On-Board Diagnostics - Pt 1
Posted by murmini Fri, 19 Jan 2007 05:45:35 GMT
The On board Diagnostics, or OBD, is yet another computer system built into all vehicles since the late 80s to assist in the monitoring of the engines performance in the interest of emissions monitoring. Well that's how it started anyway.
In the late 80's, with California pushing hard for some standardization of emission controls, the preliminary OBD standard was adopted by manufacturers to make sure that vehicles' emission were not exceeding guidelines established to meet the Clean Air Act. About this time the Society of Automotive Engineers recommended a standard connector and a number of standardized test signals.
Starting in 1996, all cars sold in California had to provide some basic engine diagnostics to make certain that they met the ever increasing demands of the California Air Resources Board or CARB standards. So between the SAE and CARB and the automobile manufacturers, a standard was adopted that provided a common connector on all vehicles and a set of standardized tests that could be run to make certain that the vehicle was meeting the manufacturer's standards.
Later, OBD-II added another layer of connectivity between the vehicle's on-board computer and peripheral test equipment for fault location and analysis. Still not completely common across all types, OBD-II has three different communications protocols. A high level view of the field is as follows: GM cars and light trucks use SAE J1850 using variable pulse width modulation or VPWM, Chrysler, all European - including MINI - and most Asian vehicles use ISO 9141 protocols and Fords use SAE J1850 pulse width modulation PWM. Each of these define the communications protocol, or the type of data signals that carry the information on the OBD connector.
Here is a diagram of all of the possible pin-outs on a generic OBD connector. On the MINI, the connection is a part of the D-Bus and has direct connections via this bus into the Engine Control Module, the Anti-Locking Brake System, the Electro-Hydraulic Steering, the Automatic Stability Control module and the Dynamic Stability Control system if fitted. If the vehicle has automatic transmission there is also connection to the Gearbox Interface Unit.
MINI uses the ISO 9141-2 protocol. This protocol has a data rate of 10.4 kbaud, and is similar to a standard RS-232 connection. Signaling between the OBD-II connector and the various computer systems in the MINi are by way of a UART, pronounced u-art, and short for universal asynchronous receiver-transmitter. This is a computer component that handles asynchronous serial communication. All computers contains a UART chip to manage the serial ports, and to send and receive the data in and out over these ports. Again, its a matter of adopting standards so thing can 'speak' the same digital language.
While OBD started life as a simple diagnostic tool for checking emissions, today's OBD-II is much more sophisticated. There are many sensors within the vehicle that report back to the Engine Control Module and subsequently the status of these sensors can be viewed with the correct diagnostic equipment, by way of the OBD connector. These sensors measure a number of dynamic functions in the following primary categories: Air Management, Fuel Management, Ignition Management. Emission Management, and Performance Management. Serving both the On Board Diagnostics and the Engine Control Module, these sensors give constant updates to the health and well being of the engine.
The data from these sensors constantly modify the way the engine is running by altering air/fuel ratios, ignition timing, and torque management, based on the sensor inputs concerning throttle opening, coolant and engine temperature, vehicle speed and of course, emission values. All this is managed by the Engine Control Module and it is here where the computations are made based on a number of tuning conditions provided in look-up tables.
If at any time the vehicle emissions exceed the Federally mandated criteria, the OBD will illuminate Malfunction Indicator Lamp on the cockpit display and store the Diagnostic Trouble Code so that it can be read by a remote diagnostic device and aid in determining what is causing the problem. Any malfunction that occurs in the four categories have OBD-II standard codes that can be read by the analyzer.
The codes are arranged in a specific format with the 1st digit identifying it to be a powertrain, body or chassis problem, the 2nd code indicates if its an SAE standard code or one specific to MINI, the 3rd code determines where the problem exists either in the total system, air/fuel, ignition, emission, vehicle speed/idle control, ECM or transmission and the 4th and 5th codes specify individual circuits or faulty components.
P = powertrain issue
0 = using an SAE standard code
3 = related to an ignition system misfire
04 indicating the misfire is on cylinder #4.
In addition to activating a dashboard warning light, these "P-codes take a snapshot of the vehicles performance at the moment when the fault first occurred. This snapshot minimally contains information about engine load, RPM, short and long term fuel trim, speed, coolant temps, intake manifold pressure, fuel pressure and the diagnostic trouble code. In addition to the standard SAE codes, MINI has implemented some additional BMW/MINI specific codes that can be read by their diagnostic scan tools.
