MAF Sensor: For the '85-89 L98, the Bosch MAF sensor outputs an analog signal, of varying voltage, typically .04-5v. The greater the airflow cooling the wire, the greater the signal - voltage or frequency. Accurate measurements depend on the mesh entry screen, used to improve laminar flow. ("Laminar" means "smooth layers".)
As a side note, if you remove the screen, you might improve airflow by a smidgen, but it may disturb your fuel injection, particularly during acceleration.
Oxygen Sensor: Corvette's engine computer applies a reference voltage (bias voltage), usually of 450mV (0.045V). This reference signal takes the place of the Oxygen sensor output, during open loop operation. In closed loop, the computer compares the reference voltage to the actual voltage, generated by the oxygen sensor. Voltage below 450mV is considered lean; above 450Mv is considered rich.
Two tips:
- The newer the sensor, the more the voltage changes, swinging from as low as 10mV to as much as 1000mV. As a sensor ages, voltage changes get smaller and slower - "lazy sensor" voltage changes may lag behind changes in the exhaust gas oxygen.
- The higher the rpm, the faster the voltage changes - more exhaust gas is flowing across the sensor.
Knock Sensor: A knock sensor is a tuned vibration sensor, mounted on the engine block. It's similar to a tuning fork, in that it vibrates most at a certain narrow band of frequencies. But when a knock sensor vibrates, its crystal generates a small voltage (about one volt) which increases with the severity of the engine vibrations associated with lnocking.
For example, the '87-88 knock sensor has a frequency of 5200 hertz, with a range of 4800-5600 hertz. Based on engine dynamometer tests, those specific vibrations are chosen because they're known to indicate engine knocking, for that engine type. So the knock sensor module is programmed to ignore any other vibrations. Retard occurs at 4° at a time, up to 20°, but with no lower than basic timing. If, after a few seconds, retard (or relaxed accelerator pressure) reduces or eliminates knock, then the control module again advances timing, in smaller 2° steps.
Idle Air Control (IAC): The IAC, in the throttle body assemby, bypasses a closed throttle, adding or subtracting bypass air to control engine idle speed. For the IAC to regulate bypass air, it must start from somewhere near the middle.
- One set of coils drive the pintle closed, to reduce rpm.
- One set of coils drive the pintle open, to increase rpm.
During idle, the computer calculates the proper position of the IAC valve (partly open), for the desired idle rpm. As the other sensor inputs vary (TPS and ECT), the IAC moves to change the rpm. The engine cannot start with a closed IAC valve, so, at engine shutdown, the IAC resets the pintle to the proper position for the next start.
The computer moves the pintle valve in small steps, which can be measired on a scan tool, plugged into the Data Link Connector (DLC) - formerly
ALDL.
You didn't mention it, but I'll throw it in anyhow...
Throttle Position Sensor: The TPS signals the computer just what you would expect - throttle position, a typical feedback signal. It also signals what the driver expects the engine to do - either change speeds, or stay the same.
The computer supplies VREF to the grounded resistor. Signals range from about .5V at idle to 4.5-5 at WOT.
