1.Interior temperature sensor
(1)Construction
The interior temperature sensor uses the thermistor and is installed in the instrument panel with the aspirator. The aspirator utilizes the air blown from the blower to suck the air inside the vehicle in order to detect the average interior temperature.
(2)Function
It detects the interior temperature that is used as the basis for the temperature control.
2.Ambient temperature sensor
(1)Construction
The ambient temperature sensor uses the thermistor and is installed at the front of the condenser. It detects the exterior temperature.
(2)Function
It detects the exterior temperature that is used for controlling the fluctuation of the interior temperature caused by the exterior temperature fluctuation.
3.Solar sensor
(1)Construction
The solar sensor uses a photodiode and is installed in the upper portion of the instrument panel. It detects the amount of sunlight.
(2)Function
It detects the amount of sunlight that is used for controlling the fluctuation of the interior temperature caused by the amount of sunlight fluctuation.
4.Evaporator temperature sensor
(1)Construction
The evaporator temperature sensor uses the thermistor and is installed on the evaporator. It detects the temperature of the air passing through the evaporator (surface temperature of the evaporator).
(2)Function
It is used for frost prevention, temperature and time-lag airflow control.
5.Water temperature sensor
(1)Construction
The water temperature sensor uses the thermistor. It detects the coolant temperature based on the engine water temperature sensor. The signal is transmitted from the engine ECU.
In some models, the water temperature sensor is installed at the heater core.
(2)Function
It is used for temperature control, warm-up control, etc.
Some vehicles are equipped with the following sensors.
Duct sensors
The duct sensors use the thermistor and are installed at inside of the side register. It detects the temperature of the air blown toward the side register and precisely controls the air temperature of the individual airflow.
Smog ventilation sensor
The smog ventilation sensor is installed in the front portion of the vehicle and detects the level of CO (carbon monoxide), HC (hydrocarbons), and NOx (oxides of nitrogen) to control switching between FRESH and RECIRC.

Never reuse the center airbag sensor assembly involved in a collision where any SRS has deployed.
The connector to the sensor should be connected to or disconnected from the center airbag sensor assembly on condition that the center airbag sensor assembly is mounted on the floor. If the connectors are connected or disconnected while the center airbag sensor assembly is not mounted on the floor, it could cause an undesired ignition of the supplemental restraint system.
Work must be started when more than 90 seconds pass after the ignition switch is turned to the LOCK position and the battery negative (-) terminal cable is disconnected, even if only loosening the set bolts of the center airbag sensor assembly.
Wire harness and connector
The SRS wire harness is integrated with the instrument panel wire harness assembly. If the SRS wire harness becomes disconnected or the connector becomes broken due to an accident, etc., repair or replace it.
Spiral cable (in combination switch)
The steering wheel must be fitted correctly to the steering column with the spiral cable at the neutral position, otherwise cable broken and other troubles may occur.

Front airbag sensor
Never reuse the front airbag sensors involved in a collision that activated the SRS front airbags or seat belt pretensioners. (Be sure to replace both left and right front airbag sensors together with center airbag sensor assembly.)
Install the front airbag sensor with its arrow mark facing toward the front of the vehicle.
The front airbag sensor set bolts have been antirust treated. When the front airbag sensor is removed and reinstalled, always use new installation bolts or bolts and nuts.

Side airbag sensor (side and curtain shield airbag sensor)
Never reuse the side airbag sensors involved in a collision that activated the SRS side airbags or curtain shield airbags. (Be sure to replace the center airbag sensor assembly together with the side airbag sensor on the collision side.)

Curtain shield airbag sensor
Never reuse the curtain shield airbag sensors involved in a collision that activated the SRS curtain shield airbags. (Be sure to replace the center airbag sensor assembly together with the curtain shield airbag sensor on the collision side.)

Replacement requirements
Airbag system components
In the following case, replace the airbag system components.
If the airbag has been deployed.
If the airbag system components have been found to be faulty in troubleshooting.
If the airbag system components have been found to be faulty during the visual check.
If the airbag system components have been dropped.

The seat position airbag sensor is used because of the adoption of the dual-stage inflator in the driver’s airbag. The seat position airbag sensor is installed on the seat rail under the driver’s seat. It detects the driver’s physique according to the seat’s sliding position, and sends the signal to the center airbag sensor assembly. The center airbag sensor assembly gently deploys the airbag when the seat position is forward and the deceleration speed is low.
The seat position airbag sensor detects two levels of position where the magnetic line of flux is cut (backward of the seat) and is not cut (forward of the seat) by the cut-off plate installed on the back of the seat rail.

