Modern fuel injection systems have many sensors and switches installed at key locations on the engine and transmission to allow the engine (and transmission) computer to operate in the ways they are intended. These sensors and switches are used to deliver the proper amount of fuel, the correct amount of spark timing, the most appropriate transmission gear, and so on. They also provide important information that is valuable for problem diagnosis. The process by which the engine and transmission controller(s) process sensor data and report errors is called on-board diagnostics (OBD). OBD has been used with GM engine controllers since the 1980s. Since that time, OBD has improved to a robust diagnostic system.
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Assembly Line Diagnostic Link
The OBD system of the first multi-port V-8 electronic fuel injection system, or Tuned Port Injection, was implemented into the 1985 ECM found in the Corvette, Camaro, and Firebird. This basic system uses sensor data to determine if unusual operating behavior exists. When the ECM determines a malfunction, it applies a ground to the service engine soon (SES) lamp in the instrument cluster to indicate the need for service. SAE recommendations have since renamed the SES lamp to the MIL for standardization among vehicle manufacturers.
When the MIL has illuminated, the repair technician must be able to see what the ECM has detected as a malfunction. The malfunction is logged in the ECM as a trouble code. A physical method for retrieving a trouble code is to connect a scan tool to the assembly line diagnostic link (ALDL) connector beneath the dashboard near the steering column. The scan tool displays the trouble code number and the repair technician can then look up the diagnostic procedure to troubleshoot through the problem.
For example, the ECM may log a “Code 33.” The GM service manual diagnostic description informs the repair technician that the MAF sensor signal voltage was too high during an engine operating condition that should not produce such high voltage. In other words, the MAF sensor reported an amount of incoming air that General Motors preconfigured the ECM to determine as improbable for the engine speed and throttle angle at which the trouble code was set.
The GM service manual diagnostic procedure leads the repair technician through a series of tests to determine why this occurred and may point to a faulty MAF sensor. This early OBD system offers a way to flash (illuminate) the MIL to represent the numerically named trouble codes that are set.
In the 1980s, automotive manufacturers had widely implemented OBD into ECMs. The OBD-I systems were used with annual California emissions testing, but without OBD standardization, the emissions testing program was not as effective as it is today.
The precursor to the standardized OBD-II system was implemented in select 1994 and 1995 GM vehicles. Although not an official name, this OBD system is now referred to as OBD-1.5 because it introduced elements of the OBD-II system. Enthusiasts recognize the 1994–1995 LT1-equipped vehicles as OBD-1.5. This OBD hybrid continues to use some of the same naming conventions as the early OBD systems, but also implements some of the newer OBD-II naming conventions. For example, with OBD-I, a “Code 15” indicates that the ECT sensor signal is too high. With OBD-II, a “P0118” indicates that the ECT sensor signal is too high.
Trouble code retrieval from OBD- 1.5 systems requires a scan tool.
In 1996, OBD-II was largely standardized and became a requirement for production automobiles. Benefits include improved diagnostics with many more parameters, ECU programming through the vehicle’s diagnostic connector, and bi-directional control of certain on-board modules.
OBD-II Data Link Connector
The SAE J1962 OBD-II standard calls for a standardized DLC that is typically located beneath the dashboard and near the steering column. The DLC is used to retrieve DTCs, retrieve real-time data from all networked ECUs, bi-directional diagnostic communication, and ECU programming. For the purposes of Gen III fuel management discussion, the DLC is pinned as follows. Unused connector cavities may be used for vehicle specific functions.
Diagnostic Trouble Code Retrieval
The standardization of OBD-II allows for DTC retrieval with any off-the-shelf OBD-II scanner. DTCs may be present within multiple onboard ECUs and are distinguished by a prefix character followed by four digits. The PCM processes and may set engine- and transmission-related DTCs. Other modules, such as the BCM, are responsible for processing non-powertrain–related DTCs. All off-the-shelf OBD-II scan tools have the capability to retrieve DTCs and clear DTCs from memory. In some cases, and through certain scan tools, freeze-frame data can be retrieved to see operating parameter that occurred when a DTC was set.
