Engine Control System - System Testing & Engine Component Tests: Overview Oldsmobile Alero I

MODEL IDENTIFICATION

Vehicle model is identified by fourth character of Vehicle Identification Number (VIN). VIN is stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION table.

MODEL IDENTIFICATION

Description

Lock-off relay is energized whenever Powertrain Control Module (PCM) supplies power to gasoline fuel pump circuit. PCM supplies power to gasoline fuel pump circuit for approximately 2 seconds at every ignition on cycle and whenever 7X reference pulses indicate engine is cranking or running. When energized, lock-off relay supplies power to high and low pressure lock-off solenoids. At initial ignition on cycle, PCM energizes gasoline fuel pump relay, supplying power to fuel pump power circuit for a two second prime pulse. When prime pulse is supplied to fuel pump power circuit, lock-off relay is energized.

When engine is started, PCM energizes gasoline fuel pump relay, supplying power to fuel pump power circuit. PCM will supply power to fuel pump power circuit, regardless of fuel system that in operation, as long as crankshaft pulses are monitored (engine is running).

High and Low Pressure Lock-Off (HPL and LPL) solenoids are energized by Alternate Fuel Engine Control Unit (AF ECU) when vehicle is operating in CNG mode. At initial ignition on request, AF ECU will close the HPL solenoid control circuit driver (completing circuit path to ground) for a one second CNG prime pulse. Prime pulse occurs at same time as gasoline fuel pump prime pulse occurs. This ensures lock-off relay will be energized and supplying power to lock-off solenoids at the same time AF ECU closes driver and supplies ground path. When engine is cranking or running, AF ECU closes BOTH lock-off solenoid control circuit drivers, allowing engine to operate on CNG.

Alternate Fuel Engine Control Unit (AF ECU) supplies power to AF fuel pump relay coil when engine starts and runs on CNG. AF fuel pump relay allows AF ECU to turn off gasoline fuel pump and supply a voltage source for lock-off relay. At initial ignition on, AF fuel pump relay is not energized by AF ECU. Powertrain Control Module (PCM) energizes gasoline fuel pump relay, supplying power to fuel pump power circuit for 2 second prime pulse. When prime pulse is supplied to fuel pump power circuit, lock-off relay is energized and gasoline fuel pump receives 2 second prime pulse power. When the engine is started, PCM energizes gasoline fuel pump relay, supplying power to fuel pump power circuit. When engine is operating on CNG, AF ECU energizes AF fuel pump relay, turning off the gasoline fuel pump. Lock-off relay continues to be energized by gasoline fuel pump relay.

Alternative Fuel Engine Control Unit (AF ECU) controls fuel system while vehicle is operating on alternative fuel. AF ECU receives signals from various engine sensors and determines correct amount of fuel required by engine. Full tank can contain fuel at 3600 psi (24,820 kPa). A High Pressure Lock-Off (HPL) solenoid in the tank and Low Pressure Lock-Off (LPL) solenoid in fuel line prevent fuel flow. AF ECU commands the HPL ONLY open for one second at every ignition ON for a CNG prime pulse. CNG prime pulse is performed charge fuel line and allow Fuel Pressure Sensor (FPS) to monitor amount of fuel pressure in tank. AF ECU commands both HPL and LPL open when engine RPM indicates engine is cranking or running on CNG.

This fuel delivery system uses 3 different pressure stages to lower fuel pressure from tank pressure down to a delivery pressure which is slightly above atmospheric pressure.

1. High Pressure (Tank) Stage - When AF ECU commands HPL open, fuel begins to flow through High Pressure Regulator (HPR). HPR reduces fuel pressure to 115-220 psi (793-1517 kPa). Outlet of HPR is intermediate pressure stage.
2. Intermediate Pressure Stage With pressure 115-220 psi (793-1517 kPa), fuel exiting HPR flows through intermediate fuel line to Low Pressure Lock-Off (LPL) solenoid. LPL is controlled by AF ECU. Fuel flows out of LPL through another portion of the intermediate fuel line and into Low Pressure Regulator (LPR). Inlet of LPR is the low pressure stage.
3. Low Pressure Stage Low pressure regulator is a 2 stage regulator. Fuel enters LPR at intermediate stage pressure of 115-220 psi (793-1517 kPa). Primary stage of LPR operation reduces fuel pressure down to 4-6 psi (28-41 kPa). Fuel pressure is then again reduced in the secondary stage of LPR, down to 2-5 in. H 2 0 (5.1-12.7 cm). Fuel exits LPR and flows through low pressure hose and into Gas Mass Sensor/Mixture Control Valve (GMS/MCV).

Alternate Fuel Engine Control Unit (AF ECU) and Powertrain Control Module (PCM) are connected by Natural Gas Operation (NGO) enable circuit. For engine to switch-over to CNG operation, PCM must be commanded to disable gasoline fuel injectors, switch to a different ignition spark calibration and disable some PCM DTCs. AF ECU performs switch-over command by grounding NGO enable circuit when CNG operation is desired.

This vehicles primary fuel source is compressed natural gas (CNG). Vehicle will only switch-over to gasoline operation if one of the following conditions is present

1. Fuel Pressure Sensor (FPS) indicates CNG tank is empty.
2. A wide-open Mixture Control Valve (MCV) is detected.
3. System fault is detected by control module (AF ECU or PCM).
4. Manual shut-off valve is closed.
5. Engine crank time exceeds 8 seconds.
6. On every 100th start and also when coolant temperature is less than 10°F (-12°C), vehicle will start and run briefly on gasoline. A switch-over to CNG will occur only during a decel condition.

Ignition relay is positive voltage supply source for Gas Mass Sensor/Mixture Control Valve (GMS/MCV) and fuel gauge select switch. Ignition relay coil is energized when ignition switch is turned to Run or Start positions. Ignition relay switch contact is supplied voltage at all times.