1999 Chevrolet/Geo Cavalier (CNG) Alternative Fuel Engine Control Unit Table 1: AF ECU Function Parameters The (AF ECU) controls alternative fuel engine operation. The control unit monitors various engine and vehicle functions to provide the correct amount of fuel under all operating conditions. This provides excellent driveability and fuel economy while maintaining exhaust emission levels. Refer to Engine Control Unit (ECU) Replacement for location views. In addition to controlling engine operation the AF ECU also runs tests (DTCs) on various sensors and outputs. If a DTC detects a problem the AF ECU will store the DTC number along with sensor information retrieved at the time of the problem (Freeze Frame). The DTC number and any Freeze Frame information can be retrieved from the AF ECU using a scan tool. AF ECU Function Parameters The AF ECU operates in several Function modes which are defined in the AF ECU Function Parameters table. AF ECU Function Parameters Function Enable Conditions Function Description Deceleration Fuel Cut-Off (DFCO) Deceleration Open Loop Speed Limiting RPM Limiting (with vehicle speed greater than 10 mph) MAP is less than 25 kpa Engine speed is greater than 1800 RPM Throttle position is less than 1 percent Above conditions present for more than 8 seconds AF ECU will exit DFCO when MAP is greater than 31 kpa MAP is less than 25 kpa Engine speed is greater than 1100 RPM Throttle position is less than 1 percent vehicle speed sensor indicates a vehicle speed of 107 or more mph. Crank sensor indicates engine speed greater than 6000 RPM During extended decel operation the vehicles exhaust catalyst temperature may exceed maximum limits. In order to cool the exhaust catalyst the AF ECU will close the low pressure lock-off (LPL) solenoid. With fuel flow stopped during the DFCO enable conditions the catalysts temperature can be reduced. During normal decel conditions the AF ECU will operate the engine in Open Loop mode. If the enable conditions are met for greater than 8 seconds the AF ECU will transition to the DFCO mode. The AF ECU limits vehicle speed. The AF ECU will close the mixture control valve (MCV) when the enable conditions are met. When vehicle speed indicates 104 mph or less, the AF ECU will open the MCV. The AF ECU will operate the engine in Open Loop mode during Speed Limiting. The AF ECU limits engine speed. The AF ECU will close the mixture control valve (MCV) when the enable conditions are met. When engine speed indicates 5800 RPM or less, the AF ECU will open the MCV. The AF ECU will operate the engine in Open Loop mode during RPM Limiting.
RPM Limiting (with vehicle speed less than 10 mph) Fuel Switch-Over to Gasoline Open Loop mode Closed Loop mode Lock-Off Solenoid operation Crank sensor indicates engine speed greater than 4000 RPM fuel pressure sensor (FPS) voltage drops below 0.85 volts (2412 kpa@21 C [350 psi@70 F]) indicating fuel level is low Engine crank time exceeds 8 seconds The AF ECU detects a fault condition (a DTC has set) A wide-open MCV is detected for the specified amount of time during following conditions: for 2 seconds during Crank for 100 ms during engine idle for 1 second during WOT WITH engine speed less than 4100 RPM Initial start-up then an engine run time of 18 to 92 seconds AF ECU operating in vehicle Speed Limiting mode AF ECU operating in engine speed RPM Limiting mode AF ECU operating in either deceleration mode DTC failure of the O2 sensor Engine is running continuously for 18 to 92 seconds. A 7 to 35 second delay timer has expired after the O2 sensor has switched Both above conditions are complete The previous ignition cycle ended with a complete AF ECU shutdown Ignition is turned ON Engine speed is 25 RPM or greater The AF ECU limits engine speed. The AF ECU will close the mixture control valve (MCV) when the enable conditions are met. When engine speed indicates 3900 RPM or less, the AF ECU will open the MCV. The AF ECU will operate the engine in Open Loop mode during RPM Limiting. The AF ECU will switch-over to gasoline operation when any ONE of the enable conditions are met. If low FPS voltage caused the fuel switch-over the AF ECU will not resume alternative fuel operation until the FPS voltage exceeds 1.50 volts (6890 kpa@21 C [1000 psi@70 F]). During Open Loop mode the AF ECU will disable both Short (EGO multiplier) and Long (Block Learn) Term Fuel Trim. Short Term Fuel Trim will be reset. The AF ECU will fuel the engine based upon Long Term Fuel Trim as it existed prior to Open Loop mode. The AF ECU will determine the amount of time the engine must continuously operate based upon coolant temperature. The colder the coolant temperature the longer the time delay. The AF ECU will determine the amount of delay time based upon the O2 sensor activity. The more active the O2 sensor the shorter the delay time. The AF ECU will open the high pressure lock-off (HPL) solenoid for 1 second at ignition ON. After engine speed of 25 RPM or more is obtained and held the AF ECU will open BOTH the HPL and the low pressure lock-off (LPL) solenoids. Every 100 engine starts or when the ECT is -12 C (10 F) or less. The AF ECU will switch-over to Alternative Fuel operation under the following deceleration conditions:
Gasoline Start strategy fuel pressure sensor (FPS) voltage indicates fuel tank pressure is 350 psi or more Engine speed is greater than 1300 RPM Throttle position is less than 6 percent for 800 ms Vehicle speed is greater than 35 mph. The AF ECU will abort the switchover if either of the following conditions occur: The throttle position has increased more than 6 percent during the 500 ms switch-over period A percent error of desired gas to actual gas has been detected for 1.5 seconds or more The AF ECU performs the gasoline start every 100 engine starts to allow the gasoline injector(s) to operate. This increases injector reliability. In addition to the gasoline start the gasoline fuel injectors are pulsed 4 times during EVERY engine start. The AF ECU performs the cold weather gasoline start to aid in startability. Most AF ECU DTCs are inhibited from testing until 5 seconds after a successful switch-over to alternative fuel operation has occurred. The AF ECU CKP Sensor Circuit performance DTC tests during gasoline operation. Diagnostic Trouble Codes (DTCs) DTCs, when set, indicate that a control module (AF ECU, VCM, PCM or ECM) has detected a malfunction in a particular circuit or system. The control modules are programmed with routines or checks that it follows only under prescribed conditions (called Conditions for Running the DTC). When these conditions exist, the control module checks certain circuits or systems for a malfunction. These checks are called Conditions for Setting the DTC. When the setting conditions are true, a malfunction is indicated and the DTC is stored as failed. Some DTCs alert the driver by illuminating the SERVICE ENGINE SOON MIL (some vehicles may also display a message). Other DTCs don't trigger a driver warning. Refer to the DTC List table in Specifications for a complete list of DTCs and what driver alerts they trigger. The control module also saves data and input parameters when most DTCs are set. The saved information is called Freeze Frame and Failure Records. The AF ECU does not support Failure Records. Trip The ability for a DTC to run depends largely upon whether or not a Trip has been completed. A Trip for a particular DTC is defined as vehicle operation, followed by an engine off period and a driving mode such that any particular DTC has had sufficient time to complete testing. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some DTCs run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run. Warm-up Cycle In addition, the execution of a DTC may also be bound by conditions which must comprehend a Warm-up cycle. A Warm-up cycle consists of engine start-up and vehicle operation such that the coolant temperature has risen greater than a certain value (typically 40 C) from start-up temperature and reached a minimum temperature of 70 C (160 F). If this condition is not met during the ignition cycle, the DTC may not run. Freeze Frame Data Government regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data captured is called Freeze Frame data. The Freeze Frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded as Freeze Frame data. A subsequent failure will not update the recorded operating conditions.
The Freeze Frame data parameters stored with a DTC failure include the following: Manifold Absolute Pressure Fuel Trim Engine Speed Engine Load Engine Coolant Temperature Vehicle Speed Loop Status Freeze Frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Data from these faults take precedence over data associated with any other fault. The Freeze Frame data will not be erased unless the associated history DTC is cleared. Failure Records Data In addition to Freeze Frame data a control module may also store Failure Records data when a DTC reports a failure. Unlike Freeze Frame data, Failure Records data can be stored by DTCs that DO NOT illuminate the MIL. The AF ECU does NOT store Failure Records. Failure Records save many more relevant data parameters than Freeze Frame records. Which data parameters are saved varies based on the DTC set. Sensor DTC Failure Records generally contain sensor related data parameters while fuel DTC Failure Records generally contain fuel related data parameters. If several DTCs are set, Failure Records for the most recently set DTCs will be saved. Freeze Frame, Failure Records data may be retrieved through the Diagnostic Trouble Code menu on scan tool. If more than one DTC is set review the odometer or engine run time data located in the Freeze Frame, Failure Records info to determine the most current failure. Keep in mind that once Freeze Frame or Failure Record is selected, the parameter and input data displayed will look just like the normal data except the parameters will not vary since it is displaying recorded data. Important Always capture the Freeze Frame and Failure Records information with the scan tool BEFORE proceeding with diagnosis. Clearing DTCs, disconnecting the battery, disconnecting control module or body connectors or procedures performed during diagnosis may ERASE or overwrite the stored Freeze Frame and Failure Records data. Loss of this data may prevent accurate diagnosis of an intermittent or difficult to set DTC. Capturing DTC Info (Capture Info) Selecting this option on the scan tool allows the technician to record the Freeze Frame data that may be stored in the control module memory. This can be useful if the control module connectors or the vehicle battery must be disconnected and later review of the stored information may be desired. DTC Types DTCs are categorized by type. The type will indicate the action the DTC will take in storing a failure and illuminating the MIL. The following table indicates what action each DTC type will take when a failure is recorded. Type C DTCs that DO NOT display a message were formerly referred to as Type D. DTC Type MIL Illumination Freeze Frame Stored Failure Records Stored A Yes Yes Yes B Yes (with two fails) Yes (on first fail) Yes (on second fail)