Real-Time Data Monitoring
OBD-II scan tools display realtime data by querying (request and response) on-board ECUs. Although scan tool interfaces vary, a repair technician specifies a parameter ID (PID) and the scan tool sends a request through the DLC for that PID (or list of PIDs). The on-board ECU containing that PID responds with the data values for the requested PID. While PIDs are technically codes, scan tools identify PIDs by their representative name, such as “Engine Speed.” Most off-the-shelf scan tools are universal in that they only retrieve the list of PIDs that automobile manufacturers are required to display. The standard, or “generic,” set of PIDs contains useful sensor data about engine and transmission operation. Automobile manufacturers also define their own, non-standard, set of PIDs. These “enhanced” or “extended” set of PIDs are proprietary and greater in number than the standard PID set. Many off-theshelf OBD-II scan tools do not query on-board ECUs for these PIDs. Scan tool manufacturers must pay a significant fee to obtain information about manufacturer-specific PIDs. Because of the extra costs involved, your basic $35 scan tool does not include the enhanced set of PIDs. Some scan tools offer enhanced PIDs for an extra fee.
Bi-Directional Diagnostic Communication
Advanced OBD-II scan tools have the ability to command control of certain engine and transmission functions. Engine bi-directional controls include, but are not limited to, electric fan operation, A/C compressor clutch operation, manual control of spark advance, commanded idle control, target air/fuel ratio, and injector performance tests. Transmission bi-directional controls include, but are not limited to, torque converter clutch solenoid control and commanded shift solenoids.
Some advanced OBD-II scan tools are capable of performing a crank angle sensor error (CASE) learn procedure (commonly referred to as “crank learn”). The GM service manual explains that due to tolerances in manufacturing, proper misfire detection requires the engine computer to learn the variances in the relationship of the CKP sensor and the crankshaft reluctor wheel. This crank learn procedure should be performed after installing a new engine computer, engine timing cover, CKP sensor, or crankshaft reluctor wheel. Should the installation of these new parts not set a P1336 DTC, the engine computer relies on the results of the previous, now incorrect, crank learn data.
The DLC is the connection point for any ECU programming interface. In the 1980s, engine computers stored calibration data in a flash PROM that was programmed outside the vehicle. All GM OBD-II ECUs have the convenience of reprogramming without removal from the vehicle. Reprogramming is required for ECU replacement, GM service updates, and custom tuning through aftermarket tuning equipment.
Mail-order tuners offer custom programming services to get your vehicle running well enough to be driven or trailered to a local chassis dyno tuning facility. To get programming access to the engine and/or transmission computer, mail-order tuners use an off-board benchtop programming solution such as EFI Connection’s Professional Series OBD-II modular benchtop programming equipment. (See Chapter 4 for more information about benchtop programming.)
Troubleshooting Diagnostic Trouble Codes
The OBD-II system’s DTC malfunction logging is a fast and convenient way to resolve a problem. With OBD-II being standardized, any offthe- shelf OBD-II scan tool displays a list of DTCs that explain why the MIL is illuminated in the dashboard. Most auto parts stores scan your vehicle for free.
I recall a customer who updated his third-generation Firebird (a TPI car) to the Gen III PCM. He participated in the Hot Rod Power Tour and was well out of reach of his tools and diagnostic equipment.
The MIL illuminated in his dash as the engine began to run poorly. He was able to stop at an auto parts store for a free scan. The scan revealed a misfirerelated DTC for a specific cylinder. He chased the spark plug wire from the distributor cap to the spark plug and found that the spark plug was loose. He tightened the spark plug, the auto parts store cleared the DTCs, and he was back on the Power Tour.
Before You Change That Sensor
In some cases the ECU sees several occurrences of a malfunction before setting a DTC. It is important to think through the problem before simply buying new parts.
As an example, let’s say a DTC has set indicating prolonged high voltage in the bank 1 O2 sensor. If you rush to the auto parts store to buy a new O2 sensor, you may waste your time, as well as about $50.
First look at what the O2 sensor does. The O2 voltage ranges from about 0 to 1V. When a lean mixture passes the sensor, the voltage is closer to 0V. When a rich mixture passes the sensor, the voltage is closer to 1V. The target voltage for a healthy running engine is about 0.5V. An O2 sensor in good working order indicates too much fuel in the air/fuel mixture exhausting from the engine. Such a DTC would not warrant an O2 sensor replacement, but rather a look at the fuel injectors on bank 1 to inspect for a faulty injector.
Follow the Manufacturer’s Procedures
The GM service manual contains step-by-step diagnostic procedures for each DTC. In some cases, the documentation includes wire schematics to help you understand the electronic circuits involved. The diagnostic procedures may call for a visual inspection, voltage test light, or multimeter to check voltage or resistance values. Advanced diagnostics may call for the use of a GM Tech2 scanner. The Tech2 is a necessity for on-board ECUs other than the engine control module and is seldom used with Gen III electronics conversions.
A good scan tool package, such as EFILive, and a GM service manual go a long way in diagnosing just about any DTC that may set.
Written by Mike Noonan and Posted with Permission of CarTechBooks
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