How the intake-air temperature sensor works
Intake-air temperature sensor
Here you will find useful basic information and important tips relating to the intake-air temperature sensor in vehicles.
The intake-air temperature sensor might be a fairly unassuming component, but it plays a key role. Its task is to supply the engine control unit with an important measurement for correcting the mixture formation and the ignition. This page explains the functional principle of the intake-air temperature sensor, the effect of faults, and what could cause the sensor to fail. You will also find step-by-step instructions on how to check the sensor.
Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for suitably qualified personnel only.
TABLE OF CONTENTS
HOW THE INTAKE-AIR TEMPERATURE SENSOR WORKS: FUNCTIONAL PRINCIPLE
The intake-air temperature sensor determines the temperature in the suction pipe and forwards the voltage signals arising from the temperature to the control unit. This evaluates the signals and influences the mixture formation and the firing angle.
The resistance of the temperature sensor changes depending on the intake-air temperature. As the temperature increases, the resistance is reduced, which reduces the voltage at the sensor. The control unit evaluates these voltage values, since they are directly related to the intake-air temperature (low temperatures result in high voltage values at the sensor, and high temperatures result in low voltage values).
Intake-air temperature sensor
INTAKE-AIR TEMPERATURE SENSOR FAULTY: SYMPTOMS
A faulty intake-air temperature sensor can manifest itself in different ways through fault detection by the control unit and the resulting emergency program strategy.
Frequent fault symptoms are:
- Fault code is stored, engine indicator lamp may come on
- Problems starting
- Reduced engine power
- Increased fuel consumption
Effects of failure
CAUSES OF A FAULTY IAT SENSOR: CAUSE OF FAILURE
Failure can be caused by different reasons:
- Internal short circuits
- Breaks in wiring
- Wiring short circuit
- Mechanical damage
- Sensor tip dirty
CHECKING THE INTAKE-AIR TEMPERATURE SENSOR: TROUBLESHOOTING
Checks are carried out using the multimeter
Test step 1
The internal resistance of the sensor is determined. The resistance is temperature-dependent. When the engine is cold it is high-impedance, and when the engine is hot it is low-impedance.
Depending on the manufacturer:
25°C 2.0 – 6 KOhm or 80°C approx. 300 Ohm
Please note the special reference value specifications.
Test step 2
Check the wiring to the control unit by checking continuity and short circuit to frame for every wire to the control unit plug.
- Connect ohmmeter between temperature sensor plug and removed control unit plug. Reference value: approx. 0 Ohm (circuit diagram required for pin assignment on control unit).
- Check the respective pin at the sensor plug against ground using an ohmmeter and with the control unit plug removed. Reference value: >30 MOhm.
Test step 3
Check the supply voltage at the removed sensor plug using the voltmeter. This is done with the control unit plugged in and the ignition turned on. Reference value: approx. 5 V.
If the voltage value is not reached, the voltage supply of the control unit and the ground supply must be checked according to the circuit diagram. If these are OK, a control unit fault comes into question.
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