Modern Vehicle Computing - On-Board Diagnostics

There was a time when engines had very simple means of controlling the fuel and spark activities needed to turn combustible gasoline into power to move our vehicles. You typically had a carburetor that metered air/fuel flow into the engine. Along with this, spark to the cylinders was often controlled by an ignition system with mechanical distributor that could advance ignition timing as rpm increased via intake vacuum and/or centrifugal weights on the distributor.

Modern Vehicle Computing On Board Diagnostics Orb Access Port

Data from the engine control module can be routed to a diagnostic scan tool plugged into the OBD access port (usually located under the dash but may also be found under the hood for OBD-I).

Data from the engine control module can be routed to a diagnostic scan tool plugged into t

In the late 1960s and early 1970s, car manufacturers started to implement basic electronic controls for fuel and spark management. However, the early systems were manufacturer specific and/or used a proprietary configuration. Fast forward to about 1988 when the California Air Resources Board (CARB) mandated some form of on-board diagnostics, and OBD (or now called OBD-I) was born. On-Board Diagnostics (OBD) is a system by which automotive drivetrain management is monitored and controlled. It also has the ability to report fault conditions to the engine control module. OBD-I was a start, but there was not yet a standardization and systems varied from manufacturer to manufacturer.

Some years later, CARB issued an OBD-II specification and mandated that 1996 vehicles use a standardized diagnostic connector and data output format. Implementation of OBD-II meant that vehicle emission performance could be checked by plugging into the vehicle diagnostic connector as opposed to having to run the vehicle through a loaded dyno roller test.

Modern Vehicle Computing On Board Diagnostics Malfunction Indicator Light

There are conditions that may trigger a fault code in the system, causing the control module to store the code. Depending on the particular fault that occurred and whether it is continually detected, the fault code may set the ‘Check Engine Light’ (CEL) or ‘Malfunction Indicator Light’ (MIL) on the dash. This signals the driver of a persistent fault in the system but often does not stop the vehicle from running. In fact, the MIL may be on but the vehicle may be running normally.

There are conditions that may trigger a fault code in the system, causing the control modu

Current OBD-II systems have expanded to allow the monitoring of numerous powertrain characteristics. The list of parameters is long (several hundred possibilities) but can include:

Engine Coolant Temperature

Throttle Position Voltage

Engine RPM

Oxygen Sensor Voltage

Vehicle Speed Sensor

Intake Air Temperature

Ignition Timing

Fuel Pressure

Mass Air Flow Sensor

Engine Fuel Rate

Evaporative System Status

Misfire Monitoring

Transmission Gear

Air Conditioning On/Off

EGR System Status

As you drive an OBD-II–equipped vehicle, it is constantly using data acquired from the various sensors to run the fuel injection system. Ambient air temperature, engine coolant temperature, throttle position, and intake air volume are all used to adjust the electrical pulse width (time injector is turned on) of fuel injectors to add more or less fuel as calculated by a data map stored in the engine computer. This estimate is further adjusted by reading the exhaust composition via oxygen sensors before and after catalytic converters.

Engine control now becomes far more sophisticated and can be optimized for better fuel economy and fewer tailpipe emissions. Additionally, the computer revises its fuel maps as it learns the specific driving patterns and performance of the specific engine it is controlling.

Modern Vehicle Computing On Board Diagnostics Handheld Code Scanners

Handheld code scanners have become popular and their price has dropped considerably in the last few years, making such tools more practical for the weekend gearhead. With these units you can typically read fault codes, monitor various drivetrain parameters (both in real-time and take snapshots at a chosen time), and clear the fault codes in memory. Once you pull a fault code with a scanner, you can reference the OBD-II code descriptions and start to get some idea as to what the problem is.

Handheld code scanners have become popular and their price has dropped considerably in the

Code Reading

As we mentioned previously, OBD have the ability to monitor drivetrain systems for failures that might occur and have some ability to report faults back to the computer. In the early days of vehicle computers, we might short two diagnostic electrical terminals and observe a flashing pattern of a “check engine” light to decipher a trouble code to aid in troubleshooting a problem. OBD reports sensor and fault data through its diagnostic connector, and a code scanner is used to report a text message to the observer.

When the engine is first started, it operates in what is referred to as “open loop” condition. In this case, the electronic controller operates the fuel injectors based on a stored data table and its best estimate of engine conditions, using some sensor data. After a brief warmup, the system switches over to “closed loop” mode. In this condition, the full array of sensors is active and provides calibrated feedback to the controller to optimize the fuel and timing settings on a continuous basis while the engine is running.

For a code example, engine control modules can detect a short or open circuit condition at a fuel injector and report the injector failure to the microprocessor in the module. From here, a technician can read the code with a scanner and work to locate the faulty component. We found useful code descriptions at www.obd-codes.com.