temp sensors -40
Senior User
Joined: Jan 2009
Posts: 353
Likes: 0
From: elk plain, wa
usally when they read -40f it is cause there is a short between the wires and if it is open it will go to the highest temp that sensor was designed for. i would unplug the pcm and intake air temp and measure resistance at the iat sensor,and at the senser put one prob on one wire and other on the other. it should be OL. or you might have an open on one of the circuits, pcm pin 39/ iat gray wire, pcm pin 91/ iat gray and red tracer,if you have a open on the gray wire you can just over lay it from the pcm to sensor, if it is in the gray/red wire you will have to find the splice and it will be between the pcm and engine conector, make sure you also check both wires for a short to ground i would guess its not the gray/red wire cause you would have alot of problems if that was shorted to ground but the gray wire at the iat could be shorted to ground
Lead Driver
Joined: Oct 2003
Posts: 8,476
Likes: 37
From: Madison, WI
Well the IAT must be used for something. They wouldn't have it as a PCM input if it wasn't used for injection timing or fuel control or something.
But the scanner may be looking for an IAT address/signature (or whatever it's called) that's different from what the PSD uses. Run AE against one of our trucks one day, with the Enhanced Ford feature activation, and you'll see at least two parameters referring to intake air temp, and one of them reads the actual value and the other defaults to -40.
But the scanner may be looking for an IAT address/signature (or whatever it's called) that's different from what the PSD uses. Run AE against one of our trucks one day, with the Enhanced Ford feature activation, and you'll see at least two parameters referring to intake air temp, and one of them reads the actual value and the other defaults to -40.
Hotshot
Joined: Aug 2006
Posts: 10,764
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1996 PCED On Board Diagnostics I SECTION 18: Diesel Diagnosis — 7.3L DI Engine
--------------------------------------------------------------------------------
C: Intake Air Temperature (IAT) Sensor
Introduction C: Pinpoint Tests
--------------------------------------------------------------------------------
Signal Functions
The intake air temperature (IAT) sensor is a thermistor-type sensor with a variable resistance that changes when exposed to different temperatures. When interfaced with the powertrain control module (PCM), it produces a 0 to 5 volt analog signal that will measure temperature.
The IAT sensor's primary function is to measure ambient air temperature in order to determine when the exhaust back pressure device is needed.
Detection/Management
An IAT signal that is detected out of range high or low by the PCM will cause the engine to ignore the IAT signal, disable exhaust back pressure operation and assume an ambient temperature of 15°C (59° F).
Note
After removing connectors, always check for damaged pins, corrosion, loose terminals, etc.
CONNECTOR CHECKS TO GROUND (B-)
(CHECK WITH SENSOR CONNECTOR DISCONNECTED AND IGNITION KEY OFF, ALL ACCESSORIES OFF) Test Points Spec. Comments
A to Grd <5 ohms Resistance to ground, if greater than 5 ohms the harness is open.
B to Grd >5 ohms Resistance less than 5 ohms indicates a short to ground.
CONNECTOR VOLTAGE CHECKS
(CHECK WITH SENSOR CONNECTOR DISCONNECTED AND IGNITION KEY ON) Test Points Spec. Comments
B to Grd 4.65-5 volts Pull up voltage, if no or low voltage circuit has open or high resistance.
A to Grd 0-.25 volts If greater than .25 volts signal wire is shorted to VREF or battery.
HARNESS RESISTANCE CHECKS
(CHECK WITH BREAKOUT BOX INSTALLED ON HARNESS ONLY) Test Points Spec. Comments
No. 39 to B <5 ohms Resistance from sensor connector to 104 pin connector.
No. 91 to A <5 ohms Resistance from sensor connector to 104 pin connector.
k Ohms IAT (Volts) Degrees C Degrees F
1.19 0.28 120 248
1.56 0.36 110 230
2.08 0.47 100 212
2.80 0.61 90 194
3.84 0.80 80 176
5.34 1.04 70 158
7.55 1.34 60 140
10.93 1.72 50 122
16.11 2.15 40 104
24.25 2.63 30 86
37.34 3.09 20 68
58.99 3.52 10 50
DTC Descriptions
0113 = Intake Air Temperature Sensor Circuit High Input.
0112 = Intake Air Temperature Sensor Circuit Low Input.
--------------------------------------------------------------------------------
C: Intake Air Temperature (IAT) Sensor
Introduction C: Pinpoint Tests
--------------------------------------------------------------------------------
Signal Functions
The intake air temperature (IAT) sensor is a thermistor-type sensor with a variable resistance that changes when exposed to different temperatures. When interfaced with the powertrain control module (PCM), it produces a 0 to 5 volt analog signal that will measure temperature.
The IAT sensor's primary function is to measure ambient air temperature in order to determine when the exhaust back pressure device is needed.
Detection/Management
An IAT signal that is detected out of range high or low by the PCM will cause the engine to ignore the IAT signal, disable exhaust back pressure operation and assume an ambient temperature of 15°C (59° F).
Note
After removing connectors, always check for damaged pins, corrosion, loose terminals, etc.
CONNECTOR CHECKS TO GROUND (B-)
(CHECK WITH SENSOR CONNECTOR DISCONNECTED AND IGNITION KEY OFF, ALL ACCESSORIES OFF) Test Points Spec. Comments
A to Grd <5 ohms Resistance to ground, if greater than 5 ohms the harness is open.
B to Grd >5 ohms Resistance less than 5 ohms indicates a short to ground.
CONNECTOR VOLTAGE CHECKS
(CHECK WITH SENSOR CONNECTOR DISCONNECTED AND IGNITION KEY ON) Test Points Spec. Comments
B to Grd 4.65-5 volts Pull up voltage, if no or low voltage circuit has open or high resistance.
A to Grd 0-.25 volts If greater than .25 volts signal wire is shorted to VREF or battery.
HARNESS RESISTANCE CHECKS
(CHECK WITH BREAKOUT BOX INSTALLED ON HARNESS ONLY) Test Points Spec. Comments
No. 39 to B <5 ohms Resistance from sensor connector to 104 pin connector.
No. 91 to A <5 ohms Resistance from sensor connector to 104 pin connector.
k Ohms IAT (Volts) Degrees C Degrees F
1.19 0.28 120 248
1.56 0.36 110 230
2.08 0.47 100 212
2.80 0.61 90 194
3.84 0.80 80 176
5.34 1.04 70 158
7.55 1.34 60 140
10.93 1.72 50 122
16.11 2.15 40 104
24.25 2.63 30 86
37.34 3.09 20 68
58.99 3.52 10 50
DTC Descriptions
0113 = Intake Air Temperature Sensor Circuit High Input.
0112 = Intake Air Temperature Sensor Circuit Low Input.
Thread Starter
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Junior User
Joined: Mar 2012
Posts: 73
Likes: 0
Very interesting. Thanks fo the information! So engine oil temp is what the PCM uses for fueling, if I am to understand all this correctly? Also do you have a link to the above reference material? Thanks
Postmaster
Joined: Sep 2007
Posts: 4,195
Likes: 6
From: Medford Oregon
This link has most of the info you might need as sensors go ...
97-15A 7.3L DI Turbo Workshop Manual
97-15A 7.3L DI Turbo Workshop Manual
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