Page 334 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

be manufacturer-specific. When a DTC is saved, the ECU sends a signal to your indicator light to state that a
problem has been found. The DTC can also be pulled by linking a sensor to the connector for the OBD system.
MIL: When the ECU collects a DTC code, it sends a signal to the vehicle dashboard to turn on the appropriate
indicator lights. These lights, known formally as Malfunction Indicator Lights or MILs, provide an early warning
system for vehicle malfunctions. Generally speaking, if the light turns on and stays on, the problem is minor. If the
light flashes, the problem is urgent.
DLC: All of the data and DTC codes collected by the ECU can be accessed via the Diagnostic Link Connector
or DLC. The DLC port is the point of access for vehicles with OBD systems and is often found beneath the
dashboard on the driver’s side of the vehicle, though it may be located elsewhere in commercial vehicles. Current
vehicles are made with a standard OBDII system so that any scan tool with a type 2 cable can connect to the type
2 connector.
How does OBD work
How Has On-Board Diagnostics Changed Over the Years?

OBD has changed significantly over the years since its introduction in the 1980s. Originally, the system would
notify the user that there was a problem using the MIL, but wouldn’t store any information as to the nature of
the problem. As cars became more advanced, the number of sensors installed in vehicles expanded, as did the
amount of information stored inside the system.
The evolution of OBD systems can be split into two distinct phases based on the type of system popular at the
time. These are described in more detail below:
1 OBD-I
The first OBD systems were proprietary in nature, so they differed between manufacturers. Prior to 1990, the
codes, systems and information gathered by each OBD system varied widely from manufacturer to manufacturer.
While these systems proved useful, they were unnecessarily complex for technicians to work with — technicians
had to purchase a new tool and cable for every vehicle make or had to invest in a scanner that had an array
of adapter cables for multiple vehicle makes. Due to the proprietary nature of these systems, users were often
forced to go to dealership technicians to diagnose issues.
The push to standardize OBD systems didn’t start until the California Air Resources Board mandated OBD
capability in all cars in 1991. The board didn’t issue any standards for these OBDs, however, causing increased
difficulties for vehicle manufacturers and users. When the OBD-II standard was implemented in 1994 in response
to this need, all previous forms of OBDs were retroactively classified as OBD-I systems.
2 OBD-II

In 1994, the California Air Resources Board issued OBD-II as a set of standards for OBD systems for all vehicles
sold in California. This mandate was officially implemented in the 1996 model year and has been in use ever
since. The Society of Automotive Engineers and the International Standardization Organization, known as the
SAE and ISO, respectively, also issued standards for how digital information should be exchanged between
ECUs and a diagnostic scan tool. The EPA further expanded the use of OBD-II following the passage of the
Clean Air Act — as of 2001, 33 states and local areas require regular vehicle inspections to ensure that they meet
emission standards, and OBD-II systems are a key part of these inspections.
The OBD-II standards are characterized by several requirements, including the following:
OBD-II Connector: Modern OBD systems use standardized DLCs called Type 2 Connectors. This allows
technicians to use the same cable, a Type 2 Cable, to access the digital communications stored in the OBD
system through a port. The location of this port is not standard, but it is usually located under the dashboard on
the driver’s side of the vehicle.
System Monitoring: The EPA requires that OBD systems monitor problems that affect vehicle emissions. Many
systems look into other metrics that are not included in this scope as a way to make it easier to find and fix vehicle
issues, but the minimum requirement is set.
With this set of standards in place, technicians can service a wider variety of vehicles quickly and easily without
the need for manufacturer-specific tools.

321
CITS : Automotive - Mechanic Diesel - Lesson 98 - 103 CITS : Automotive - Mechanic Diesel - Lesson 98 - 103