C NO A Message may display NO Varies In order for a type B DTC to request MIL illumination the DTC must fail in two consecutive drive trips in which the DTC tests. Refer to the Diagnostic Trouble Code (DTC) List in Specifications for the type of each AF ECU DTC. Always refer to the diagnostic support information within each DTC to obtain the Action Taken when the DTC sets and the Conditions for Clearing the DTC. These will indicate any variations from the general type A, B and C actions. Special Cases Of Type B Diagnostic Tests Fuel trim and misfire are special cases of type B DTCs. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperatures are recorded. In order for the fuel trim or misfire DTCs to report a PASS the load conditions must be within 10 percent, the speed conditions must be within 375 rpm, and the coolant temperatures must be in the same calibrated high or low range at the time the diagnostic test last reported a failure When the ignition is turned off, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a malfunction occurs during two consecutive trips, the control module treats the failure as a normal type B diagnostic, and does not use the stored conditions. However, if a malfunction occurs on two non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions: When the engine load conditions are within 10 percent of the previous test that failed. Engine speed is within 375 rpm, of the previous test that failed. Engine coolant temperature is in the same range as the previous test that failed. Unique to the misfire diagnostic, the misfire DTC has the ability of alerting the vehicle operator to potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the control module will command the MIL to flash at a rate of once per second during the time that the catalyst damaging misfire condition is present. Snapshot Using A Scan Tool Snapshot data may also be taken and retrieved using a diagnostic tool by selecting the Snapshot option. These parameters can then be reviewed at any time. Keep in mind that once a Snapshot is triggered, the parameter and input data displayed will look just like the normal control module parameter and input data with the same parameter numbers except it will not vary since it is displaying recorded data. Diagnostic Trouble Code Display DTCs can only be displayed with the use of a scan tool. Scan Tool Parameter - DTC Status This selection will display any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition cycle. DTC tests which run and pass will cause that DTC number to be removed from this scan tool display. Scan Tool DTC Information
The scan tool will display any DTCs which have failed at any time since DTCs were last cleared. The display will indicate a DTCs status for the LAST TEST, during THIS IGNITION and SINCE CLEAR. Clearing Diagnostic Trouble Codes ALWAYS use a scan tool to clear DTCs from control module memory. Disconnecting the vehicle battery MAY also clear the control module memory, however most other system memories will also be cleared. This is generally undesirable. Disconnecting the control module connectors solely for clearing DTCs is not recommended since this unnecessarily disturbs the connections. Before clearing DTCs the scan tool has the capability to save any data stored with the DTCs and then display that data at a later time. Refer to Capture Info. Once a problem has been corrected and verified it is a good idea to clear DTCs so that any future service work is not needlessly confused. Many DTCs have complex running and setting conditions. Therefore, simply clearing DTCs and watching to see if the DTC sets again may not indicate whether a problem has been corrected. To verify a repair after it is complete, you must look up the test conditions and duplicate those conditions. If the DTC runs and passes, chances are good that the problem is fixed. AF ECU Communications The provision for communicating with the control module is the data link connector (DLC). It is usually located under the instrument panel. The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below: Identifying stored Diagnostic Trouble Codes (DTCs). Clearing DTCs Performing output control tests. Reading serial data. U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that it provides scan tool manufacturers with the capability of accessing data from any make or model vehicle sold in the United States. Check Engine or Service Engine Soon (SES) Malfunction Indicator Lamp (MIL) Operation The CHECK ENGINE or the SERVICE ENGINE SOON (varies by model) Malfunction Indicator Lamp (or MIL) is located in the instrument panel cluster (IPC). The MIL is controlled both by the AF ECU and the gasoline Control Module (VCM / PCM or ECM). The MIL is used to indicate that a control module (AF ECU, VCM / PCM or ECM) has detected a problem that affects vehicle emissions, may cause powertrain damage, or severely impacts driveability. Refer to Malfunction Indicator Lamp Circuit Check for diagnosis of MIL. Additional Description Information Refer to the Engine Controls Description and Operation section in the J Platform service manual for further information that may apply to this vehicle.