The side door sensors are only set in the two-door or three-door hatchback vehicle with the side airbag, and are installed inside the front doors. The side door sensor detects the impact from the side and sends the deceleration signal to the center airbag sensor assembly. Based on the signal, the center airbag sensor assembly activates the side airbag and curtain shield airbag.

The occupant detection sensor, which is enclosed in the seat cushion of the front passenger seat, is used to detect whether or not the front passenger seat is occupied.

The sensor shown in the illustration consists of a construction in which two sheets of electrodes sandwich a spacer. When the occupant is seated, the electrode sheets come in contact with each other through the hole that is provided in the spacer portion, thus enabling the current to flow. As a result, the center airbag sensor assembly detects the presence of the occupant. Using this signal, some models do not operate the airbag when there is no one in the front passenger seat. This signal also controls the front passenger seat seatbelt warning light lighting (when there is no one on the front passenger seat, the warning light does not come on the warning light).

The side airbag sensors (side and curtain shield airbag sensor) are mounted on the right and left center pillars, and the curtain shield airbag sensors are mounted on the right and left rear pillars.
The side airbag sensors (side and curtain shield airbag sensor) and curtain shield airbag sensor consist of deceleration sensor, safing sensor, ignition control circuit and diagnosis circuit.
The side airbag sensors (side and curtain shield airbag sensor) detects the impact from the side and sends the deceleration signal to the center airbag sensor assembly. The center airbag sensor assembly activates the side airbag and curtain shield airbag based on the deceleration signal from the side airbag sensor.
The curtain shield airbag sensor detects the impact from the side and sends the deceleration signal to the center airbag sensor assembly. The center airbag sensor assembly activates the curtain shield airbag based on the deceleration signal from the curtain shield airbag sensor.
HINT:
There are two types of side airbag sensors (side and curtain shield airbag sensor) and curtain shield airbag sensor: One with a built-in safing sensor and one without.

The front airbag sensors are mounted on the right and left front side members.
The sensors cannot be disassembled.
The front airbag sensors detect the impact from the front and send the deceleration signal to the center airbag sensor assembly.
The deceleration sensor is stored in the front airbag sensor.
There are two types of deceleration sensor: a type made from a semi-conductor and a mechanical type that uses an eccentric rotor.
There used to be a type that had no front airbag sensor and controlled the front airbag system only with the center airbag sensor assembly.

The center airbag sensor assembly is mounted on the center floor under the instrument panel and consists of diagnosis circuit, ignition control circuit, deceleration sensor, safing sensor, etc.
Front airbags and seat belt pretensioners circuit
Side and curtain shield airbags circuit (When the safing sensor is stored in the side airbag sensor and curtain shield airbag sensor)
(1)Diagnosis circuit
This circuit constantly diagnoses the system for any malfunction. When a malfunction is detected, it lights up or blinks the SRS warning light to warn the driver.
(2)Ignition control circuit
The ignition control circuit performs a prescribed calculation based on the signal output by the deceleration sensors of the airbag sensor assembly and front airbag sensor assembly. If these calculated values are larger than a predetermined value, it activates the ignition operation.

(3)Deceleration sensor
Based on the deceleration of the vehicle that occurs during a frontal collision, the distortion that is created in the sensor is converted into an electric signal. This signal is a linear representation of the deceleration rate.
(4)Safing sensor
The safing sensor, which is built in the center airbag sensor assembly, turns on if a deceleration force that is higher than a predetermined value is applied to the safing sensor as a result of a collision.
(5)Back-up power source
The back-up power source consists of a power supply capacitor and a DC-DC converter. In case of a power system failure during a collision, the power supply capacitor discharges and supplies electric power to the system. The DC-DC converter is a boosting transformer when the battery voltage drops below a certain level.
(6)Memory circuit
When a malfunction is detected by the diagnosis circuit, it is coded and stored in this memory circuit.
The codes can be retrieved later to identify the location of the malfunction and provide quicker troubleshooting. Depending on the model, this memory circuit is either the type in which the memory contents are erased when power is interrupted, or the type in which the memory contents are retained even if the power is interrupted.
In some models, center airbag sensor assembly transmits the signal that deploys the airbags to the body ECU and unlocks all the doors, which makes it easier for the occupants to get out of the vehicle or to be rescued in case of an accident.
At this time, center airbag sensor assembly also transmits signal to the engine ECU to stop the fuel pump.

