Where would you put the Ambient Air Temp Sensor on this car?

[OldRanger]

First, Can I just delete it?

Would you put it in the headlight duct? (Assume I close it off the opening with wire mesh). That would be relatively simple.

That’s a plywood splitter, supported by the AJ Hartman chassis mounted kit. I can put a hole in it anywhere if the OEM position is the best place. The challenge under there would be creating a little pocket. I’d have to fabricate that

[bigjae46]

If you don't have the HVAC then you don't need the ambient temp sensor.

[OldRanger]

It’s a goner!

I’ll pull the wire out of the engine compartment along with the wires for the secondary water pump or whatever that was.

thanks bigjae46

[heinzboehmer]

Agreed with bigjae46, with one caveat.

TIS says the cluster uses the reading from that sensor as one of the inputs for the oil temp gauge reading. I don't know what the logic looks like, just know that ambient temp will have an effect on the value displayed on the gauge.

If you rely on that gauge, some testing would be wise to verify that it's still accurate. Otherwise, that sensor isn't needed.

Edit: Actually more than one caveat, see karter16's reply below.

[bigjae46]

Agreed with bigjae46, with one caveat.

TIS says the cluster uses the reading from that sensor as one of the inputs for the oil temp gauge reading. I don't know what the logic looks like, just know that ambient temp will have an effect on the value displayed on the gauge.

If you rely on that gauge, some testing would be wise to verify that it's still accurate. Otherwise, that sensor isn't needed.

I think deleting the ambient temp sensor might help the oil gauge work better. I used to have an aftermarket oil temp guage and the dash was always really close.

[CrookedCommie]

I left mine in just to get a read on the gauge of air temps. I used a dremel on the driver side bumper plastic and cut a slot to fit it in. And everytime I go to remove the quick disconnect bumper I snag it on the wires and remember that's where I put it

[karter16]

First, Can I just delete it?

Would you put it in the headlight duct? (Assume I close it off the opening with wire mesh). That would be relatively simple.

That’s a plywood splitter, supported by the AJ Hartman chassis mounted kit. I can put a hole in it anywhere if the OEM position is the best place. The challenge under there would be creating a little pocket. I’d have to fabricate that

The key thing to know is that if the DME doesn't receive a valid t_umg (ambient air temp) value it reverts to using tan (intake air temp) as a back up.

t_umg is used in the following modules:

All M3
KATH (Cat heating) - informs some decisioning-making around catalyst heating abort, seems to be when air temps are in negative degrees C so doubt this is a real impact
SLV (Secondary air valve) - informs decisions around use of secondary air valve on restarts
TOG (Thermal oil level/temp sensor) - used in multiple functions (Oil temp model, ambient correction to model, adaption)
LA (Lambda) - Determines validity of lambda conversion diagnostic states
LDP (Tank Leak Diagnosis) - used in a bunch of places to assess state for when diagnosis and error monitoring does/doesn't occur
SMG - The SMG control unit is sent t_umg over CAN. I don't know what it's used for so don't know to what degree (if any) it matters.

(t_umg is also used in the modules that provide diagnostics over DS2, logging of DTCs etc, but only in that it is a data point that can be captured and logged, so haven't included those uses in this list.



For CSL conversions (running 0401 program)
GKS (Snorkel Flap) - Informs states where the flap stays closed due to low temperatures, doubt this is an impact for most people
TABG (Exhaust Gas Temp) - Used in construction of the modelled TABGM that is used by the CSL cylinder filling calculations.

[heinzboehmer]

The key thing to know is that if the DME doesn't receive a valid t_umg (ambient air temp) value it reverts to using tan (intake air temp) as a back up.

t_umg is used in the following modules:

All M3
KATH (Cat heating) - informs some decisioning-making around catalyst heating abort, seems to be when air temps are in negative degrees C so doubt this is a real impact
SLV (Secondary air valve) - informs decisions around use of secondary air valve on restarts
TOG (Thermal oil level/temp sensor) - used in multiple functions (Oil temp model, ambient correction to model, adaption)
LA (Lambda) - Determines validity of lambda conversion diagnostic states
LDP (Tank Leak Diagnosis) - used in a bunch of places to assess state for when diagnosis and error monitoring does/doesn't occur
SMG - The SMG control unit is sent t_umg over CAN. I don't know what it's used for so don't know to what degree (if any) it matters.

(t_umg is also used in the modules that provide diagnostics over DS2, logging of DTCs etc, but only in that it is a data point that can be captured and logged, so haven't included those uses in this list.



For CSL conversions (running 0401 program)
GKS (Snorkel Flap) - Informs states where the flap stays closed due to low temperatures, doubt this is an impact for most people
TABG (Exhaust Gas Temp) - Used in construction of the modelled TABGM that is used by the CSL cylinder filling calculations.

I was hoping you'd chime in

Great info to have, seems like you should keep the sensor in most cases.

[George Hill]

A lot of the newer cars put it in the duct for the radiator, that is where I am planning to mount mine.

[bigjae46]

All M3
KATH (Cat heating) - informs some decisioning-making around catalyst heating abort, seems to be when air temps are in negative degrees C so doubt this is a real impact
SLV (Secondary air valve) - informs decisions around use of secondary air valve on restarts
TOG (Thermal oil level/temp sensor) - used in multiple functions (Oil temp model, ambient correction to model, adaption)
LA (Lambda) - Determines validity of lambda conversion diagnostic states
LDP (Tank Leak Diagnosis) - used in a bunch of places to assess state for when diagnosis and error monitoring does/doesn't occur
SMG - The SMG control unit is sent t_umg over CAN. I don't know what it's used for so don't know to what degree (if any) it matters.

Ahhh...makes sense. Does the interaction with the lamda values only affect the rear O2 sensors?

Would my observation that the oil temp gauge is more accurate after deleting the ambient temp sensor make sense?

[karter16]

Does the interaction with the lamda values only affect the rear O2 sensors?

The function that's impacted is calculation of the catalytic converter efficiency. To do this the DME is looking at the pre and post cat data to calculate how efficiently the cats are running. I think your question is "is this relevant if I've got rid of the post cat O2 sensors?" in which case the answer is you don't need to worry about this use of t_umg.

Would my observation that the oil temp gauge is more accurate after deleting the ambient temp sensor make sense?

So the way the TOG determines toel (oil temp) isn't simply by measuring it, it applies a complex model of compensations.

With some of the inputs to those compensations disabled (like t_umg) the model defaults to the calculated/modelled toel_wanne (temperature of oil in the sump) more directly. So in some cases I think yes it's absolutely possible that it seems more accurate. But equally it means there's going to be a bunch of cases where it's less accurate as well, so I don't think you could say it's a universal way to make the oil gauge more accurate.

If this is something that's of interest I can definitely dig into it in more detail at some point and map out the behaviour more fully