FUEL TRIM(MING) DIAGNOSTIC TIME
No matter what the driveability issue happens to be, beginning by checking the PCM's fuel trim decisions can get you pointed in the right direction, and may end up cutting your diagnostic time in half.
The year is 2006, and for those who may have overlooked it, this is the 10th anniversary of On-Board Diagnosties II (OBD II). I believe its cause for celebration. Here's an example of how it used to be: I was recently called to help a shop with a 1992 Subaru. In order to retrieve the diagnostic trouble codes (DTCs), I had to remove the driver's kick panel, visit a vehicle repair information source for the instructions on how to jumper the diagnostic connector, then count the flashes of the malfunction indicator light (MIL). The final step was looking up the DTC description. Total time from start to finish was approximately 15 minutes. If this had been an OBD II vehicle, I would have had the information in under 30 seconds. The standardization associated with OBD II, which gives us easy access to fuel trim data, has really simplified the diagnostic process.
What is fuel trim? Fuel trim is a window that allows you to see what the computer is doing to control fuel delivery and determine how the PCM's adaptive strategy is operating.
Why was fuel trim created? In order for vehicle manufacturers to comply with EPA emissions regulations, catalytic converters were added to reduce tailpipe emissions. Catalytic converters need a stoichiometric air/fuel ratio of approximately 14.7:1 to obtain the greatest emissions reductions. Vehicle engineers designed closed-loop engine control systems to maintain that ratio, adjusting injector pulse width based on information from die oxygen sensor and other inputs. Short-term fuel trim (STFT) and long-term fuel trim (LTFT) are expressed as a percentage, and the ideal range should be within ±5%.
Positive fuel trim percentages indicate that the powertrain control module (PCM) is attempting to richen the fuel mixture, to compensate for a perceived lean condition. Negative fuel trim percentages indicate the PCM is attempting to lean out the fuel mixture, to compensate for a perceived rich condition. STFT and LTFT percentages are the adjustments made by the PCM to maintain the 14.7:1 ratio.
No matter what the driveability issue happens to be, the fuel trim window should be used first to check the STFT and LTFT parameters.
There are two basic fuel control systems used on most vehicles: speed density systems, which use rpm, manifold absolute pressure (MAP) and barometric pressure (BARO) to calculate engine load, and mass airflow systems, which use the mass airflow sensor (MAF) and rpm. to calculate engine load. In both cases, the PCM begins with a standard injector pulse width calculation, based on various inputs and internal fuel cell tables.
The equation used by early Chrysler speed density OBD II vehicles to establish initial pulse width is: Injector Pulse Width = (RPM × MAP/BARO) × TPS × ECT × IAT × Battery Volts × O2 (Short Term x Long Term). Once the vehicle is running and the engine control system enters closed-loop, the PCM relies primarily on feedback from, the oxygen sensor to determine if the stoichiometric air/fuel ratio is being maintained.
Think of closed-loop operation as a Sense-Decide-React sequence. The Closed Loop System Operation sequence in Fig. 1 on page 66 provides an explanation of the Sense-DecideReact process. The PCM determines the base injector pulse width as described above. Once the system enters closed-loop, the Sense phase begins, and is handled by the oxygen sensor. In the Decide phase, the PCM uses the oxygen sensor data to determine if the proper 14.7:1 air/fuel ratio is being maintained. If the ratio is correct, the PCM decides that no change should be made to the injector pulse width. In this scenario, the React phase maintains the same injector pulse width. However, if the air/fuel ratio is 16.1:1 (lean) during the Sense phase, the PCM makes the decision to increase the injector pulse width to correct the lean air/fuel ratio condition. In the React phase, the PCM commands the fuel injector to stay open longer. The Sense-Decide-React sequence continues throughout closed-loop operation, maintaining the proper air/fuel ratio.
During closed-loop operation, the PCM reports changes in fuel trim calculations via the OBD II generic data parameters short-term and long-term fuel trim. STFT for most vehicles will normally sweep rapidly in response to the oxygen sensor. In many cases, if you graph Bank 1 STFT and BlSl O2 sensor, you'll see the oxygen sensor go rich and STFT go lean to adjust the air/fuel ratio. The oxygen sensor will then go lean and STFT will go rich.
LTFT for most vehicles will remain more stable, adjusting over a longer period of time. On some vehicles, if STFT has reached the specified limit, LTFT will change in a few seconds. On other vehicles it may take 15 to 20 seconds before a change occurs. The LTFT calculation is normally kept in memory, so the PCM is ready to use the last known injector pulse width following a restart. STFT will normally begin at 0% and adjust to the current conditions. Both STFT and LTFT will normally reset when all trouble codes are cleared.
Affichage des articles dont le libellé est Auto. Afficher tous les articles
Affichage des articles dont le libellé est Auto. Afficher tous les articles
mercredi 28 octobre 2009
TRANSFER CASE STUDIES: DIAGNOSING GM's 4WD SYSTEM
TRANSFER CASE STUDIES: DIAGNOSING GM's 4WD SYSTEM
Access to DTCs can simplify any system diagnosis. But as two separate cases on GM's 4WD system reveal, these codes are not always 100% accurate.
