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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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Diagnosing Catalytic Converter Failure Symptoms

Although construction can vary according to engine application, the common three-way catalytic converter contains a reduction and oxidation stage. To create maximum surface area, each stage is generally a ­ceramic or stainless steel honeycomb substrate...

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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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2007-2011 Ford Expedition and Navigator Air Suspension

The Ford Expedition air suspension does more than just lift and raise the vehicle. The system levels the vehicle under loads and when a trailer is attached. The system uses only two air bags on the rear axle unlike previous models with air bags on all...

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MAZDA: Timing Belt & Chain Replacement

This month, we’re going to be looking at the ­timing components on the Mazda line of vehicles. We’ll be focusing on timing belts since they are considered a service item and will present the greater amount of opportunity for replacement. Then, we’ll...

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Honda Element Brake Job

It may look like a car that was never removed from the box it came in, but the Honda Element isn’t boxy when it comes to the brakes. Based on the CR-V platform, there is also nothing tricky when it comes to service. But, its brake system is hardware...

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The Changing Maintenance Market: New Technologies Mean More Opportunities

Most of us wake up each morning, not ­realizing that our professional world has changed even as we slept. Our first job of the day is to service a ­vehicle equipped with an oil life monitor. Not only do we discover that modern oil life monitors can...

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Maintaining Your Spray Guns

If there’s one piece of equipment that epitomizes the painter and the paint shop, it’s the spray gun. Over the years we’ve seen many spray guns. Although there are operating principles and functions that remain the same, some have been improved...

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Wheel Bearing Adjustment Tools & Equipment

A recent survey showed that more than half of the bearings on the road today are adjusted incorrectly. A wheel bearing that’s out of adjustment can reduce bearing life and can affect more than just the bearing. An out-of-adjustment bearing affects...

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ETI's ToolTech 2015 Focuses on the Connected Vehicle

ETI’s Annual ToolTech conference remains the premiere event in the tool and equipment industry with more than 115 professionals from over 50 companies in attendance at this year’s ToolTech 2015 in Austin, TX. The Equipment and Tool Institute's...

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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 and Servicio Automotriz magazines. He has been with Babcox Media for more than 12 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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