Does the ABS Affect the OBD Drive Cycle?

There's a ghost in this machine -- and it's controlling your brakes.

Things used to be so simple; engines were what made the car go, and brakes were what made them stop. Now, in this age of computer controls and cross-system networking, it's completely fair to wonder if one system can affect the other. While this generally isn't the case, you might be surprised just how smart and cooperative modern chassis- and engine-control computers are getting.

Engine Management Basics

Your car's computer controls are basically a system that monitors what goes into the engine and what comes out, then subtracts the difference to figure out what's going on inside. Using that information, the computer determines how much fuel it should inject and the degree of timing advance or retard it should use to keep things running at a steady equilibrium. The computer uses a number of multi-factor algorithms that mechanics can download and view as three-dimensional "maps." The computer uses several different maps to tailor fuel injection and ignition control to adapt to ever-changing internal and external conditions.

ABS Control Basics

Antilock brakes have been around for a long time, and the basics haven't really changed much in that time. Modern ABS systems use a magnetic wheel-speed sensor on each wheel to figure out how fast each wheel spins relative to another. Above or below a certain threshold, if the computer determines that one wheel is spinning significantly slower than another is, then it assumes that wheel has locked and releases braking pressure to it. A fluid pressure accumulator supplies quick bursts of pressure to wheels that are spinning too fast, or to re-supply pressure to a wheel previously released.

Normal ABS and OBD Computer Interaction

In a standard automobile, the engine control and ABS systems are completely separate entities; neither one knows or cares that the other one even exists. The traditional approach has been to use one computer to control the engine -- and sometimes transmission -- and another to control the ABS system. The only time these computers even come into contact is in the circuit board that supplies power to both or behind the dash where the readouts are. So, no -- in most cars, the ABS system cannot and will not affect the engine's programming map or on-board diagnostic drive cycle.

Special Cases

The above is true for most cars, particularly older ones, but computer networking, traction control, stability control and computer-controlled differentials can throw a sizable monkey wrench into that particular theory. All of these systems use the ABS system's wheel-speed sensors for input, and all may be networked with the engine-control computer. Traditional traction control systems used input from the driven-wheels' wheel supply systems to determine if the wheels were spinning; if it detected spinning, the traction control would physically close the throttle plate with a servo. Now, however, TC and SC are just as likely to control wheel-spin by retarding the ignition timing, which certainly does affect the OBD drive cycle. With computer networking, manufacturers can reprogram the engine to do almost anything on the fly, including increase or decrease engine braking to assist the ABS system.

References

• "Engine Management: Advanced tuning"; Greg Banish; 2006
• "Auto Electricity and Electronics: Principles, Diagnosis, Testing, and Service of All Major Electrical, Electronic, and Computer Control Systems"; James E Duffy; 2004
• "Automotive Computer Control Systems: Fundamentals and Service"; William L. Husselbee; 1996