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When it comes to data bus diagnostics, a lot can be done with just an OBDII connector breakout box, meter and a scope.
The OBDII connector is more than 20 years old. 1996 was the first year all vehicles sold in the U.S. had to have this standard 16-pin connector. Most technicians now take this port for granted, and some do not even remember the days when a scan tool was more about the cables and ID cards than the tool itself.
The Society of Automotive Engineers (SAE) is an organization for engineers to develop standards for vehicles of all kinds. The mission of SAE International is to enable voluntary consensus on standards development. If you see the letter “J” followed by four numbers, chances are it is an SAE standard for vehicles.
One of the greatest benefits to come from SAE was the standardization of the language concerning diagnostics and diagnostic connectors. SAE J1930/ISO 15031-2 standardized system diagnostic terms, abbreviations and acronyms, and SAE J1962 standardized the On-Board-Diagnostic (OBD) port or Diagnostic Logic Connector (DLC). The invocation of these standards means that the EVAP system is called an EVAP system by all of the OEMs, and that they have the same diagnostic connector in the same location.
Unlike previous diagnostic connections, the OBDII 16-pin trapezoidal connector was based on an SAE standard. Automakers all had to comply with this standard or they wouldn’t be able to sell a car in 1996. The truly revolutionary part was how the new language used between the scan tool and the vehicle standardized the protocols for emissions information.
For two decades now, the DLC has remained under the dash and changed the aftermarket in ways people never anticipated. Imagine a world without parts stores scanning for codes. This would be a blow to oxygen sensor and EGR valve manufacturers. These parking lot diagnostics have changed the way people perceive technicians and diagnostic time.
The SAE J1962 OBD connector is more than just the location a scan tool plugs into. These pins are where networks, power and sensor (signal) grounds can be accessed by the technician. It is also the connection point for serial data networks. These connections are what scan tools use to “talk” to the vehicle.
Some pin assignments are standardized across all vehicles. Some pin assignments are left up to the manufacturer. While some pin assignments will change, the shape of the connector will stay the same.
PINS 2+10: SAE J1850
SAE J1850 comes in two forms: a single wire and a dual wire. J1850 is a class B communications network interface for class 2 communications — what GM calls their single wire J1850. Chrysler calls their single wire J1850, PCI. Ford has a two-wire J1850 standard that is a SCI bus. ISO 9141 (K-Line) is an Asian communication bus similar to J1850.
PINS 1+14: SAE J2284/ISO15765-4
If you see a code with these standards, you are dealing with a CAN bus system.
SAE J2534 refers to recommended practices for Pass-Thru reprogramming. This standardizes practices so universal re-programmers can be used to reprogram certain modules. This can be performed through multiple pins, and some of the protocols are model specific.
Probing and Diagnostics
When it comes to data bus diagnostics, a lot can be done with just an OBDII connector breakout box, meter and a scope. A breakout box is an essential tool that plugs into the DLC and has a connector for a scan tool. The box has meter connections for the pins and LED lights that show activity in the circuit.
With a breakout box, it is possible to back-probe female terminals in the DLC without any damage. Just by plugging it in, it becomes a diagnostic tool by telling if there is power and ground, and even if there is bus activity. This can be extremely helpful if you are trying to diagnose a parasitic battery draw caused by a module that will not go to sleep.
If you attach a meter to a serial data bus in the breakout box (set to DC voltage), the meter will jump all over the place if the bus is active. If you have a meter with a min-max feature, you can see maybe a 0-volt minimum and a 5- or 6-volt maximum. You won’t be able to determine if the HVAC module is telling ECM to turn on the A/C compressor, but you will be able to check if the bus is active.
With a scope, it is possible to see the voltage toggling as modules are communicating on the bus. On this bus, it is possible to see two signals with different voltages. One signal is 0 and 5 volts, and the other 0 and 7 volts. The 5-volt pattern is the BCM responding to the scan tool. The 7-volt pattern is the scan tool responding. Don’t be alarmed, you may have two different levels. Sometimes the scan tool’s voltage is a little higher on the output. But, the important thing is that there are consistent square waves.
As the vehicle is powered down by turning the key off, and the scan tool is backed out of the datastream, you will see less activity on the scope. You can watch the bus power down and look for signs a module is staying awake that could run the battery down.
It is a good diagnostics practice to look for the presence of a signal and the quality of a signal with either a meter or scope. These tools and a DLC breakout box are essential tools in diagnosing and finding shorts to power and ground.
The FBI Issues Warning About OBDII Devices
The Federal Bureau of Investigation (FBI) recently issued a public service announcement advising both consumers and manufacturers to maintain awareness of potential cybersecurity threats pertaining to aftermarket devices:
“All modern vehicles feature a standardized diagnostics port, OBDII, which provides some level of connectivity to the in-vehicle communication networks. This port is typically accessed by vehicle maintenance technicians, using publicly available diagnostic tools, to assess the status of various vehicle systems, as well as to test emissions performance. More recently, there has been a significant increase in the availability of third-party devices that can be plugged directly into the diagnostic port. These devices, which may be designed independent of the vehicle manufacturer, include insurance dongles and other telematics and vehicle monitoring tools. The security of these devices is important, as it can provide an attacker with a means of accessing vehicle systems and driver data remotely.
“While in the past accessing automotive systems through this OBDII port would typically require an attacker to be physically present in the vehicle, it may be possible for an attacker to indirectly connect to the vehicle by exploiting vulnerabilities in these aftermarket devices. Vehicle owners should check with the security and privacy policies of the third-party device manufacturers and service providers, and they should not connect any unknown or untrusted devices to the OBDII port.”
The FBI advised that technicians should report suspected hacking attempts and perceived anomalous vehicle behavior that could result in safety concerns to the NHTSA by filing a Vehicle Safety Complaint at www.odi.nhtsa.dot.gov.
Article courtesy Underhood Service.