The ISC system is not functional when the vehicle is moving. The ECU monitors the SPD signal from the vehicle speed sensor to determine when to operate the ISCV. If the IDL contact is closed and no SPD signal is detected, the ECU will output a signal to the ISCV.
Photocoupler Type: The Photocoupler vehicle speed sensor is also located in the combination meter and operated by the speedometer cable. The sensor consists of a photocoupler circuit and a 20-slot trigger wheel.
The photocoupler circuit is a simple electronic device which uses a phototransistor and a light emitting diode (LED) to generate a digital electrical signal (see article on Karman vortex air flow meter in Chapter 5 for operation theory of photocoupler circuit). As the slotted trigger wheel moves between the LED and phototransistor, it intermittently blocks and passes light at the photo-transistor. When the wheel blocks the LED, the transistor turns off and when the wheel passes the light, the transistor turns on.
With 20 slots, this sensor generates 20 digital pulses per speedometer revolution. An electronic circuit in the combination meter conditions this signal into four pulses which are sensed by the SPD circuit in the ECU.
Electrically, both the Reed type and Photocoupler type speed sensors work the same. The sensor is, in fact, a switch. By switching on and off, the sensor pulls a reference voltage from the ECU to ground. The resulting voltage drop is monitored by the ECU as the SPD signal.
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