Just sharing..........

I have always assumed, ambient air temp sensor in a car is only for driver personal "weather" information. Like one in our room.

Since I am doing bumper removal to do 530mm LED BAR install under registration plate, my bumper been removed for a more than a week now.
During this time I was also doing lots of other things to the engine, example intake valve carbon cleaning and installing Banks Gauge various pressure and temperature sensors...etc etc.
So I have to test my engine with bumper removed.

The bumper itself has 2 connectors only. 1 multipin one at the right side ( mine is a facelift W212 ) which connects to the parking/proximity 6 sensors and 1 of 2 pin connector direct to the Ambient Air Temp sensor,
which I shall now call AAT sensor.

Depending on your car engine model and other comfort options, missing an AAT can create stupid DTC

Engine has not run yet afer the mechanical things I did on it, but I need to scan and cleared all codes. Stubbord DTC below. I can't clear it.



So, it seems, some components or modules in our car needs Ambient Temperature data as a reference temperature.
without AAT, the computers in our car also can not calculate temperature delta, for example the aftercooler or what not.
I am sure the HVAC system will be impacted too, without an AAT.

Last nite I re-installed the bumber for testing purpose and disconnected the AAT while doing so, suddenly both my 641 LED ILS fan started running .
This one I think is a fail safe for 641 LED ILS, it deemed no AAT data means it is too hot a temperature and the fan need to run.

So, keep this information on the back of your mind, when the AAT started to drift its value or super slow to response, best replaced it with a new one.

Xentry will overtime and after a few drive cycle, will report on the AAT status for sure.
What we worry is, if the AAT make other modules starting to trigger DTCs and not mentioning AAT is the culprit, you could be pulling your hair wondering what went wrong.

DTCs are great, but they are not as direct or as accurate as we expect it to be, at least some DTC sentences/explanation at certain conditions.

Have fun maintaining ur car.....

 

Thank you for showing us what weird disfunctions are linked to a missing ambient air temp sensor.

ISM... fake overheat !

The MB logic assumes crazy temp values (ISM Shifter, ILS LED,..) instead of just flagging the missing sensor. That's really good to know!

Ambient Temp:
We knew the cluster display was showing doctored outside temperature.

- When the car is stopped it shows temp when car was last moving at x-speed.
- When the car is started it shows temp when car was last stopped.

In the end it's a LIAR like the coolant gauge that tells it's own story...

It's almost surprising the radiator fan does not go supersonic with bad Temp, like it does on bad A/C pressure.....

 

AAT is used by the HVAC system to command fan speed in AUTO climate control setting.

IAT (intake air temperature) is another (very) high priority sensor as it calculates air density for fuel injection calculations. My IAT sensor for a 3.5L NA M276 is in the air filter housing assembly, upstream of the throttle servo.

 

Most common is the IAT as a part of MAF.
I had AAT malfunction on my Sprinter and I had to set AC to LOW for it to start cooling. Replaced the sensor and now it works set to 75F.

 

Came across this tread. Interesting.

Chasing a P2279 and occasionally a P2277 code (engine lean, part throttle bank 1 and 2 respectively). No vacuum leaks that I can find and no other codes except today...

Now getting IAT sensor code that the circuit is High, meaning either open or shorted to 12V. I know I have two intake air temp sensors on my M276. One just after the air filter, and one on the intake plenum. Same part number. Was getting 46 deg. C today with the engine fully warmed up. But the outside temp was around 13 deg. C. Is it possible that the intake air would be that high on a running engine that's always bringing fresh air in?

But this thread will have me keep an eye on the AAT also, which I noted seems showing an accurate number.

 

Non turbo engine : 46C intake air temperature when car is not moving is normal if engine components got heat soaked already by being stationary long enough.
It will be very cool like +5C above ambient temperature when you speed 100KM/H or more.

My turbo charged M276 can be as cool as only +15C above ambient if I run 120KM/h long enough on the highway.
If in creeping slow traffic it can be as bad as 65C or more, while ambient temp is 30-32C.

 

I was looking at the data stream on my icarsoft MBII scanner today, and it seems that the intake air temperature sensor is reading correctly considering what you said above and so is the ambient air temperature sensor. For the intake air temperature sensor, I still don't know whether the computer is looking at the one on the intake plenum or the one after the air filter, so I'll probably try unplugging one and then maybe swapping them. I don't know if there are any adaptations that I can do to bring my fuel trims down. They're around plus 30 on bank 1 and about 22 on bank 2. So either too much unmetered air going in or not enough fuel coming out of the injectors or an issue with some sensor even though I don't have any other codes. But under load, I don't have any issues. So I figured, if the injectors weren't putting out enough fuel, the problem would be more evident at higher loads, which it's not. Even in open loop, any problems with injectors should show itself, I would think. I have a 144,000 miles, and other than the code, the engine runs well. I even changed the oil separator and pcv valve trying to see if that was the cause for the lean condition.

Ignore the tensioner lol. IAT is #10 on top left (pic is mirrored in the parts diagram), and #165 on bottom right.

 

To test any temperature sensor is quite easy.
When engine cold , OFF already +24 hours, all temperature sensors will read ambient temperature of your location, +-2C error only at the most.

Looking at wiring diagram, you got two IAT sensors




If you want to ID them, disconnect 1 of them and run a scan, DTC will show open circuit or short circuit on that sensor.

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At 144,000 miles, you best replaced all 4 oxygen sensors, as they are already at its end of service.
The front ones ( wide band ) are what decides the fuel trim, based on the sensing they done for AFR.
If they are not accurate, aka have drifted, you will may end up getting bad fuel trim.

It is a wear item, supposedly 150,000 KM good life in standard car. They live in exhaust stream.... super hot.
This is the family we use on M276 wide band ( front ) oxygen sensors , Bosch LSU4.9 :
https://www.bosch-motorsport.com/con...7199306612107/
https://www.milspecwiring.com/DATA%2...U4.9Sensor.pdf


Regardless of other possible issues your M276 may have at this matured mileage, replace these 4 oxygen sensors, the rear one is very cheap because they are narrow band ones.
Get Bosch one from FCP Euro, genuine Bosch without the MB logo...much cheaper and it is what MB uses too.
My M276.8 3.0 Turbo, rear sensor comes from NTK, front one from Bosch.

I have replaced mine all 4 at 10th year age of my car and at only 36,00KM or so. I often exceeded 930C exhaust temperature too when I run hard.
Mine was decent, but the rear ones are slow response already.
They get slow response as they aged.

 

Really appreciate the quick reply. Will go over the items you mentioned one by one.

I did notice that the code started popping up gradually over the past four or five years.

It may very well be the oxygen sensors.