GM has used many different four-wheeldrive control systems in its vehicles over the years, ranging from pure mechanical to electronically controlled. Each has its own diagnostic procedures, and the best source for understanding how these systems work will always be your information system.
Some later model 4WD systems have scan tool capabilities along with diagnostic trouble codes (DTCs) to help locate and diagnose many of the problems associated with them.
We looked at twu GM K 1500 trucks that have the NVG 246 two-speed transfer case. One is a 1999 model, the other a 2000. Both use the same controls and happen to have stored the same DTC. The basic system consists of a transfer case motor and encoder, Front differential actuator, trans for case module, speed sensors, switches and park input and output to the PCM. It seems logical to use case studies to show how these systems can he repaired.
The transfer case module data can be accessed with a scan tool such as the Tech 2, which is GM's official scan tool. To get to the proper screens, you must choose the Powertrain selection, then navigate your way to the ATC selections. ATC is the GM acronym for acMoe transfer case. As with any module/computer-based diagnostic strategy, it's always a good idea to make checking DTCs one of the first steps in your diagnosis. These two trucks have 19 possible DTCs, including those for input, motor, encoder and even VIN codes.
Selecting "F1" on the menu got us where we needed to be. Both trucks had one DTC stored: B2725 (active transfer case mode switch malf'unction). A Tech 2 information screen will also let you know whether certain 4WD systems do not allow scan tool access.
The dash-mounted transfer case mode control is a set of normally open switches that vary the voltage supplied by the transfer case module when closed. The transfer case mod- ule sends 8 volts to the switches, then monitors the return circuit to deter- mine whether any of the switches is depressed. It can determine this because each switch has a different resistor in series with the circuit. Since both trucks have mode switch trouble codes, it seems logical to start there.
On the 2000 GM truck, the com- plaint was no four-wheel drive, and the 4WD light in the IPC (instrument panel cluster) came on at times. (This warning light, located in the left-side LED indicator, can display various messages, from "Security" to "Low Fuel" and even "4WD/AWD.") Any ATC system malfunction will turn on this light to alert the driver of a problem. A trouble code is also stored at that time.
Armed with the complaint and DTC B2725, I was ready to continue my diagnosis. I simply pressed the buttons while watching the scan data. As I pressed various buttons, the scan data would not indicate the proper input. Pressing "4HI" or "4LO" would not always result in the data stream reflecting that condition. Also, the LEDs on the switch would randomly go on and off.
The easiest way to confirm this problem is to tap on the button and watch the scan data along with the switch LEDs. When we did this, it caused all kinds of random lights and data readings. At one point, it even turned on the 4WD warning indicator. It was obvious the switch had a mind of its own and needed replacement.
It's always nice to have such an easy diagnosis, but should we assume that all B2725 DTCs indicate the need for a switch replacement? After all, General Motors TSB No. 02-0421-004 indicates a known problem with this switch assembly.
Before we look at the other truck, it might be useful to see what GM's ESI website (www.acdelcotds.com) information says about B2725, to get a better understanding of the systems design characteristics. The following description comes from document ID No. 375434:
"The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit. If no buttons are pressed, and the transfer case shift control module detects a voltage level outside the possible range (approx. 0.5-1.0 volts) for longer than 5 minutes, the transfer case shift control module will set this DTC if a button is held down or sticks for a period longer than 5 minutes. When each of the switches is depressed they will complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the 4HI, AUTO 4WD, 2HI, and 4LO button selections are made by the driver."
Now we move on to the 1999 model K 1500 truck. The customer complained of no 4WD operation. He did not mention any warning lights, but it seemed like a good idea to check DTCs anyway. As mentioned earlier, this one had the same DTC B2725 stored. A quick check of the switches did not indicate any problem with the inputs from the switch to the module while monitoring scan data. However, the front wheels would not engage when the 4WD button was selected. A problem was obviously occurring, but the DTC did not seem to match the system malfunction.
Access to DTCs can simplify any system diagnosis. But as two separate cases on GM's 4WD system reveal, these codes are not always 100% accurate.
GM has used many different four-wheeldrive control systems in its vehicles over the years, ranging from pure mechanical to electronically controlled. Each has its own diagnostic procedures, and the best source for understanding how these systems work will always be your information system.
Some later model 4WD systems have scan tool capabilities along with diagnostic trouble codes (DTCs) to help locate and diagnose many of the problems associated with them.
We looked at twu GM K 1500 trucks that have the NVG 246 two-speed transfer case. One is a 1999 model, the other a 2000. Both use the same controls and happen to have stored the same DTC. The basic system consists of a transfer case motor and encoder, Front differential actuator, trans for case module, speed sensors, switches and park input and output to the PCM. It seems logical to use case studies to show how these systems can he repaired.
The transfer case module data can be accessed with a scan tool such as the Tech 2, which is GM's official scan tool. To get to the proper screens, you must choose the Powertrain selection, then navigate your way to the ATC selections. ATC is the GM acronym for acMoe transfer case. As with any module/computer-based diagnostic strategy, it's always a good idea to make checking DTCs one of the first steps in your diagnosis. These two trucks have 19 possible DTCs, including those for input, motor, encoder and even VIN codes.
