140 Auto Engine Performance and Driveability
Data Link Connector Location
and Design
The OBD II protocol requires manufacturers to place
the data link connector in a location where it is out of
visual sight, but is easily accessible from the driver’s seat.
The protocol also requires the use of a standardized, 16-pin
data link connector, Figure 7-38. The standardized data
link connector and codes allows the use of a generic scan
tool to read and clear the trouble codes on all vehicles.
Note: Some OBD I equipped vehicles use
the 16-pin data link connector. However,
these vehicles are not ODB II compliant.
Failure Types
Unlike OBD I systems, OBD II systems can determine
the type of problem in a circuit. These failure codes fall into
one of four categories:
p General-circuit failure.
p Low-input failure.
p High-input failure.
p Range/performance failure.
A general-circuit failure is caused by disconnected or
damaged wires and connectors, grounds and shorts, or a
component that is constantly operating out of range. This
type of circuit failure is often the easiest to locate, since in
almost all cases it sets a code.
A low-input failure is set when the ECM receives a
weak or abnormally low voltage, current, or operational
signal. This failure is caused by high resistance, poor elec-
trical connection, or a contaminated or defective sensor.
A high-input failure is caused by a sensor failure or
mechanical fault. This type of signal supplies the ECM with
excess voltage or current. A false signal may also be trans-
mitted to the ECM.
A range/performance failure occurs when a sensor is
reading slightly lower or higher than normal. This can be
due to a contaminated sensor, sensor wear, or poor electri-
cal connections.
Freeze Frame and Failure Record
When a type A or B trouble code is set in the ECM
memory, certain vehicle operations at the time the code
is set are stored in memory. This information is commonly
referred to as the freeze frame record. Most ECMs store the
failure information for only one code that sets a DTC and
illuminates the MIL. For example, if a second type B DTC
occurs, this information is not updated. However, if a type
A fuel trim or misfire DTC occurs, it overwrites the freeze
frame information stored for the type B DTC.
Some ECMs have a failure record that can be accessed
in the case of multiple DTCs. It is updated whenever a code
is set. ECMs can store several failure records. A failure record
is stored anytime a type B, C, or D code is set. Type A codes
are not stored in the failure record as the information for
these codes is stored in the freeze frame record.
Summary
The most important advance in the areas of driveabil-
ity, fuel economy, and performance is the development of
the on-board computer to control the fuel and ignition sys-
tems. These systems are composed of electrical, electronic,
and mechanical devices that control engine operation. On-
board computers also control parts of the emission control
system, drive train, and accessory equipment such as the
air conditioner. Most vehicles have several computers,
connected by a bus or a controller area network.
The basic components of the computer control system
are the computer (ECM), inputs, and outputs. The computer
receives electrical inputs, decide on the actions to be taken,
and provides instructions to output devices.
There are two kinds of memory in the computer: fixed
and volatile. The fixed memory circuits are installed at the
factory and cannot be changed. The volatile memory is
Figure 7-38. Sixteen-pin data link connector. This connector is
used on all OBD II vehicles. Manufacturers usually install sensor
leads for other ECMs to the terminals assigned “manufacturer’s
discretion.” Terminals 2, 4, 5, 7, 10, 15, and 16 are assigned by
SAE and cannot be changed. (Chrysler)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
*Pins 2, 7, 10, and 15 are used for external communications.
On some vehicles, these pins may have
alternate assignments.
Manufacturer’s Discretion
J1850 Bus + Line*
Manufacturer’s Discretion
Chassis Ground
Signal Ground
Manufacturer’s Discretion
ISO 9141 K Line*
Manufacturer’s Discretion
Manufacturer’s Discretion
J1850 Bus-Line*
Manufacturer’s Discretion
Manufacturer’s Discretion
Manufacturer’s Discretion
Manufacturer’s Discretion
ISO 9141 K Line*
Battery Positive Voltage
Terminal General Description
1
8
9 16