Countertalk: Knowledge Building: OBD II Tech Overview
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While OBD II has been with us for more than a decade, there continues to be a misunderstanding of what it is, and what it is not.
It is a “standard” method for communicating which emissions system component or components might be failing, causing vehicle emissions to rise. What it is not is a method of spoon-feeding a required repair to the technician.
OBD II differs from its predecessor OBD I which required a code to be set whenever a system condition resulted in an emissions or driveability fault.
With OBD II, the deterioration of the emission system sets a Diagnostic Trouble Code (DTC) and the Malfunction Indicator Light (MIL) is illuminated to identify the need for service. The deterioration of the system is usually determined if the emissions, fueling, etc., exceeds the federal limits by 1.5 times.
While there is a standardization of sorts in terms of the codes set, there is less of this in terms of how this information is communicated. Different manufacturers use different communications protocols, made all the more complicated by the fact that each references a standard.
GM cars and light trucks generally use SAE J1850 Variable Pulse Width Modulation (VPW), Ford uses SAE J1850 Pulse Width Modulation (PWM), while Chrysler, European, and most Asian imports use the ISO 9141 communication protocol. There are some variations among domestic badged imports such as the Cadillac Catera, a German Opel derivative, for example, which employs the European ISO 9141 protocol.
A quick method to determine which protocol is in use is to check the connector which is standardized, will be somewhere within three feet of the driver’s position, and accessible without tools.
On the 16-pin connector, metallic contacts on pins 2, 4, 5, and 16, but not 10, indicate the J1850 VPW protocol is in use whereas 2, 4, 5, 16 and 10 are used when J1850 PWM is utilized; metallic contacts on pins 4, 5, 7, 15, and 16 flag the international ISO 9141-2 standard protocol.
PINDescription:
1Specific to manufacturer
2J-1850 BUS+
3Specific to manufacturer
4Vehicle ground (GND)
5Signal ground (GND)
6Controller Area Network (CAN) High (J-2284)
7ISO 9141-2 K-Line
8Specific to manufacturer
9Specific to manufacturer
10J-1850 BUS-
11Specific to manufacturer
12Specific to manufacturer
13Specific to manufacturer
14Controller Area Network (CAN) Low (J-2284)
15ISO 9141-2 L-Line or 2. K-Line
16+12V (+12V Battery)
Perhaps the most important part of the OBD II method of communication is the standardization of the codes generated. While there remains work to be done to be truly standard across all manufacturers, and notwithstanding the fact that the inability for the aftermarket in Canada to obtain all manufacturer-specific scan tools and access to repair and calibration information, the first order standardization of the DTC menu does help to speed repairs.
OBD II regulations require that test modes and DTCs be standardized between all automotive manufacturers. All OBD II DTCs will use five-digit designator codes, as opposed to the two-digit codes used by many OBD I systems.
Each DTC is directly related to a diagnostic test. The Diagnostic Management System sets DTCs based on the failure of the tests during a trip or trips. Certain tests must fail two consecutive trips before the DTC is set. The following are the five types of DTCs and the characteristics of those codes.
Type A
* Emissions related.
* Requests illumination of the MIL on the first trip with fail.
* Stores a History DTC on the first trip with a fail.
* Stores a Freeze Frame on the first trip with a fail (if empty).
* Stores a Fail Record.
* Updates the Fail Record each time the test fails each ignition cycle.
Type B
* Emissions related.
* “Armed” after one trip with a fail.
* “Disarmed” after one trip with a pass.
* Requests illumination of the MIL on the second consecutive trip with a fail.
* Stores a History DTC on the second consecutive trip with a fail (the DTC is armed after the first fail).
* Stores a Freeze Frame on the second consecutive trip with a fail (if empty).
* Stores Fail Record when the first test fails (not dependent on consecutive trip fails).
* Updates the Fail Record the first time the test fails each ignition cycle.
Note: Some special conditions apply to misfire and fuel trim DTCs.
Type C
* Non-Emissions related.*
* Requests illumination of the Service Lamp (not the MIL) or the service message on the DIC on the first trip with a fail.
* Stores a History DTC on the first trip with a fail.
* Does not store a Freeze Frame.
* Stores Fail Record when test fails.
* Updates the Fail Record the first time the test fails each ignition cycle.
Type D
* Non-Emissions related.*
* Does not request illumination of any lamps.
* Stores a History DTC on the first trip with a fail.
* Does not store a Freeze Frame.
* Stores a Fail Record when test fails.
* Updates the Fail Record the first time the test fails each ignition cycle.
* Failure condition does not cause emissions greater than 1-1/2 times FTP standards.
Type X
* Diagnostics that are coded into the software, but are disabled and will not run. This is mostly for export vehicles that do not require MIL illumination or DTC storing.
* Some domestic vehicles use X DTCs.
* Even though the codes are not stored, the DTC chart can still be used as a resource when diagnosing problems with systems that are associated with the Type X DTCs.
Many of the DTC tables include a functional check of the system that may pinpoint a problem. However, the tables are specifically designed for use only when a DTC is set. As this is not always the case, a thorough understanding of the normal operation of the system being diagnosed is necessary.
A limited number, usually five or less, of Fail Records can be stored. Each Fail Record is for a different DTC. It is possible that there will not be Fail Records for every DTC, if multiple DTCs are set.
It is important to note that not all Diagnostic Trouble Codes illuminate the Check Engine light. Sometimes the vehicle needs to see an error a few times to ensure the problem isn’t just a temporary glitch. Other times the computer determines the problem isn’t emission related and the driver is not notified. Therefore, periodic checking of the Diagnostic Trouble Codes helps detect problems early before becoming more costly to repair. For instance, the computer may detect a problem with the automatic transmission but since the failure is not emission related the computer may not illuminate the Check Engine light. The transmission failure may also be difficult for the driver to detect just by driving the vehicle. Only a scan tool can reveal that a problem exists and the cause.
This is a point not often appreciated; experienced technicians know that a DTC may point to the failure, or be the consequence of another, initial failure. What follows–it is hoped–is an organized diagnostic procedure. Failure to employ one can result in ineffective repairs.
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