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A/C Oil Service: Avoid Compressor Failure With Proper Maintenance

HVAC systems often fail because the air conditioning compressor fails. Not following the proper service procedures or compressor oil recommendations could be factors that contribute to premature compressor failure. Refrigerant Since the mid-‘90s,...

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How Ford's New Grille Shutters Control Airflow

Some new Ford vehicles feature active grille shutters — a new system that optimizes aerodynamics by using vents to control airflow through the grille to the cooling system and engine compartment. If air is required to cool the engine, the vents...

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How Engine Coolant Temperature Sensors Influence The Modern Powertrain

  Considering that roughly 1,500 or more ­different vehicle models are introduced into our domestic market each year, it’s becoming more difficult to predict how a powertrain control module (PCM) will utilize data from a particular sensor or...

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Nissan Maxima Brake Job Tips

Called the four-door sports car by Nissan, the 2004-2008 Maxima brake system is easy to service with very few complaints of brake noise or pulsation. The system used the same brake pads, rotors and calipers on all models. There were some changes in...

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Get Inside the Mind of a Modern Transmission

The transmission game has changed. Solenoids, sensors and computers have replaced vacuum lines, governors and kick down cables on modern automatic transmissions. The tools have also changed: Scan tools, scopes and meters have replaced pressure and...

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Saturn: Hesitation Between 35-50 mph

Condition Some customers may comment on a flat spot or hesitation between 35-50 mph (56-80 km/h) speed range. Cause The existing transmission calibration is programmed to maximize fuel economy and requires a greater throttle input than some customers...

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Audi: Vibrates at Idle

Complaint The customer says that the check engine light is on and that the vehicle vibrates at idle. Cause The customer’s complaint is confirmed, with the vehicle vibrating at idle, but the vibration was not felt at high speeds. A scan tool found...

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Step-by-Step Lexus Fuel Pump Replacement

We will use a third-generation 2006 Lexus LS430 as our case study for this fuel pump assembly replacement. First, discharge the fuel system pressure, disconnect the negative battery terminal and drain the fuel. Be sure to remove the rear seat cushion...

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Nissan: Squeak From Fuel Pump

Customer may report a squeak noise that is: • Coming from inside the vehicle; • Happens after driving for 30 minutes; • Is coming from under the vehicle or from the left rear area of the vehicle. If you confirm that a “squeak” noise...

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Midtronics Launches Informational Microsite

  Midtronics, Inc. has launched an informational microsite — or mini website — to provide customers with detailed information about the company’s newest diagnostic platform — the DSS-7000 Battery Service Diagnostic System. The microsite...

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German Tool Specialist KC Tool Announces Addition of Gedore Tools

As of August 1 KC Tool became the first certified reseller of Gedore Tools in the United States already adding more than 2,500 Gedore tools to its selection of German made tools. KC Tool plans to add an additional 6,000 Gedore Tools over the next few...

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United Stationers Expands Into Auto Aftermarket

United Stationers announced Sept. 11 that its wholly owned subsidiary, United Stationers Supply Co., signed an agreement to acquire MEDCO, a U.S. wholesaler of automotive aftermarket tools and supplies, and its affiliates including G2S Equipment de Fabrication...

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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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A/C Oil Service: Avoid Compressor Failure With Proper Maintenance

HVAC systems often fail because the air conditioning compressor fails. Not following the proper service procedures or compressor oil recommendations could be factors that contribute to premature compressor...More

How Ford's New Grille Shutters Control Airflow

Some new Ford vehicles feature active grille shutters — a new system that optimizes aerodynamics by using vents to control airflow through the grille to the cooling system and engine compartment. If...More

Nissan Maxima Brake Job Tips

Called the four-door sports car by Nissan, the 2004-2008 Maxima brake system is easy to service with very few complaints of brake noise or pulsation. The system used the same brake pads, rotors and...More

Get Inside the Mind of a Modern Transmission

The transmission game has changed. Solenoids, sensors and computers have replaced vacuum lines, governors and kick down cables on modern automatic transmissions. The tools have also changed: Scan tools,...More

Audi sees boost in U.S. sales during first eight months of 2014

The Volkswagen Group saw another jump in deliveries in August – 2.7 percent increase year over year – with sales topping 740,500. From the start of the year through January, deliveries are up 5.6 percent...More

Audi: Vibrates at Idle

Complaint The customer says that the check engine light is on and that the vehicle vibrates at idle. Cause The customer’s complaint is confirmed, with the vehicle vibrating at idle, but the vibration...More

ACDelco Impact Wrench Offers Five Preset Torque Settings

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