Below is a list of the main sensors and how they function:
COOLANT SENSOR - used to monitor the temperature of the engine coolant. Its resistance changes in relationship to the changing coolant temperature. The sensor on the MINI is located in cylinder head and tells the computer the engine temperature.
OXYGEN (O2) SENSOR - this is a crucial sensor involved in the fuel mixture feedback control loop. There are two in the MINI engine, one located either side of the exhaust catalytic converter. They look at how much unburned oxygen is contained in the exhaust gases. Again, the voltage coming from these sensors is proportional to the amount of unburned oxygen in the exhaust. The 'post' catalytic sensor is used to determine the efficiency of the catalytic converter. When the fuel mixture is rich, most of the oxygen is consumed during combustion so there is little unburned oxygen in the exhaust.
The signal coming from the primary oxygen sensor is actually vacillating between high and low. When it reads "lean" the PCM increases the 'on-time' of the injectors to make the fuel mixture go rich. Conversely, when the sensor reads "rich" the PCM shortens the on-time of the injectors to make the fuel mixture go lean. This causes a rapid back-and-forth switching from rich to lean and back again as the engine is running. These even waves result in an "average" mixture that is almost perfectly balanced for clean combustion. When the oxygen sensor is controlling the fuel sent to the injectors, its referred to as 'closed loop' operation. when specific demands like wide open throttle, or engine braking occurs, this changes to "open loop" operation.
TEMPERATURE AND MANIFOLD (T-MAP) SENSOR - Is located in the air stream next to the electronic throttle valve. Measuring the pressure and the temperature of airflow into the engine, it is capable of determining the volume of air being used by the engine as well as the engine load and the intake temperature (outside air temperature). In the Cooper S, it determines the pressure differential across the supercharger in cooperation with the MAP sensor.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR This sensor is mounted on the left side of the cylinder head and measures intake vacuum from a line connected to the supercharger supply ducting. It changes voltage as intake pressure changes. The computer uses this information to measure engine load so ignition timing can be advanced and retarded as needed. It performs essentially the same job as the vacuum advance diaphragm on an old fashioned mechanical distributor that changed the ignition timing by rotating the distributer under heavy manifold vacuum. WHen the ignition is turned on, it measures atmospheric pressure. Once the engine is running, it measures the absolute manifold pressure which is the barometric air pressure minus the vacuum created by the pistons.
THROTTLE POSITION SENSOR - Mounted on the electronic throttle body, there are actually two sensors here, a primary and a backup. As the MINI uses a 'drive by wire' throttle, (see here) this approach provide a fail-over in the event of a problem. The throttle position sensor (TPS) changes resistance as the throttle opens and closes. The computer uses this information to monitor engine load, acceleration, deceleration based on throttle position. The sensor's signal is used by the to compute how much to enrich the fuel mixture during acceleration, as well as to retard and advance ignition timing.
CRANKSHAFT POSITION SENSOR - Located in front of the crankcase at the flywheel end of the engine the crankshaft position sensor serves essentially the same purpose as the ignition pickup in an electronic distributor. It uses a Hall effect circuit and "sees" the presence of a reluctor ring bolted to the end of the crankshaft and as it rotates, there are 58 targets and two missing "teeth" on the ring. It generates a signal that the ECU can use to determine the position of the crankshaft and the piston position in number one cylinder. This information is necessary to control ignition timing and for the accurate operation of the fuel injectors.
CAMSHAFT POSITION SENSOR - This is located in the front of the cylinder head just under the valve cover and again uses a Hall effect circuit with a reluctor ring to determine the exact position of the camshaft. Viewed in correlation with the crankshaft position sensor this is used to determine injector timing in both the open and closed loop and has a clever design that controls the four injectors completely individually. So that if one injector circuit fails, the other three can continue to provide power, albeit limited.
KNOCK SENSOR - The knock sensor detects engine vibrations that indicate incorrect fuel detonation is occurring. If this occurs, the computer can momentarily retard the ignition timing. It is actually an acoustic or vibration sensor like a small microphone and listens for unusual vibrations typical of partially or spontaneously ignited fuel - called pinging. It is bolted to the crankcase just below the intake manifold.
There are many other sensors at play in this complex and highly evolved system. Many feed other subsystems and their appropriate control modules which in turn feed data back to mission central - the engine control module. Pt 2 of this OBD article will look at the signals coming from the OBD connector and evaluate their contribution to a smoothly running, well tuned and happy MINI engine.