The Alternate Fuel Engine Control Unit (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.
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.
Function | Enable Conditions | Function Description | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Deceleration Fuel Cut-Off (DFCO) |
AF ECU will exit DFCO when MAP is greater than 31 kPa. | 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. | ||||||||||||||
Deceleration Open Loop |
| 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. | ||||||||||||||
Speed Limiting | Vehicle speed sensor indicates a vehicle speed of 172.2 km/h (107 mph) or more. | 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 167.4 km/h (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. | ||||||||||||||
RPM Limiting - with vehicle speed greater than 16.1 km/h (10 mph) | Crank sensor indicates engine speed greater than 6000 RPM. | 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 16.1 km/h (10 mph) | Crank sensor indicates engine speed greater than 4000 RPM. | 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. | ||||||||||||||
Fuel Switch-Over to Gasoline |
| 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]). | ||||||||||||||
Open Loop mode |
| 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. | ||||||||||||||
Closed Loop mode |
Both above conditions are complete. | 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. | ||||||||||||||
Lock-Off Solenoid operation |
| 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. | ||||||||||||||
Gasoline Start strategy | 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:
The AF ECU will abort the switch-over if either of the following conditions occur:
| 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. |
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.
Additional Description Information
Refer to the Powertrain Control Module Description in Engine Controls-2.2L (LN2) in the J Platform Service Manual for further information that may apply to this vehicle.
