Frequently Asked Questions
- Frequently Asked Questions
- What is OBD II?
- Where is the connector? What does it look like?
- What is CAN?
- What is OBD2 data?
- Which electrical OBD2 interface does my vehicle have?
- More frequently asked questions
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- Q. What is OBD II?
- A. OBD stands for On Board Diagnostics. The œII signifies the version of the OBD specification being used. OBD’s roots go all the way back to early 1970s. In order to combat smog problems in LA, the EPA started requiring car manufacturers to provide emission control systems on the vehicles they manufactured. In 1988 a set of standards was developed by the SAE (Society of Automotive Engineers) including a standard connector plug and set of diagnostic test signals. This was OBDI.
- OBD II is a more sophisticated standard that was introduced in the mid-90s. It monitors parts of the chassis, body and accessory devices, as well as the diagnostic control network of the car. On-Board Diagnostic systems are in all 1996 and newer cars and light trucks
- Q. Where is the connector? What does it look like?
- A. The OBDII connector is most frequently located on the driver’s side at the bottom of the dash.
- The pins are used in the following way:
- Pin 2 – J1850 Bus+
- Pin 4 – Chassis Ground
- Pin 5 – Signal Ground
- Pin 6 – CAN High (J-2284)
- Pin 7 – ISO 9141-2 K Line and ISO/ DIS 14230-4
- Pin 10 – J1850 Bus
- Pin 14 – CAN Low (J-2284)
- Pin 15 – ISO 9141-2 L Line and ISO/ DIS 14230-4
- Pin 16 – Battery Power
- If your connector has extra pins, don’t worry. Manufacturers can use them as they see fit.
- Q. What is CAN?
- A. CAN is short for Controller Area Network. CAN is the newest addition to the OBDII standard and will be the required electrical interface for all vehicles beginning in the model year of 2008. AutoTap supports all OBDII interfaces including CAN.
- Q. What is OBD2 data?
- A. The generic OBD2/ODBII data is used extensively in modern engine control. The information below attempts to demonstrate some of the more common uses for the data.
Please note that this information is presented without warranty of accuracy. Although we will continue to strive to provide accurate, complete information, all use of information on these pages is at your own risk. This information attempts to pertain to a large number of vehicles from different manufacturers, and some variations may be omitted for overall clarity.
- Calculated Engine Load
- California Air Resource Board (CARB) defines this as airflow divided by peak air flow. Peak air flow may be adjusted for altitude via barometric means if available. For diesel engines, the number is current output torque divided by peak output torque at current rpm. We have noted that vacuum leaks can have dramatic effects on this value.
- Coolant Temperature
- Modern engine performance depends heavily on the accuracy of the sensor monitoring coolant temperature. This sensor is often independent of the measurement reported on the dash. It should read close to ambient temperature at startup, and run to between 180°F-220°F after warmup. Injector pulse width is heavily dependent on this sensor, causing engines to run very rich at startup (remember the manual choke?). Fuel enrichment is usually tapered off around 150°F
- Fuel Trim
- Fuel trim refers to adjustments being made dynamically to the base fuel table to get the proper ratio of fuel to air. Short term fuel trim refers to adjustments being made in response to temporary conditions. Long term fuel trim is used to compensate for issues that seem to be present over a much longer period. Fuel trims are expressed in percentages; positive values indicate lean (add fuel) and negative values indicate rich (subtract fuel). Fuel trim banks refer to the cylinder banks in a V style engine. Cylinder #1 is always in bank 1. Fuel trim is generally calculated by using a wide set of data values, including front O2 sensors, intake air temperature/pressure (or the more elegent air mass sensor), engine (coolant) temp, anti-knock sensors, engine load, throttle position (and change in throttle position), and even battery voltage can effect fuel trim. Long term fuel trim generally should not exceed +- 10%
- Manifold Pressure
- Manifold Air (or Absolute) Pressure (MAP) sensor measures intake vacuum, used to calculate engine load and timing advance. It may also contribute to fuel trim.
- O2 Sensors
- O2 sensors monitor exhaust gases for the presence of oxygen. Heated sensors are commonly referred to as HO2S, which allows sensors to begin giving valid data much earlier in the warm up cycle. Most sensors output a voltage between 0 and 1.0 volts (usually .10 to .90), 0v – .45v for lean conditions, and .55v – 1.00v for rich conditions. Upstream sensors are primarily used to adjust engine air and fuel flow, while downstream (of the catalytic converter) sensors are used to monitor the effect of the catalytic converter. Sensors do not usually run at constant voltages, rather oscillate rapidly (at least once every few seconds) in a nice sine wave. The frequency is referred to as switching frequency, or as switches. The rear sensor should display a much slower switching frequency than the front sensor, the difference indicating the catalyst capability. These sensors are invaluable in the diagnostic effort, and can indicate conditions ranging from fuel delivery issues, vacuum leaks, to engine oil overfill conditions.
- Q. Which electrical OBD2 interface does my vehicle have?
- A. Although OBD2 is a standard, each manufacturer is free to implement it in their own way, so as to not stifle innovation. (I can’t tell if I am being sarcastic here) If you are going to build your own converter, or buy a converter that isn’t universal, you must determine which of the 3 or 4 types of OBD2 electrical interfaces is in your vehicle.
- Q. Where is the OBD2 interface in my vehicle?
- A. Usually under the driver side dash.
- Q. What data does OBD2 provide?
- A. Theoretically, the OBD2 could provide any data that any of the sensors are capable of reporting. However, the manufacturers do not seem especially anxious to publish request codes specific to their vehicles to get this data. The OBD2 spec mandates a functional addressing mode and the request codes for about 30 parameters directly related to emissions. Support for most of these “generic” data points is common, in both the tools and the vehicles.
- Q. What is included in the “generic” OBD2 data?
- A. Generic data includes calculated engine load, coolant temp, data about short and long term fuel trim (whether the engine is adjusting lean or rich), fuel pressure, manifold air pressure, engine RPM, vehicle speed, ignition advance, intake air temperature, air flow, throttle position, and O2 sensor data. The ability to get/clear trouble codes, and the above data values present when the trouble code is set.
- Q. What is “extended” OBD2 data?
- A. This includes any data that is not in the above “generic” set. Almost every manufacturer provides many more data points that their own proprietary scantools are capable of reading.
- Q. My station has an OBDII scan tool. Can I use the same scan tool to do the OBDII test?
- A. No. You must conduct the OBDII test with the equipment that is provided. You can use your current OBDII scan tool for other diagnosis and repair work.
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