The detection capability of the oxygen sensor and A/F (air-fuel) ratio sensor decline at low temperatures (under 400 °C). Therefore, some oxygen sensor or A/F ratio sensor are provided with a heater to heat the elements. The engine ECU controls the amount of the current of the heater in accordance with the intake air mass and engine speed. In other words, when the engine load is small and the exhaust gas temperature is low, the amount of the current flowing to the heater increases in order to maintain the sensor efficiency. However, when the engine load and exhaust gas temperature increase, the heater is stopped operating or the amount of the current flowing to the heater is decreased.
Air Conditioner Control System
The engine ECU turns OFF the A/C compressor in accordance with vehicle conditions to maintain good drivability and acceleration performance. For example, when rapidly accelerating from a low engine speed, the engine ECU turns OFF the A/C compressor in accordance with the vehicle speed, engine speed, throttle valve position, and intake manifold pressure or intake air mass. There are two types of air conditioner control systems. One type indirectly controls the air conditioner operation via the A/C amplifier. The engine ECU sends an ACT signal to the A/C amplifier to disengage the magnetic clutch of the A/C compressor. In the other type, the engine ECU directly controls the air conditioner operation by operating the magnetic clutch relay. With some engine models, after the air conditioner switch is turned ON, the magnetic clutch operation is delayed for a moment. At this time, the engine ECU opens the ISC valve to increase the engine speed to prevent the engine speed from dropping when the air compressor operates. This delay control function is called air conditioner compressor delay control.
Cooling Fan Control
There are various types of cooling fan controls in addition to the one shown in the illustration. Up to now, the fan speed was controlled by having the water temperature switch control the fan relay. Currently, some engine ECUs control the fan relay to control the fan speed, or the cooling fan ECU to control the fan speed.
HINT:
As shown in the illustration, low speed operation lowers the voltage applied to the motor using a resistor placed in series in the circuit to reduce the cooling fan speed, or two motors are connected in series to reduce the fan speed.

The knock sensor is attached to the cylinder block, and sends a KNK signal to the engine ECU when engine knocking is detected. The engine ECU receives the KNK signal and retards the ignition timing to suppress the knocking. This sensor contains a piezoelectric element, which generates an AC voltage when knocking causes vibration in the cylinder block and deforms the element. The engine knock frequency is in the range of 6 to 13 kHz depending on the engine model. The proper knock sensor is used in accordance with the knocking generated by each engine. There are two types of knock sensors. As can be seen from the graph, one type generates a high voltage in a narrow vibration frequency range, and the other generates a high voltage in a wide vibration frequency range. Recently some sensors that detect open and short circuits, as shown in the illustration, have come into use. In this type of circuit, 2.5 V is constantly supplied so the KNK signal is also output with a 2.5 V base frequency. STA (Starter) Signal/NSW (Neutral Start Switch) Signal
The STA signal is used to detect whether or not the engine is cranking. The main role of the signal is to gain approval from the engine ECU to increase the fuel injection volume during cranking. As can be seen from the circuit diagram, the STA signal detects in the engine ECU the same voltage that is supplied to the starter. NSW (Neutral Start Switch) signal This signal is only used in vehicles with an automatic transaxle, and is used to detect the shift lever position. The engine ECU uses this signal to determine if the shift lever is in the Por Nposition, or other position. The NSW signal is mainly used to control the ISC system.
A/C (Air Conditioner) Signal / Electrical Load Signal and Variable Resistor

A/C (Air Conditioner) signal
The A/C signal differs depending on the vehicle model, but it detects if the magnetic clutch of the air conditioner or the air conditioner switch is ON. The A/C signal is used by the ignition timing control during idling, ISC system control, fuel cut-off, and other functions.
Electrical load signal
The electrical load signal is used to detect if the headlights, rear window defogger, or other devices are ON. As can be seen in the circuit diagram, this signal circuit has several electric load signals. Depending on the vehicle model, these are compiled together and sent to the engine ECU as a single signal, or each signal is sent individually to the engine ECU. Electric load signals are used to control the ISC system.
Variable Resistor
The variable resistor is used to change the air-fuel ratio during idling and to adjust the idling CO. The variable resistor is installed in models without an oxygen sensor or air fuel ratio sensor. When the idle mixture adjusting screw is turned to the R direction, the contact inside the resistor moves to increase the VAF terminal voltage. Conversely, when the screw is turned to the L direction, the VAF terminal voltage is decreased. When the VAF terminal voltage increases, the engine ECU slightly increases the fuel injection volume to make the air-fuel mixture a little richer.
HINTAs the vane type air flow meter has an idle mixture adjusting screw in its body, a variable resistor is not required even if there is no oxygen sensor.