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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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Ford: ABS Light On With DTCs C1175, C1236, P0500 or P0503

Model: 2005-’07 Super Duty Some 2005-‘07 Ford Super Duty vehicles may exhibit an ABS light on and possible diagnostic trouble codes (DTCs) C1175, C1236, P0500 and/or P0503. This condition may be due to faulty vehicle wiring, a failed wheel speed...

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Composite Rotors Are Back

Veteran technicians will remember the problems with composite rotors in the late 1990s. These rotors had hats of stamped steel cast into the iron rotor. They saved weight, but they were prone to runout and installation errors. These types of rotors...

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Power Steering Dynamics and Diagnostics

Twenty-five years ago, when the power steering failed, diagnosing the system was easy. The most difficult problem to diagnose was the “morning sickness” that plagued some ­vehicles when they were cold. Today, the introduction of speed sensitive steering,...

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Saab: Fuel Sending Unit Replacement

The 2006 Saab 9-3 Sport sedan came in on the hook and the tow truck driver said, “I think it needs a fuel pump.” The gas gauge was reading less than a quarter of a tank and the low fuel warning light wasn’t on, so a quick fuel pressure check was...

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KIA: Brake Light On, Tachometer And Speedometer Inoperative

Applicable Models: 2001 Kia Optima LX, 2.4L Complaint The customer states the red brake light is illuminated on the instrument cluster. The ­customer also states the tachometer and speedometer intermittently stop working. Cause Confirmed the customer’s...

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Mazda: Fuel System Servicing Precaution

Applicable Models: 2004-’07 Mazda3 2005-’07 Mazda6 2006-’07 MX-5 vehicles During service/removal of fuel system parts on the above vehicles, the memory of the malfunctions and the long-term fuel trim need to be cleared by either removing the...

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Using Volumetric Efficiency to Determine the Health of an Engine

Measuring voumetric efficiency can diagnose problems like blocked catalytic converters, bad MAF sensors or vacuum leaks. Glen Beanard show you how to do it with a scan tool. For the complete article, click here.  ...

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Phoenix Systems Sends Facebook Giveaway Winner to Meet Larry McReynolds for All-Star Race

Phoenix Systems, maker BrakeShot, BrakeStrip and Reverse Brake & Clutch Bleeders, announced the winner of its 2015 Win Race Tickets Facebook Giveaway. Suzanne Cleary Drews from Edgewater, FL, is this year's grand-prize winner. Phoenix Systems will...

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Intermittent Engine Misfire Analysis

Even for an experienced diagnostic technician, attempting to diagnose an intermittent misfire condition that occurs only under specific driving conditions can be a frustrating exercise. Contributor Gary Goms breaks it down, and includes tips for using...

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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 amarkel@babcox.com.
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