Page 365 - Understanding Automotive Electronics

2735 | CH 10 Page 352 Tuesday, March 10, 1998 1:27 PM

10 DIAGNOSTICS

automotive emission control regulations, has proposed a new, relatively severe
requirement for on-board diagnosis that is known as OBDII (on-board
diagnosis II). This requirement is intended to ensure that the emission control
system is functioning as intended.
Automotive emission control systems, which have been discussed in
Chapters 5 and 7, consist of fuel and ignition control, the three-way catalytic
converter, EGR, secondary air injection, and evaporative emission. The OBDII
regulations require real-time monitoring of the health of the emission control
system components. For example, the performance of the catalytic converter
must be monitored using a temperature sensor for measuring converter
temperature and a pair of EGO sensors (one before and one after the converter).
Another requirement for OBDII is a misfire detection system. It is known
that under misfiring conditions (failure of the mixture to ignite), exhaust emissions
increase. In severe cases, the catalytic converter itself can be irreversibly damaged.
The only cost-effective means of meeting OBDII requirements involves
electronic instrumentation. For example, one possible means of detecting
misfire is based on measurements of the crankshaft instantaneous speed. That
speed fluctuates about the average RPM in response to each cylinder firing
event. Misfire can be detected in most cases by monitoring the crankshaft speed
fluctuations using some relatively sophisticated electronic signal processing.
Off-board Diagnosis
An alternative to the on-board diagnostics is available in the form of a
service bay diagnostic system. This system uses a computer that has a greater
diagnostic capability than the vehicle-based system because its computer is
typically much larger and has only a single task to perform—that of diagnosing
problems in engine control systems.
Special-purpose digital An example of a service bay diagnostic system is General Motors’ CAMS
computers are coming (Computerized Automotive Maintenance System). Although the system
into use in service bay discussed here is essentially obsolete, it is at leats representative of this level of
diagnosis systems. diagnosis. The GM-CAMS used an IBM PC/AT computer that had
considerable computational capability for its time. Its memory included 640K
of RAM, 1.2 million bytes on a 5.75 inch diskette drive and 20 million bytes
on a fixed disk drive. This system was capable of detecting, analyzing, and
isolating faults in late-model GM vehicles that are equipped with a digital
engine control system. This system, commonly called the technicians’ terminal,
has a modem equivalent that operates in essentially the same way as the CAMS.
The technicians’ terminal is mounted on a rugged portable cart (Figure
10.12) suitable for use in the garage. It connects to the vehicle through the
assembly line data link (ALDL). The data required to perform diagnostics are
obtained by the terminal through this link. The terminal has a color CRT
monitor (similar to that of a typical home computer) that displays the data and
procedures. It has a touch-sensitive screen for technician input to the system.
The terminal features a keyboard for data entry, printer for hard copy output,

352 UNDERSTANDING AUTOMOTIVE ELECTRONICS