IAT sensor...does anyone know what this does for the car? - PeachParts Mercedes-Benz Forum

mushedroom · #1 02-04-2003, 06:51 PM

hey y'all...

does anyone know what an IAT does for your car...aside from triggering the check engine light....

mine is dead and i'm not sure if it's vital for me to replace asap!
or if it can be done whenever....
thanks

joe p · #2 02-05-2003, 08:01 AM

It measures intake air temp. Its purpose is for input signals to the hfm/lh/me control unit for fuel trim. Personally, I'd replace it as cat converters aren't cheap.

Joe

mbdoc · #3 02-05-2003, 08:15 AM

On the 1992 300CE the sensor "helps" in the timing curve adjustment. AN open circuit sensor will cause the timing to be retarded by at least 6-8deg.

#4 02-05-2003, 08:39 AM

In general, the IAT sensor is a thermistor that measures cahnges in air temperature. As the temperature of the engine increases the resistance of the sensor decreases. The thermistor usually receives a 5 volt reference voltage through a set of pull up resisters in the ECM. The sensor will vary from about 4.7V cold to 0.4V hot. This signal is used to control fule delivery and sprk timing.

Most vehicles have a minimum and maximum temperature operating range. If the ECM detects sensor readings out of these parameters, a code will set for an open or short in that circuit. An example is an IAT sensor that is unplugged will result in a 5V signal to the ECM. As this is out of range, the ECM will set a code for an open circuit.

To diagnose, first check the elctrical connections to the IAT, then when the engine is cold (= ambient), a DVM should read about 3.5V for 50*F ambient, 3.07V for 68*F, etc, the voltage should decrease as temperature rises.

You should replace ASAP if it is bad.

This is a general diagnosis for most IAT sensors.

Arthur Dalton · #5 02-05-2003, 10:10 AM

To add to Inspectors well done post, here is a chart showing the values of the Sensor in relationship to temp.

Note that the temp/resistance are inverse [ these are called Negative Thermistors]
An ohm meter and temp comparision will tell if you are in spec.

Temp C -- Ohms
0-5900
20-2500
40-1170
50-830
60-600
70-435
80-325
90-245
100-185

So, basically, the car leans out as temp increases [ along with ignition changes per the ECU mapping]

mushedroom · #6 02-05-2003, 12:35 PM

unfortunately i don't have the luxury of all the electrical testing equipment...however my mechanic did a diagnostic and the computer gave the IAT bad error...
so i guess i'll have to replace that this weekend.
thanks you guys...

stevebfl · #7 02-05-2003, 01:09 PM

I would clear the code and see if it comes back. The code is often set from someone turning the key on with the sensor disconnected. The code stays till its cleared.

As Doc says most of its imput is to ignition timing. It also affects evaporative purging.

Arthur Dalton · #8 02-06-2003, 07:13 PM

Quote:

Originally posted by Arthur Dalton
To add to Inspectors well done post, here is a chart showing the values of the Sensor in relationship to temp.

< An ohm meter and temp comparision will tell if you are in spec.

Temp C -- Ohms
0-5900
20-2500
40-1170
50-830
60-600
70-435
80-325
90-245
100-185

So, basically, the car leans out as temp increases [ along with ignition changes per the ECU mapping]

>>

CORRECTION NOTICE:
The above specs are for ECT sensor. My All-Data has a mistake
on the IAT/ECT sensor charts.....
Here are the correct IAT specs , in case someone has copied the above error ones:

C -Ohms
10-9670
20-6060
30-3900
40-2600
50-1760
60-1220
70-860
80-640
These are +/- 5%

willjohn123 · #9 02-13-2008, 05:58 PM

If a car is started without the IAT plugged in and it trips the CEL, will the ECU know if it is plugged back in? Or does the CEL code have to be cleared for the ECU to reconize that it does have a working sensor again?

bobs · #10 02-13-2008, 06:39 PM

Stevebfl just answered that question about two posts up. The code stays until cleared.

willjohn123 · #11 02-14-2008, 11:06 AM

I know that the code stays until cleared, however my problem is that I cannot clear my codes from the DM. I was wondering if the code prevents the ECU from knowing that it has a working sensor.

willjohn123 · #12 02-18-2008, 05:57 PM

I gave up and took it to the shop today. It was that air intake sensor after all. I still do not know why I could not read or clear the codes. The ECU does not know the sensor is actually hooked up until the code is cleared.