1984-88 CARBURETED ENGINES
The computer consists of one electronic printed circuit board which simultaneously receives signals from all the sensors and, within milliseconds, analyzes them to determine how the engine is operating. The computer then advances or retards ignition timing by signaling the ignition coil to produce electrical impulses which fire the spark plugs at the exact moment when ignition is required.Microprocessor Electronic Spark Control
The microprocessor is an electronic module located within the computer that processes signals from the engine sensors for accurate engine spark timing. Its digital electronic circuitry offers more operating precision and programming flexibility than the voltage-dependent analog system used previously.Pickup Sensors
The start and run pickup sensors are located inside the distributor, supplying a signal to the computer to provide a fixed timing point used for starting (start pickup) and for normal engine operation (run pickup). The start pickup also has a back-up function of taking over engine timing in case the run pickup fails. Since the timing in this pickup is fixed at one point, the truck will be able to run, but not very well. The run pickup sensor also monitors engine speed and helps the computer decide when the piston is reaching the top of its compression stroke.
These two sensors will not operate at the same time.Coolant Sensor
The coolant temperature sensor, located in the intake manifold, informs the computer when the coolant temperature reaches a predetermined operating level. Its signals to the computer also help to control the amount of spark advance for a cold engine.Carburetor Switch
The carburetor switch sensor is located on the end of the idle stop solenoid and signals the computer when the engine is at idle or off-idle. With the carburetor switch grounding out at idle, the computer cancels the spark advance and the idle control of the air/fuel ratio at the carburetor.Vacuum Transducer
See Figure 1
The vacuum transducer, located on the computer, monitors the amount of intake manifold vacuum present in the engine. Engine vacuum is one of the factors that determines how the computer will advance/retard the ignition timing and, in conjunction with a feedback carburetor, change the air/fuel ratio.
The detonation sensor is mounted in the No. 2 branch of the intake manifold, and is tuned to the frequency characteristic of engine knocking. When detonation (knocking) occurs, the sensor sends a low voltage signal to the computer, which retards ignition timing in proportion to the strength and frequency of the signal. The maximum amount of retard is 11° for 1984 models, or 20° for 1985 and later models. When the detonation has ceased, the computer advances timing to the original value.
Oxygen (O2) Sensor
The oxygen sensor is used when the engine is equipped with a feedback carburetor. The sensor is located in the exhaust manifold and, through the use of a self-produced electrical current, signals the computer as to the oxygen content within the exhaust gases flowing past it. Since the electrical output of the oxygen sensor reflects the amount of oxygen in the exhaust, the results are proportional to the rich and lean mixture of the air/fuel ratio. The computer then adjusts the air/fuel ratio to a level that maintains the operating efficiency of the three-way catalytic converter and the engine.Charge Temperature Switch (CTS)
See Figure 2
The CTS is located in the No. 6 runner of the intake manifold on 6-cylinder engines, or in the No. 8 runner of the intake manifold on 8-cylinder engines. When the intake air temperature is below approximately 60°F (15°C), the CTS is closed, allowing no EGR timer function or valve operation. The injected air is switched to the exhaust manifold (upstream). The CTS opens when the intake air temperature is above approximately 60°F (15°C), thus allowing the EGR timer to time out and the EGR valve to operate, and diverts injected air to the catalytic converter (downstream).
1988 FUEL INJECTED ENGINES
The computer consists of one electronic printed circuit board which simultaneously receives signals from all the sensors and within milliseconds, analyzes them to determine how the engine is operating. The computer then advances or retards ignition timing by signaling the ignition coil to produce electrical impulses which fire the spark plugs at the exact moment when ignition is required.Microprocessor Electronic Spark Control
The microprocessor is an electronic module located within the computer that processes the signals from the engine sensors for accurate engine spark timing. Its digital electronic circuitry offers more operating precision and programming flexibility than the voltage-dependent analog system used previously.Coolant Sensor
See Figures 3, 4 and 5
The coolant temperature sensor is a device which monitors coolant temperature. This sensor provides the data on engine operating temperature to the Single Module Engine Control (SMEC). This allows the SMEC to demand slightly richer air-fuel mixtures and higher idle speeds until normal operating temperatures are reached.Five Port Coolant Vacuum Switch
See Figure 6
The five port coolant vacuum switch combines the functions of the Coolant Controlled Engine Vacuum Switch (CCEVS) and the three port Coolant Vacuum Switch (CVS3P).
This valve can be best described as two entirely separate switches. One half of the part behaves as a two port CCEVS. The switch is normally closed up to 150°F (65°C); above this temperature the switch opens, supplying vacuum to open the EGR. Above 150°F (65°C), the other half of the part acts as a CVS3P. At idle, full distributor advance is ensured by supplying manifold vacuum to the distributor under 150°F (65°C).
Oxygen (O2) Sensor
The sensor is located in the exhaust system and is electrically heated internally for faster switching when the engine is running. When there is a large amount of oxygen present, the sensor produces a low voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich/lean switch. Voltage is supplied by the SMEC computer, which then adjusts the fuel delivery accordingly.