BimmerFest BMW Forum banner
1 - 10 of 10 Posts

·
Registered
Joined
·
5 Posts
Discussion Starter · #1 ·
Afternoon all,

I have a couple of issues with my second hand E39 temperature control. I've spent several days going through relivant posts but have been unable to locate an answer to the following. I was hoping one of you might be kind enough to help if you know the answer.

When I bought my E39, the ambiant temp sensor was registring -40, so I replaced it. It still registers -40!. This weekend I'll try shorting the connection to see if I can force it to re-calibrate.

However, does anyone know if the incorrect reading of -40 is why the fan controls allways seem to be on auto all the time regardless of if I switch it to manual or auto?.

Even if I increase or decrease the fan initially, sooner or later the fan ramps up and apart from turnning off periodically for a few seconds, always ends up on full pelt.

Its the last electrical problem I have to resolve and its driving me nuts. Well except it drowns out my wife nagging!.

Any help greatfully received.

Regards.
Nobby.
 

·
Registered
Joined
·
15,745 Posts
The external temp sensor is an input to the auto HVAC (IHKA), so you need to sort that out. It may, or may not, fix the fan problem. There may be a defect in the IHKA system.

Shorting will not "recalibrate" it. You need to troubleshoot the circuit.

Was the sensor missing? Was there a connector into which you plugged the new sensor? Or did you actually have to splice the wires together?
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #3 ·
Hi edJack,

Thanks for the response.

My father-in-law did the work for me as he is a BMW owning mechanic!. He said the original sensor was missing. The new one attached to a connector.

I guess I need to hunt down a circuit diagram and get my old amp meter out this weekend.

Regards.
Nobby.
 

·
Registered
Joined
·
214 Posts
Had a similar problem when the external temp probe came loose and dragged on the highway for a few miles. The result: crazy fan and -40.
 

·
Registered
Joined
·
3,362 Posts
Bad outside temp sensor will indeed create major havoc and automatically adjust the blower, as the computer "thinks" it is indeed -40 F outside and needs to keep you alive.... Did you try driving a bit after the new sensor was installed? Generally, the temp sensor will NOT reset to current ambient temperature readings unless you drive around a bit. If still bad, then first double-check the connections to the new sensor, and then look to finding and replacing the fuse (I don't know where it is, but probably in the fuse bloom behind the glove compartment).
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #6 ·
Waveho,

Thanks for the reply. Your reply in conjunction with that from edJack confirm what I thought with regard to the fan.

I've driven about 250 miles since replacing the sensor so I figure if it was going to re-calibrate it would have done so by now.

On occasion the temperature reading is initally different (never correct) but allways goes back to -40. Pretty much the same as it did before.

According to the BMW service dept where the sensor came from, -40 indicates the sensor is faulty!.

As you suggest, I guess I need to check the fuse and the wiring for breaks. Hopefully thats all it is!.

Regards.
Nobby.
 

·
Registered
Joined
·
3,362 Posts
Good luck! My understanding is that the -40 reading is the "default" where the computer is no longer receiving any data from the sensor. My guess is that the connection is bad.
 

·
livin the 'Fest
Joined
·
4,280 Posts
is it possible that maybe your father inlaw, may not put it back in the right place, or i hope this didn't happen but maybe the new one fell off.
mine tryied to read -40, so i took off cleaned the tip put it back on worked fine.
 

·
Freude am Fahren
Joined
·
6,378 Posts
Good luck! My understanding is that the -40 reading is the "default" where the computer is no longer receiving any data from the sensor. My guess is that the connection is bad.
The sensor is a variable resistor. Completely open is -40, completely closed is... +40°C or something.
 
1 - 10 of 10 Posts
Top