The Intake Air Temperature (IAT) sensor is a thermistor type sensor that has a variable resistance that changes when exposed to different temperatures. When interfaced with the ECM, it produces a 0–5 volt analog signal that will deduce temperature. For IAT sensor function diagram, see Figure
“Intake Air Temperature Sensor Function Diagram”
.
Figure 1. Intake Air Temperature Sensor Function Diagram
Timing and Fuel Rate — The primary function of the IAT sensor is to measure intake air temperature to control timing and fuel rate while starting the engine in cold weather to limit smoke emissions.
Section 30.1.1
Fault Detection Management
An IAT signal that is detected out of range high or low by the ECM will cause the engine to ignore the IAT signal, and assume an ambient temperature of 77°F (25°C). For IAT sensor, see Figure
“Intake Air Temperature Sensor “
.
Figure 2. Intake Air Temperature Sensor
Connector voltage checks are listed in Table
“Connector Voltage Checks”
. These checks are performed with sensor connector disconnected and the ignition key ON.
| Test Points | Spec | Comments |
| A to Grd | 0 – .25v | If greater than 0.25 volts, signal ground wire is shorted to VREF or battery. |
| B to Grd | 4.8 – 5.0v | Pull up voltage; if no voltage, circuit has open or high resistance or short to ground. |
Connector checks to chassis ground are listed in Table
“Connector Checks to Chassis Ground”
. These checks are performed with the sensor connector disconnected, the ignition key OFF, and the positive battery cable disconnected.
| Test Points | Spec | Comments |
| A to Grd | < 5 Ω | Resistance to chassis ground, check with key OFF; if > 5 Ω, the harness is open. |
| B to Grd | > 1,000 Ω | Resistance less than 1,000 Ω indicates a short to ground. |
Harness resistance checks are listed in Table
“Harness Resistance Checks”
. These checks are performed with the breakout box installed on the chassis harness only.
| Test Points | Spec | Comments |
| #11 to A | < 5 Ω | Resistance from sensor connector to 60-pin connector – Signal ground. |
| #12 to B | < 5 Ω | Resistance from sensor connector to 60-pin connector – IAT signal. |
Operational signal checks are listed in Table
“Operational Signal Checks”
. These checks are performed with the breakout box installed in line with the ECM.
| Test Points
(+) #12 to (-) #19 |
Operational Signal Checks
(Check with breakout box installed in line w/ECM) |
|||
| Voltage | Temp °F | Temp °C | Resistance | Comments |
| 1.72v | 122 | 50 | 10.9 K Ω | Resistance checks must be taken across sensor only with the battery disconnected. |
| 3.09v | 68 | 20 | 37.34 K Ω | |
| 3.897v | 32 | 0 | 68.75 K Ω | |
| 4.33v | 0 | -18 | 120.9 K Ω | |
| 4.53v | -40 | -40 | 194.3 K Ω | |
Fault code descriptions are listed in Table
“Fault Code Descriptions”
.
| Circuit Faults | Descriptions |
| 154 | Signal voltage was less than .127 volts; more than 0.2 seconds. |
| 155 | Signal voltage was greater than 4.6 volts for more than 0.2 seconds. |
Section 30.1.2
Function
The engine control system includes an Intake Air Temperature sensor (IAT). The ECM measures the signal from the IAT sensor to determine the temperature of the air entering the engine. The ECM uses this data to adjust timing and fuel rate for starting in cold weather, to limit smoke emissions.
Section 30.1.3
Operation
The IAT is a thermistor type sensor that changes resistance when exposed to different air temperatures.
When the temperature of the intake air decreases, the resistance of thermistor increases, which causes the signal voltage to increase. When the air temperature increases, the resistance of the thermistor decreases causing the signal voltage to decrease.
The IAT sensor is supplied a regulated 5 volt reference signal at terminal B from the ECM. A return circuit (ground) is supplied at terminal A from the ECM. As the air temperature increases or decreases, the sensor changes resistance and provides the ECM with the air temperature signal voltage reading.
Section 30.1.4
ECM Diagnostics
With the ignition key ON, the ECM continuously monitors the IAT signal to determine if it is within expected values. If the signal voltage is above or below the expected levels, the ECM will set a fault code.
If the IAT sensor is not sending a correct signal, the ECM will default to 77°F (25°C).
IAT faults can be retrieved using the EST or by reading the flash codes from the warning light using the STI diagnostic switch located on the vehicle dash. If the ignition key is shut off, the code will become an inactive code. IAT codes will cause the Engine Warning light to be illuminated.
Section 30.1.5
Flash Codes
While troubleshooting the Intake Air Temperature sensor, the following codes could set.
Section 30.1.5.1
Flash Code 154
ATA Code PID 171 FMI 4
Intake Air Temp Signal out of range LOW.
An out of range low code will be set if the ECM detects the signal voltage to be less than 0.127 volts for more than 0.2 seconds. If this fault is active, the ECM will default to a value of 77°F (25°C) for starting.
Flash Code 154 may be set to a short to ground in the signal circuit or a defective sensor.
Section 30.1.5.2
Flash Code 155
ATA Code PID 171 FMI 3
Intake Air Temp Signal out of range HIGH.
An out of range high code will be set if the ECM detects the signal voltage to be more than 4.6 volts for more than 0.2 seconds. If this fault is active, the ECM will default to a value of 77°F (25°C) for starting.
Flash Code 155 may be set due to an open signal circuit between the ECM and the sensor or a short to a voltage source. A defective sensor may also cause Code 155 to set.
| Series 40E Troubleshooting Manual – 6SE241 |
| Generated on 10-13-2008 |