Selecting "F1" on the menu got us where we needed to be. Both trucks had one DTC stored: B2725 (active transfer case mode switch malf'unction). A Tech 2 information screen will also let you know whether certain 4WD systems do not allow scan tool access.
The dash-mounted transfer case mode control is a set of normally open switches that vary the voltage supplied by the transfer case module when closed. The transfer case mod- ule sends 8 volts to the switches, then monitors the return circuit to deter- mine whether any of the switches is depressed. It can determine this because each switch has a different resistor in series with the circuit. Since both trucks have mode switch trouble codes, it seems logical to start there.
On the 2000 GM truck, the com- plaint was no four-wheel drive, and the 4WD light in the IPC (instrument panel cluster) came on at times. (This warning light, located in the left-side LED indicator, can display various messages, from "Security" to "Low Fuel" and even "4WD/AWD.") Any ATC system malfunction will turn on this light to alert the driver of a problem. A trouble code is also stored at that time.
Armed with the complaint and DTC B2725, I was ready to continue my diagnosis. I simply pressed the buttons while watching the scan data. As I pressed various buttons, the scan data would not indicate the proper input. Pressing "4HI" or "4LO" would not always result in the data stream reflecting that condition. Also, the LEDs on the switch would randomly go on and off.
The easiest way to confirm this problem is to tap on the button and watch the scan data along with the switch LEDs. When we did this, it caused all kinds of random lights and data readings. At one point, it even turned on the 4WD warning indicator. It was obvious the switch had a mind of its own and needed replacement.
It's always nice to have such an easy diagnosis, but should we assume that all B2725 DTCs indicate the need for a switch replacement? After all, General Motors TSB No. 02-0421-004 indicates a known problem with this switch assembly.
Before we look at the other truck, it might be useful to see what GM's ESI website (www.acdelcotds.com) information says about B2725, to get a better understanding of the systems design characteristics. The following description comes from document ID No. 375434:
"The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit. If no buttons are pressed, and the transfer case shift control module detects a voltage level outside the possible range (approx. 0.5-1.0 volts) for longer than 5 minutes, the transfer case shift control module will set this DTC if a button is held down or sticks for a period longer than 5 minutes. When each of the switches is depressed they will complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the 4HI, AUTO 4WD, 2HI, and 4LO button selections are made by the driver."
Now we move on to the 1999 model K 1500 truck. The customer complained of no 4WD operation. He did not mention any warning lights, but it seemed like a good idea to check DTCs anyway. As mentioned earlier, this one had the same DTC B2725 stored. A quick check of the switches did not indicate any problem with the inputs from the switch to the module while monitoring scan data. However, the front wheels would not engage when the 4WD button was selected. A problem was obviously occurring, but the DTC did not seem to match the system malfunction.
Pep Boys Now Sells Auto Parts and Accessories Online
Pep Boys Now Sells Auto Parts and Accessories Online
The Pep Boys-- Manny, Moe & Jack, said it is now selling automotive parts and accessories on its website, pepboys.com. The Pep Boys online shopping experience enables customers to conduct a detailed search by either the name brand or specific part they desire. There is also a section featuring the latest in car and truck accessories, the company said.
There are approximately 400 major brands and over 120,000 parts available through the website. Brands include: American Racing, manufacturer of custom wheels; APC, manufacturer of appearance accessories such as gauges and carbon fiber hoods; along with traditional lines such as Edelbrock, manufacturer of engines and intake manifolds and Westin, a truck and SUV manufacturer of bumpers, grill guards, running boards and tail light guards, Pep Boys said.
According to Mitchell G. Leibovitz, Pep Boys Chief Executive Officer, "In addition to satisfying the needs of our in-store customers, pepboys.com greatly expands our geographic reach and broadens our customer appeal."
Pep Boys has 629 stores and more than 6,500 service bays in 36 states and Puerto Rico.
COPYRIGHT 2002 Ron DeMarines
COPYRIGHT 2008 Gale, Cengage Learning
The Pep Boys-- Manny, Moe & Jack, said it is now selling automotive parts and accessories on its website, pepboys.com. The Pep Boys online shopping experience enables customers to conduct a detailed search by either the name brand or specific part they desire. There is also a section featuring the latest in car and truck accessories, the company said.
There are approximately 400 major brands and over 120,000 parts available through the website. Brands include: American Racing, manufacturer of custom wheels; APC, manufacturer of appearance accessories such as gauges and carbon fiber hoods; along with traditional lines such as Edelbrock, manufacturer of engines and intake manifolds and Westin, a truck and SUV manufacturer of bumpers, grill guards, running boards and tail light guards, Pep Boys said.
According to Mitchell G. Leibovitz, Pep Boys Chief Executive Officer, "In addition to satisfying the needs of our in-store customers, pepboys.com greatly expands our geographic reach and broadens our customer appeal."
Pep Boys has 629 stores and more than 6,500 service bays in 36 states and Puerto Rico.
COPYRIGHT 2002 Ron DeMarines
COPYRIGHT 2008 Gale, Cengage Learning
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