheel balancer at the show. Before SEMA, two shops can win a trip for two to SEMA 2015 by following Hunter...

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Ingersoll Rand's 'Real Work Real Play' Sweepstakes With Gas Monkey Garage And NASCAR Rewards A Tool User With The Ultimate Fan Weekend

Ingersoll Rand, the Official Power Tools of NASCAR and a preferred tool provider for Gas Monkey Garage, has announced the “Real Work Real Play” sweepstakes to reward automotive fans who “get it done” with a weekend of fun. Ingersoll Rand is working...

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Bosch Relaunches Boschdiagnostics.com With Mobile-Responsive Design, New Layout For North America

Bosch has announced the re-launch of boschdiagnostics.com in North America, continuing to update all of its sites to a mobile-responsive, intuitive design. The URL contains three separate sites, featuring DIY diagnostic tools (DIY), professional diagnostic...

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REAPPLY
Since pressure from the master cylinder has been bleed off, the pump in the HCU will spool up and apply pressure. The outlet valve is closed and the inlet valve is opened. The pump applies pressure to the wheel. 
If the wheel is still outside the wheel slip parameters, the cycle will start over. This happens very quickly. The operation of the solenoids and pump will cause a “kick back” or pulsation in the pedal. 
 

HCU Mechanical Problems
Mechanical issues with the HCU are rare, but they can happen. Valve seats and pintles can become stuck or not seat properly due to debris, corrosion or contaminated brake fluid. 
 
If the inlet/isolation valve is stuck open, it will not affect normal braking in any way. It will only hurt the ABS system. This could lead to a pulling condition during ABS activation. 
 
If an outlet/dump valve is stuck open in one circuit, this could cause a pull condition during normal braking. This is due to the loss of brake pressure at a wheel. Typically, this is not discovered until brake hoses, calipers and other parts have been replaced.

Testing Solenoids Electrically
Sometimes a stuck or defective solenoid or pump will set a code. A solenoid has a resistance between 2 and 8 ohms. On some units, it is impossible to access the individual solenoids. 
 
Testing of the unit with a scan tool with bi-directional control might be the best way to confirm the condition of the HCU.
 
Most vehicles equipped with ESC will have 12 valves or solenoids in the HCU. Eight solenoid control the wheels. Four additional solenoids can block off the master cylinder and allow the pump to send pressure to a specific wheel.
 
Understeer is a condition where the wheels are turned, but the vehicle continues to travel in a straight line. This is sometimes described as a push. 
 
The ESC computer would see this event through the sensors. The wheel speed sensors in the front typically read slower than the rears. The computer would also see that the steering angle is greater than the intended path.
 
The ESC system needs to intervene before the event occurs. It needs to anticipate the problem and correct as the vehicle travels.
 
This is what the ESC sees during an understeer event. The SAS angle is at +52º, this means that the customer has the wheel turned to the right at a significant angle. Even with the steering wheel turned, the yaw and accelerometer read like the vehicle is going straight. 
 
 The APPS or throttle pedal position sensor shows the driver is off the gas and the brake pedal is not pressed.
 
The deciding information for the system is in the wheel speed sensor inputs. Between the front and rear there is a 6 to 9 mph difference between the front and rear speeds. The front wheels are traveling slower than the rears.
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Andrew Markel

Andrew Markel is the editor of Brake & Front End, Underhood Service and Servicio Automotriz magazines. He has been with Babcox Media for more than 15 years. He is a technician and former service writer and holds several automotive certifications from ASE and ­aftermarket manufacturers. He can be reached at [email protected]
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